New specimens of Acyon myctoderos (Metatheria, Sparassodonta) from Quebrada Honda, Bolivia

NEW SPECIMENS OF ACYON MYCTODEROS (METATHERIA, SPARASSODONTA) FROM QUEBRADA HONDA, BOLIVIA RUSSELL K. ENGELMAN1 FEDERICO ANAYA2 DARIN A. CROFT3

Department of Biology, Case Western Reserve University, 10900 Euclid Ave., Cleveland, Ohio 44106, USA. Facultad de Ingeniería Geológica, Universidad Autónoma Tomás Frías, Av. del Maestro s/n, Potosí, Bolivia. 3 Department of Anatomy, Case Western Reserve University School of Medicine, 10900 Euclid Ave., Cleveland, Ohio 44106, USA. 1 2

Submitted: July 15th, 2014 - Accepted: November 19th, 2014

To cite this article: Russell K. Engelman, Federico Anaya, and Darin A. Croft (2015). New specimens of Acyon myctoderos (Metatheria, Sparassodonta) from Quebrada Honda, Bolivia. Ameghiniana 52: 204–225. To link to this article: http://dx.doi.org/10.5710/AMGH.19.11.2014.2803

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Marine and terrestrial palynomorphs and the Silurian–Devonian age of marine deposits of western Argentina.

ARTICLES

AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

ISSN 0002-7014

NEW SPECIMENS OF ACYON MYCTODEROS (METATHERIA, SPARASSODONTA) FROM QUEBRADA HONDA, BOLIVIA RUSSELL K. ENGELMAN1, FEDERICO ANAYA2, AND DARIN A. CROFT3 1

Department of Biology, Case Western Reserve University, 10900 Euclid Ave., Cleveland, Ohio 44106, USA. [email protected]

2

Facultad de Ingeniería Geológica, Universidad Autónoma Tomás Frías, Av. del Maestro s/n, Potosí, Bolivia. [email protected]

3

Department of Anatomy, Case Western Reserve University School of Medicine, 10900 Euclid Ave., Cleveland, Ohio 44106, USA. [email protected]

Abstract. Despite being the dominant group of South American mammalian carnivores for much of the Neogene, most post-early Miocene sparassodonts (Metatheria) are poorly known. Here, we describe new specimens of the hathliacynid sparassodont Acyon myctoderos Forasiepi et al. from the middle Miocene locality of Quebrada Honda, Bolivia, including a juvenile specimen that preserves the first known hathliacynid DP3. This specimen is unique among sparassodonts in preserving both upper and lower deciduous premolars and suggests that hathliacynids may have differed from borhyaenoid sparassodonts in having simultaneous eruption of M3 and m4 rather than M4/m4. Another specimen from Quebrada Honda tentatively assigned to A. myctoderos preserves the first known long bones (femur, tibia) of this genus, allowing us to estimate its body mass (13–17.5 kg) based on postcranial data. These new specimens document further morphological and ontogenetic variation within A. myctoderos as well as the Sparassodonta as a whole. Key words. Sparassodonta. Hathliacynidae. Metatheria. Bolivia. Miocene.

Resumen. NUEVOS EJEMPLARES DE ACYON MYCTODEROS (METATHERIA, SPARASSODONTA) DEL MIOCENO MEDIO DE QUEBRADA HONDA, BOLIVIA. A pesar de ser el grupo dominante de mamíferos carnívoros de América del Sur durante gran parte del Neógeno, la mayoría de los esparasodontes de post-Mioceno temprano son poco conocidos. Aquí, describimos nuevos ejemplares del esparasodonte hatliacínido Acyon myctoderos Forasiepi et al. del Mioceno medio de la localidad de Quebrada Honda, Bolivia, incluyendo un ejemplar juvenil que conserva el primer DP3 hatliacínido conocido. Este ejemplar es único entre los esparasodontes en preservar los premolares de leche superiores e inferiores y sugiere que los hatliacínidos se podrían haber diferenciado de los esparasodontes borienoideos en la erupción simultánea del M3 y m4 en vez del M4/m4. Otro ejemplar de Quebrada Honda, asignado tentativamente a A. myctoderos, conserva los primeros huesos largos conocidos (fémur, tibia) para este género, los que nos permiten estimar su masa corporal (13–17,5 kg) con base en datos postcraneos. Estos nuevos ejemplares documentan variación morfológica y ontogenética adicional dentro de A. myctoderos así como de Sparassodonta en su conjunto. Palabras clave. Sparassodonta. Hathliacynidae. Metatheria. Bolivia. Mioceno.

DURING South America’s period of ‘splendid isolation’, most

poor after the late early Miocene (Santacrucian South

terrestrial carnivorous mammal niches were occupied by

American Land Mammal Age or SALMA). Most species of

the Sparassodonta, an endemic group of metatherian

late Neogene (middle Miocene–early Pliocene) hathliacynids

mammals that were an important component of the conti-

are only known from their type specimens, many of which

nent’s predator guild for much of the Cenozoic (Marshall,

are fragmentary or poorly preserved (e.g., Notocynus her-

1977a; Marshall and Cifelli, 1990; Prevosti et al., 2013). The

mosicus Mercerat, 1891a, Notictis ortizi Ameghino, 1889),

most abundant group of sparassodonts at most South

and as a result almost nothing is known about the anatomy

American fossil localities are the Hathliacynidae (e.g., Sin-

or paleobiology of these species.

clair, 1906; Marshall, 1981; Villarroel and Marshall, 1982;

One of the few South American fossil localities known

Prevosti et al., 2012), a group of weasel or civet-like species

to produce well-preserved late Neogene hathliacynid re-

that occupied a number of small carnivore niches from the

mains is Quebrada Honda, Bolivia (Fig. 1). Quebrada Honda

late Oligocene to the Pliocene (Argot, 2003a; Forasiepi,

has produced one of the most diverse pre-Pleistocene

2009; Prevosti et al., 2013). However, despite their relative

mammal faunas outside of southern South America (>30° S;

abundance, the fossil record of hathliacynids is extremely

Flynn et al., 2012) since its discovery in 1976 (Hoffstetter,

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ENGELMAN ET AL.: QUEBRADA HONDA ACYON

Argentino de Ciencias Naturales “Bernardino Rivadavia”, Colección Nacional Ameghino, CABA, Argentina; MLP, Museo de La Plata, La Plata, Buenos Aires, Argentina; MNHN-Bol, Museo Nacional de Historia Natural, La Paz, Bolivia; MNHN SCZ, Muséum National d’Histoire Naturelle, Paris, France; MPEF-PV, Museo Paleontológico Edigio Ferugulio, Trelew, Chubut, Argentina; MPM-PV, Museo Regional Provincial Padre M. Jesús M. Río Gallegos, Santa Cruz, Argentina; Figure 1. Relative position of Quebrada Honda in time (left) and space (right), modified from Croft (2007). Approximate amount of time represented by the fossil-bearing sediments at Quebrada Honda is highlighted in black. The position of Quebrada Honda in Bolivia is indicated with a star.

UATF-V, Universidad Autónoma Tomás Frías, Potosí, Bolivia; UCMP, University of California Museum of Paleontology, Berkeley, USA; UF, Florida Museum of Natural History, University of Florida, Gainesville, USA; YPM-VPPU, Yale Peabody Museum of Natural History, Vertebrate Paleonto-

1977), including numerous species of metatherians, xe-

logy, Princeton University Collection, New Haven, Connecti-

narthrans, rodents, and endemic ungulates (see Croft, 2007;

cut, USA.

Croft et al., 2011, 2013). Sparassodonts were first noted at

Anatomical abbreviations. P/p, upper and lower premolars;

Quebrada Honda in 1981 (Frailey, 1981; MacFadden and

M/m, upper and lower molars; I/i, upper and lower incisives;

Wolff, 1981) based on specimens collected at this locality

D/d, upper and lower deciduous teeth; R/L, right and left;

in 1978 by an expedition from the University of Florida

StA–E, stylar cusps.

(MacFadden and Wolff, 1981). However, these specimens were only identified in a faunal list as ‘Cladosictis’ (Frailey,

MATERIALS AND METHODS

1981) or ‘Borhyaenidae’ (MacFadden and Wolff, 1981) and

Measurements of specimens were taken with digital

were never described nor figured. Nothing more was known

calipers to the nearest 0.01 mm, unless otherwise noted.

about the sparassodonts of Quebrada Honda until Fora-

Additional data on the holotype of A. myctoderos and other

siepi et al. (2006) described a new species of large-bodied

species of sparassodonts was taken from the published

hathliacynid from the site, Acyon myctoderos Forasiepi,

literature or personal observations by one or more of the

Sánchez-Villagra, Goin, Takai, Shigehara, and Kay, 2006,

authors. The taxonomy of Sparassodonta and Hathliacyni-

based on a specimen in the collections of the Museo Na-

dae used in this paper follows Forasiepi (2009).

cional de Historia Natural in La Paz. A second small-bodied

Body mass for UATF-V-001400 (identified here as cf.

sparassodont species unique to Quebrada Honda was de-

Acyon myctoderos) was estimated using measurements of

scribed in 2014 (Engelman and Croft, 2014), and a third

the femur based on two equations from Anyonge (1993)

species is currently under study (Croft et al., 2013). In this

and also used by Argot (2003a) to estimate the body mass

paper, we describe new specimens of Acyon myctoderos dis-

of Cladosictis patagonica Ameghino, 1887, and Sipalocyon

covered through our research group’s ongoing field investi-

gracilis Ameghino, 1887:

gations at Quebrada Honda as well as several previously undescribed specimens from the Florida Museum of Natural History that we believe correspond to previous references

(Equation 1) body mass = e (2.92*

log(proximodistal length of femur)–5.27)

(Equation 2) body mass = e (2.88*log(circumference of femur at midshaft)–3.40)

of sparassodonts at this locality. These new specimens pro-

These equations were chosen to facilitate comparisons

vide additional insights into the paleobiology of A. mycto-

with previous estimates, though we note their use in esti-

deros, including its evolutionary relationships, ontogeny,

mating body mass in carnivorous marsupials has been criti-

individual variation, and estimated body mass.

cized (Wroe in Argot, 2004a; Ercoli and Prevosti, 2011).

Institutional abbreviations. AMNH, American Museum of

Shape-based morphometric postcranial body mass esti-

Natural History, New York, USA; FMNH, Field Museum of

mates of this specimen (i.e., as in Ercoli and Prevosti, 2011)

Natural History, Chicago, Illinois, USA; MACN-A, Museo

are beyond the scope of this paper.

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AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

Abundance data for Quebrada Honda were calculated

24), suggesting that the two taxa may not necessarily be

by surveying 873 fossil vertebrate specimens in the collec-

conspecific. With the description of the new specimens of

tions of the University of Florida and the Universidad Autó-

Acyon from Quebrada Honda, more specimens of Acyon are

noma Tomás Frías that could be identified to family level.

known from this locality than from all other localities in

Data for specimens in other collections (e.g., MNHN-Bol)

South America combined.

were not available. Given that Quebrada Honda represents

Another specimen that has previously been referred to

an attritional fossil assemblage rather than a bonebed, we

the genus Acyon is MACN-A 5988, the holotype of “Ictio-

calculated abundance based on NISP (Number of Identified

borus destructor” Ameghino, 1894, which was assigned to

Specimens) where each specimen with a separate specimen

“Anatherium defossus” Ameghino, 1887, by Marshall (1981,

number was considered to represent a distinct individual

p. 104) but only tentatively referred to Acyon tricuspidatus

with the exception of the UF Acyon (as detailed below), which

by Forasiepi et al. (2006, p. 671). Our examination of the pre-

was the only individual represented by multiple specimens.

molar region of MACN-A 5988 suggests that this specimen probably does not belong in Acyon. The diastema between

SYSTEMATIC PALEONTOLOGY

p1–2 (represented by the exposed dorsal margin of the dentary) is less than half the length of a single premolar

MAMMALIA Linnaeus, 1758

root, and p2–3 are very close to one another, with only a

METATHERIA Huxley, 1880

thin wall of bone separating their alveoli. Similarly, the

SPARASSODONTA Ameghino, 1894 HATHLIACYNIDAE Ameghino, 1894 Genus Acyon Ameghino, 1887

diastema between p1–2 is less than half the length of a premolar root. Although premolar diastemata are reduced or absent in some specimens of Acyon and Cladosictis (see below), this feature is related to the juvenile status of these specimens. By contrast, MACN-A 5988 represents a very

Type species. Acyon tricuspidatus Ameghino, 1887 p. 8; original designation.

old individual (Marshall, 1981, pp. 106–108), and the morphology of this specimen does not compare favorably to

Included species. The type species, A. myctoderos Forasiepi,

adult specimens of Acyon and Cladosictis. The mandibular

Sánchez-Villagra, Goin, Takai, Shigehara, and Kay, 2006, and

symphysis is more extensive in this specimen than in speci-

Acyon herrerae Marshall, 1981.

mens of Acyon, reaching the p3/m1 embrasure (Marshall,

Stratigraphical and geographical distribution. Sarmiento

1981: fig. 77), and the dentary is relatively deeper (see table

Formation, Colhue-Huapi Member (Gran Barranca), early

on mandibular ramus height in large-bodied hathliacynids,

Miocene (Burdigalian age; Colhuehuapian SALMA), Patago-

below).

nia, Argentina (Marshall, 1981); Santa Cruz Formation, late early Miocene age (Burdigalian age; Santacrucian SALMA),

Acyon myctoderos Forasiepi, Sánchez-Villagra,

Patagonia, Argentina (Marshall, 1981); unnamed formation

Goin, Takai, Shigehara, and Kay, 2006.

of the Honda Group (Quebrada Honda), middle Miocene age

Figures 4–6

(Serravallian age; Laventan SALMA), Department of Tarija, Bolivia (Forasiepi et al., 2006).

Type specimen. MNHN-Bol-V-003668, a partial skull pre-

Remarks. In addition to the Acyon material from Quebrada

serving the left side of the cranium and a complete mandible,

Honda, only three other specimens have been referred to

atlas, axis, four additional vertebral bodies, and a partial pes.

this genus, two of which, MLP 11-64 and FMNH P13521,

Referred specimens. Three UF specimens belonging to a

represent the holotypes of A. tricuspidatus and A. herrerae,

single individual (the UF Acyon): UF 26921, partial right

respectively. A. herrerae may be a junior synonym of A. tri-

maxilla preserving M3–4; UF 26933, partial left maxilla pre-

cuspidatus (Forasiepi et al., 2006, p. 671). However, the holo-

serving M2–4, left dentary fragment preserving broken

type of A. herrerae is much closer in size to A. myctoderos

roots of three teeth, axis vertebra, and a partial distal

than the holotype of A. tricuspidatus (Marshall, 1981: tab.

metapodial; UF 26941, partial right maxilla preserving M2

206

ENGELMAN ET AL.: QUEBRADA HONDA ACYON

and the alveoli of M1. UATF-V-000926, partial juvenile skull

species of Cladosictis (premolar morphology is unknown in

preserving a right dentary with root of i3, partial lower ca-

A. tricuspidatus).

nine in two pieces, complete postcanine dentition with dp3

Remarks. The specimen label of UF 26921 reads “with

and erupting p3 and m4, isolated LDP3, and RM1–2 and

26933”, indicating that these two specimens were found

erupting RM3 missing protocone. Originally, the RM1–3 of

together. They represent right and left parts of the upper

this specimen were preserved in life position by the sur-

dentition, respectively, and compare favorably in size, struc-

rounding matrix, but these teeth became separated during

ture, and wear. UF 26941 was catalogued as an RM3 but

preparation.

actually represents an RM2, and the posterior end of this

Provisionally referred specimen. UATF-V-001400, articulated

tooth fits perfectly with the anterior end of UF 26933, indi-

partial skeleton preserving the right femur, right tibia, right

cating that they are pieces of the same specimen. For sim-

fibula, right astragalus, left and right calcanei, much of the

plicity, we refer to these specimens as the UF Acyon.

spinal column (unprepared), many rib fragments, and small

The UF Acyon was collected during a University of Flo-

cranial fragments (including one small, otherwise unidenti-

rida expedition to Quebrada Honda in June of 1978 and was

fiable tooth fragment).

listed in UF collections as pertaining to the late early

Localities and horizons of referred specimens. The UF Acyon

Miocene (Santacrucian SALMA) genus Cladosictis (Croft,

was collected from Section 1, Unit 2 of MacFadden and Wolff

2007). Cladosictis was also included in the faunal list of

(1981) from exposures near the town of Quebrada Honda

Frailey (1981: tab. I-1), presumably in reference to the same

(i.e., the Quebrada Honda Local Fauna sensu Croft et al.,

specimens, as no other sparassodont fossils from Que-

2011). UATF-V-000926 was also collected from exposures

brada Honda (excluding UF 27881, see Engelman and Croft,

near Quebrada Honda at an equivalent stratigraphic level.

2014) are listed in the UF collections database. As detailed

UATF-V-001400 was collected approximately 3 km south

below, these specimens pertain to A. myctoderos and thus,

of the town of Papachacra from exposures on the opposite

Cladosictis is no longer known to occur at Quebrada Honda.

(north) side of the river from the Río Rosario section of Mac-

All parts of UATF-V-000926 pertain to an eroded skull

Fadden et al. (1990). We consider fossils from this area to

of one individual and were collected in a single plaster jacket

comprise the Papachacra Local Fauna. This is the same

by one of the authors (F. A.).

area that produced the holotype of A. myctoderos (Forasiepi et al., 2006, and M. Takai pers. comm., January, 2013).

Description

The stratigraphy of the Papachacra area has not yet been

The UF Acyon. The right maxilla of the UF Acyon, preserved in

described but is currently under study by our team. Thus,

two parts, includes the roots of M1 and complete M2–4 (Fig.

the precise stratigraphic level of UATF-V-001400 relative

2.1; Tab. 1). The left maxilla of the UF Acyon preserves the

to the scheme proposed by MacFadden and Wolff (1981)

protocone and posterior root of M2 as well as M3–4 (Fig.

is still uncertain. Significant slumping of higher levels is

2.2). Like the holotype (Forasiepi et al., 2006, p. 674), but un-

present on the Papachacra side of the river, which raises

like some other hathliacynids such as Cladosictis and Sipa-

the possibility that this specimen, along with the holotype of

locyon (Forasiepi, 2009), neither of these maxilla fragments

A. myctoderos (MNHN-Bol-V-003668), derive from higher

exhibit maxillary ‘cheeks’. Two pairs of palatal pits are visi-

(younger) levels than the bulk of the Quebrada Honda Fauna

ble in the UF Acyon, one lingual to M2–3 and the other lingual

(D. Croft, pers. obs.).

to M3–4. The posterior pair of palatal pits is deeper than the

Emended diagnosis. Differs from Cladosictis and other species

anterior one, creating noticeable depressions in the surface

of Acyon by its larger size, shallower dentary, and longer

of the palate. Similar pits have been reported in many other

premolar diastemata in adults. Differs from A. herrerae by the

sparassodonts (Forasiepi, 2009; Engelman and Croft, 2014),

presence of a distinct posterior cusp on p2 and from both A.

although only a single pair was reported in the holotype of

herrerae and A. tricuspidatus by having larger hypoconids and

A. myctoderos (Forasiepi et al., 2006). Similar variation in the

more vertically oriented hypoconulids on m1–3. Premolars

number of palatal pits has been observed in the closely re-

less sectorial than in A. herrerae but slightly more so than in

lated Cladosictis patagonica (Forasiepi, 2009), suggesting

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AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

Figure 2. UF Acyon, cranio-dental remains; 1, UF 26921, UF 26941, right maxilla preserving M2–4; 2, UF 26933, left maxilla preserving M3– 4; 3–4, UF 26921, dentary fragment; 3, labial view; 4, lingual views. Anterior is to the right in 1 and 3 and to the left in 2 and 4. Abbreviation: pp, palatal pit. Scale bars= 10 mm.

TABLE 1. Measurements of the upper dentition of the new specimens of Acyon myctoderos compared to the holotype of the species (MNHN-Bol-V003668) and representatives of the large-bodied hathliacynids Chasicostylus castroi and Cladosictis patagonica. Specimen

M1

M2

M3

M4

L

W

L

W

L

W

L

W

UF 26921-26941 (left)

-

-

11.66*

-

12.07

9.00

3.93

8.76

UF 26921-26941 (right)

10.76*

-

11.78

7.39

10.65

8.45

3.89

9.09

UATF-V-000926

10.71

7.47

12.20

7.86

12.45

-

-

-

MNHN-Bol-V-003668 (left)

11.85*

7.45*

10.60*

3.05

10.85

3.15

5.50

7.20

MNHN-Bol-V-003668 (right)

10.04*

5.07*

-

-

-

-

-

-

8.60

4.40

8.90

4.80

-

-

-

-

MACN-A 9350

6.90

4.70

7.40

5.40

8.00

6.10

4.70

2.50

MACN-A 5927 (left)

7.00

5.30

8.00

6.00

9.00

6.60

5.70

2.80

MACN-A 5927 (right)

7.20

5.20

8.10

5.80

8.80

6.50

5.50

2.80

Acyon myctoderos

Chasicostyluscastroi MLP 57-X-9-2 Cladosictis patagonica

Estimated measurements are demarcated by an asterisk. Measurements of the holotype of Acyon myctoderos were taken from Forasiepi et al., 2006, and measurements of Cladosictis patagonica and Chasicostylus castroi were taken from Marshall, 1981. All measurements in millimeters.

208

ENGELMAN ET AL.: QUEBRADA HONDA ACYON

that the observed differences in specimens of Acyon are due

structure of the talon suggests that a paraconule and meta-

to individual variation.

conule were also present on these teeth, but the prepro-

The M2–3 are very similar to one another, with some

tocrista and postprotocrista are worn on both teeth so the

minor structural differences. On both teeth the metacone

presence of these cusps cannot be determined with cer-

and paracone are heavily worn, with the two cusps being

tainty. In the UF Acyon, M4 is oriented such that the long

almost indistinguishable on M2. By contrast, the metacone

axis of the tooth is nearly perpendicular to the rest of the

is taller and less worn than the paracone on M3. The proto-

tooth row, as in some other hathliacynids (e.g., Sipalocyon).

cone is heavily worn and located slightly more anterior to

However, in both the holotype of A. myctoderos (Forasiepi

the paracone than in specimens of Cladosictis. The proto-

et al., 2006) and some specimens of Cladosictis patagonica

cone of M3 is larger than in Cladosictis but smaller than in

(Marshall, 1981), the long axis of M4 is rotated such that

Sipalocyon and is basined. A paraconule and metaconule are

the protocone is posterolingual to the paracone.

present on M3 but are difficult to distinguish due to wear.

In contrast to the upper dentition, very little of the

Like in most sparassodonts, the upper molars of the UF

lower dentition of the UF Acyon is preserved (Fig. 2.3–4). No

Acyon have a reduced stylar shelf with only two stylar

tooth crowns are preserved and much of the roots are also

cusps, StA–B. In M2, StA is anterolabial to the paracone

missing. The ventral edge of the mandible is broken and

and connects to this cusp by a short, heavily worn pre-

remnants of the mandibular canal are visible in ventral

paracrista. StB resembles the holotype of A. myctoderos,

view. There is no evidence of diastemata between the teeth,

where this cusp is represented by a swelling almost at the

suggesting that some of the roots may pertain to molars.

level of the midpoint between the paracone and metacone

The most anterior roots are implanted at an angle, similar to

(Forasiepi et al., 2006, p. 678) and is not connected to StA by

the premolars of other hathliacynids, whereas the posterior

an ectocingulum. StA is more posteriorly located in M3, and

roots are more vertical, suggesting that this portion of the

is connected to StB by an ectocingulum. The preparacrista of

dentary housed p3–m2. This interpretation is supported

the RM3 is almost perpendicular to the long axis of the

by an anterodorsal-posteroventrally inclined ridge that

tooth, whereas that of LM3 is oriented more anterolabially.

ends above the anterior root of the possible p3 and proba-

The anterolabial cingulum of this tooth is highly reduced,

bly represents the posteriormost part of the mandibular

being restricted to a small portion of the anterior face of the

symphysis. In most other hathliacynids, including the holo-

paracone on M2 and virtually absent. The ectoflexus does

type of A. myctoderos, the mandibular symphysis termi-

not seem to be as well developed as in observable speci-

nates posteriorly below the anterior root of p3 (Marshall,

mens of Cladosictis. The postmetacrista is well-developed,

1981; Forasiepi et al., 2006). Two mental foramina are pre-

being more than twice the length of the preparacrista.

served in this specimen, one below the posterior root of p3

The M4 of the UF Acyon is well preserved on both sides

and the other below the midpoint of m1. The positions of

of the jaw. This tooth has only two roots and three main

these foramina are similar to those reported for the holo-

cusps; the paracone, protocone, and StB. The M4 of the UF

type of A. myctoderos (Forasiepi et al., 2006).

Acyon lacks any vestige of a metacone, in contrast to the

The atlas vertebra of the UF Acyon is relatively well-

condition in the holotype of A. myctoderos (Forasiepi et al.,

preserved, missing the distal portion of the transverse

2006). Similar variation is present in Sipalocyon gracilis;

processes, the dorsal portion of the neural arch, and the

the metacone is absent in YPM-VPPU 15373 but present

dorsal edge of the cranial facets (Fig. 3; Tab. 2). The neural

–yet not entirely distinct from the cingulum– in MACN-A

arch is fused to the intercentrum, as in the holotype of A.

692. In the UF Acyon, the protocone is aligned with the para-

myctoderos and specimens of Cladosictis. The anterior ar-

cone and preparacrista in RM4, as in the holotype of A.

ticular facets are distinctly curved, whereas the posterior

myctoderos, but is anterolingual to the paracone in LM4. The

articular facets are flatter and face posteromedially. It is not

protocone is basined in both RM4 and LM4, as in the holo-

possible to determine if the dorsal border of the anterior

type (Forasiepi et al., 2006; see also Fig. S1) and is larger than

facets were curved as in other hathliacynids and Hondadel-

in Cladosictis patagonica (where this cusp is basinless). The

phys Marshall, 1976a (Forasiepi et al., 2006; Forasiepi,

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AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

deros, but in this specimen only one foramen is present on each side. These foramina are also present in Sipalocyon and Borhyaena Ameghino, 1887 and may have transmitted branches of the vertebral artery (Forasiepi et al., 2006). In addition to the material described above, the UF Acyon includes a distal portion of a metapodial. The holotype of A. myctoderos only preserves proximal metapodial fragments, precluding direct comparison. The shape of this metapodial fragment resembles the proximal end of metacarpal III of Cladosictis (Argot, 2003a) and may represent the same element, though this is not certain. UATF-V-000926. UATF-V-000926 is unusual among sparassodonts in preserving part of the deciduous dentition. Few juvenile sparassodont specimens are known from the fossil record, only five of which preserve deciduous teeth (Tab. 3). Most of these deciduous teeth represent dp3, with DP3 only being known from a specimen of cf. Pharsophorus (AMNH 29591; Marshall, 1976b; Forasiepi and Sánchez-Villagra, 2014) and the thylacosmilids Patagosmilus Forasiepi and Carlini, 2010, and Thylacosmilus Riggs, 1933, which retain DP3 into adulthood (Goin and Pascual, 1987; Forasiepi Figure 3. UF Acyon, UF 26933, atlas; 1, dorsal view; 2, ventral view. Anterior is to the top in both images. Scale bar= 10 mm.

and Carlini, 2010; Forasiepi and Sánchez-Villagra, 2014). An isolated tooth (MPEF-PV 4766) from the early Miocene (Colhuehuapian SALMA) of Patagonia may also represent a DP3, given its strong resemblance to the DP3 of AMNH

TABLE 2. Postcranial measurements of the UF Acyon.

29591 (Goin et al., 2007). Although no DP3s have previously

Greatest length of atlas

27.19

been described for hathliacynids (and the only comparable

Greatest width of atlas

45.06

teeth are either damaged [cf. Pharsophorus] or represent a

Greatest length of metapodial

10.49

highly derived condition relative to other species; Forasiepi

Greatest width of metapodial

9.20

and Sánchez-Villagra, 2014), this tooth can be identified as

All measurements in millimeters.

DP3 based on several lines of evidence: 1) this tooth is only about 3/4 the length of M1 (roughly 7.06 mm long by 4.42 mm wide, compare with measurements in Tab. 1), relatively

2009). The anterior articular facet is separated from the

the same size as the DP3/dp3s of other sparassodonts

transverse process dorsally by the sulcus of the atlantal

(Forasiepi and Sánchez-Villagra, 2014, p. 232); 2) this tooth

foramen, but because of breakage it is not possible to de-

has three roots, two labial and one lingual, as in the DP3s

termine if the atlantal foramen was completely enclosed as

of thylacosmilids (Forasiepi and Carlini, 2010; Forasiepi

in Cladosictis, Sipalocyon, and the holotype of A. myctoderos.

and Sánchez-Villagra, 2014). The lingual root is smaller than

The large transverse foramen is located on the ventral side

the anterolabial root, but the posterolabial root is not well-

of the atlas from the atlantal foramen, just posterior to the

preserved, making it impossible to determine if this root

anterior cranial facets. Several small foramina are present

was the largest root of DP3 as in thylacosmilids (Forasiepi

at the dorsal junction between the transverse process and

and Carlini, 2010); and 3) there are wear facets on the post-

the neural arch, two on the left side and three on the right.

metacrista and metacone, whereas M2–M3 are almost

Similar foramina are present in the holotype of A. mycto-

unworn.

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ENGELMAN ET AL.: QUEBRADA HONDA ACYON

TABLE 3. List of known juvenile sparassodont specimens. Specimen

Taxon

Reference

MPEF-PV 1620

Acrocyon riggsi

Goin et al., 2007

MACN-A 52-385

Acrocyon riggsi

Marshall, 1978

MLP 11-70

Acrocyon sectorius

Marshall, 1978

UATF-V-000926*

Acyon myctoderos

Present Study

MLP 82-V-1-1

Arminiheringia sp.

Forasiepi and Sánchez-Villagra, 2014

MPM-PV 3554*

Borhyaena tuberata

Forasiepi and Sánchez-Villagra, 2014

YPM-VPPU 15097*

Cladosictis patagonica

Sinclair, 1906; Marshall, 1981

MNHN SCZ 145

Cladosictis patagonica

Marshall et al., 1984

MPM-PV 3646*

Cladosictis patagonica

Forasiepi and Sánchez-Villagra, 2014

UCMP 39250

cf. Dukecynus sp.

Marshall, 1978; Forasiepi et al., 2004

UCMP 39251

Hondadelphys fieldsi

Marshall, 1976

UCMP 38061

'Lycopsis' longirostrus

Marshall, 1977

AMNH 29591*

cf. Pharsophorus sp.

Marshall, 1978

MACN-A 642*

Prothylacynus patagonicus

Forasiepi and Sánchez-Villagra, 2014

MACN-A 5931-5937

Prothylacynus patagonicus

Marshall, 1976, 1979

MLP 11-39

Sipalocyon gracilis

Marshall, 1981

Specimens were considered to represent juvenile individuals if at least one tooth was still in the process of eruption at the time of death. Specimens preserving dP3 or dp3 are demarcated by an asterisk.

As in most marsupials, the DP3 of A. myctoderos re-

either side of this cusp (Fig. S2). The preparacrista runs from

sembles a smaller, simplified version of the upper molars.

the paracone to the stylar shelf, which has two very small

The DP3 of A. myctoderos is more molariform than that of cf.

stylar cusps (StA–B) and forms a short but conspicuous ec-

Pharsophorus or thylacosmilids, as would be expected given

tocingulum. A small swelling of enamel anterior to the para-

the more generalized dental morphology of hathliacynids.

cone may represent the anterolabial cingulum (Fig. S2).

The paracone is much smaller and much more strongly ap-

The RM1–3 of UATF-V-000926 are much less worn

pressed to the metacone than in any of the upper molars,

than those of most sparassodont specimens (e.g., the UF

to the point that the paracone is nearly subsumed into the

Acyon described above) and thus provide a more detailed

metacone. The paracone and metacone are aligned with

picture of the dental anatomy of this species (Fig. 4). M1–3

the postmetacrista, as in Patagosmilus and Thylacosmilus

are similar in shape, with a large metacone and a slightly

(Forasiepi and Sánchez-Villagra, 2014), but unlike the DP3 of

smaller paracone. These two cusps are adjoined at their

AMNH 29591 where the postmetacrista is oblique. How-

base, but their tips are separate and connected by a linear

ever, the DP3 of A. myctoderos is much less sectorial than in

centrocrista. This contrasts with the condition in most

Patagosmilus and Thylacosmilus, and the postmetacrista is

sparassodont specimens, including the UF Acyon, in which

less developed. The protocone of DP3 is much more reduced

the bases of the paracone and metacone are nearly com-

than the protocone of the upper molars, as in AMNH 29591,

pletely fused and the centrocrista is obliterated by wear.

and there is a rudimentary paraconule and metaconule on

The protocones of M1–2 are comparable in size to those

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Figure 4. Acyon myctoderos, UATF-V-000926, photographs and line drawings of the upper dentition. 1–2, left DP3; 3–4, right M1; 5–6, right M2; 7–8, right M3, all in occlusal view. Light grey represents bone, dark grey represents broken bone or matrix. Anterior is to the left in 1 and to the right in 2–4. Abbreviations: anc, anterolabial cingulum; mc, metacone; mec, metaconule; pac, paracone; par, paraconule; pmc, postmetacrista; ppc, preparacrista; prc, protocone; StA, stylar cusp A; StB, stylar cusp B. Scale bar= 10 mm.

seen in Cladosictis patagonica and the holotype of A. myc-

ble that this cusp could represent StA, in which case the

toderos (Forasiepi et al., 2006: fig. 8), but smaller than in C.

other might be an anteriorly displaced StB. A cast of the

centralis Ameghino, 1902a (Goin et al., 2007). These cusps

holotype of A. myctoderos at the MLP exhibits a similar mor-

are distinctly basined, as in most hathliacynids (Forasiepi,

phology, with a worn stylar cusp anterior and slightly labial

2009), and bear a distinct metaconule and paraconule posi-

to the paracone on LM1. The presence of stylar cusps lo-

tioned lingual to the bases of the metacone and paracone,

cated anterior to the paracone and metacone is a feature

respectively. The M3 is broken near the base of the para-

characteristic of, and more prominently developed in, some

cone and metacone, and so the morphology of the trigon of

late Miocene hathliacynids such as Chasicostylus castroi

the tooth is unknown.

Reig, 1957 (Marshall, 1981), suggesting a possible relation-

The stylar shelf of UATF-V-000926 is highly reduced,

ship between this species and A. myctoderos (see Discus-

especially labial to the paracone, and has only two stylar

sion). The stylar shelf morphology of M3 is unknown, as this

cusps. In M1, both cusps are anterior to the paracone,

tooth is still partially encrypted (see below).

whereas in M2 StA is anterior to the paracone and forms a

The ectoflexus of M1–2 is very shallow, to the point

small ectocingulum with StB, which is positioned postero-

where this structure is almost absent on M1 and the labial

lingual to the paracone, as in the holotype of A. myctoderos

edge of this tooth is nearly straight (Fig. 4.1). By contrast,

(Forasiepi et al., 2006, p. 678). By contrast, the identities of

the ectoflexus is deeper on M3, similar to the meristic gra-

the two cusps on M1 are harder to determine. One stylar

dient seen in the holotype of A. myctoderos (Forasiepi et

cusp is small and is aligned anteroposteriorly with the para-

al., 2006) and other sparassodonts. The enamel layer is

cone and metacone, whereas the other cusp is located an-

exposed in cross-section near the ectoflexus of M3 and is

terolabial to this cusp in the parastylar corner. Based on the

relatively thin (~0.1 mm), similar to what has been docu-

former cusp’s connection with the preparacrista, it is possi-

mented in other hathliacynids (von Koenigswald and Goin,

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ENGELMAN ET AL.: QUEBRADA HONDA ACYON

Figure 5. Acyon myctoderos, UATF-V-000926, left dentary; 1, labial view; 2, occlusal view; 3, lingual view. Anterior is to the right in 1–2 and to the left in 3. Scale bar= 10 mm.

2000). The anterolabial cingulum is visible on M1–2, but is

sodonts. The postmetacrista is longer than the preparacrista

larger on M2. On M1 the cingulum is primarily present be-

in M1–2, similar to the postmetacrista of other sparas-

tween the base of the paracone and the anteriorly directed

sodonts. Like the UF Acyon, the upper molar roots of UATF-

stylar cusps, whereas on M2 it extends towards, but does

V-000926 are short, only about as long as the crown.

not contact, the paraconule. The visible portion of M3 sug-

The M3 of UATF-V-000926 is attached to and still par-

gests that the anterolabial cingulum was highly reduced, as

tially covered by a large portion of the labial alveolar wall,

in the UF Acyon. The preparacrista of M1 is initially aligned

indicating that this tooth had apparently not fully erupted

with the paracone and metacone, but turns anterolabially

by the time of the animal’s death. Only the apices of the

after contacting the more lingual of the two stylar cusps. By

paracone, metacone, and the anterior portion of the post-

contrast, the entire preparacrista of M2 is directed antero-

metacrista are exposed above the edge of the alveolar frag-

labially, as in many other hathliacynids. The preparacrista of

ment in labial view. Furthermore, the base of the crown is

M3 is more obliquely oriented than in M1–2 (but not fully

still covered by a layer of bone, indicating that the roots of

perpendicular), similar to what is observed in other sparas-

this tooth had not yet begun to form. Combined with the

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AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

TABLE 4. Relative proportions of the dentary of UATF-V-000926 compared to other species of large-bodied hathliacynids. Specimen

Dm1

Lm1

Dm1/Lm1

Lm2

Dm1/Lm2

MNHN-Bol-V-003668 (left)

17.07

8.80

1.94

10.05

1.70

UATF-V-000926

14.51

9.83

1.48

10.52

1.38

FMNH P13521 (left)

21.00

8.40

2.50

9.22

2.28

FMNH P13521 (right)

21.50

8.90

2.42

8.90

2.42

18.70

-

-

8.00

2.34

20.50

-

-

8.00

2.56

MACN-A 664

14.50

-

-

7.00

2.07

MACN-A 6288 (left)

20.20

7.00

2.89

7.20

2.81

MACN-A 6288 (right)

21.00

7.00

3.00

-

-

MACN-A 9360

14.00

6.20

2.26

6.60

2.12

MLP 11-10

14.50

6.90

2.10

7.20

2.01

Acyon myctoderos

Acyon herrerae

Acyon tricuspidatus MLP 11-64 Sparassodonta indet. MACN-A 5988 Cladosictis patagonica

Measurements of the holotype of Acyon myctoderos from Forasiepi et al. (2006). Measurements of other large-bodied hathliacynids from Marshall (1981). All measurements in millimeters. Abbreviations: Dm1, depth below m1; Lm1, length of m1; Lm2, length of m2.

partially erupted m4 of this specimen, this suggests that

Villagra, 2014). It remains to be seen if this pattern of dental

M3 and m4 probably erupted simultaneously in A. mycto-

eruption was more widely distributed among sparassodonts

deros. This type of eruption pattern also occurs in several

or was unique to Acyon, given that eruption sequences are

other groups of omnivorous and carnivorous metatherians,

unknown for other hathliacynids.

most notably the recently extinct thylacine (Forasiepi and

The dentary of UATF-V-000926 is very shallow and

Sánchez-Villagra, 2014) but has not previously been docu-

of uniform depth, measuring no more than 17 mm at its

mented in sparassodonts. In other sparassodonts for which

greatest preserved height (Fig. 5; Tab. 4). The mandibular

juvenile specimens are known, M4 and m4 erupt almost si-

ramus is shallower than in closely-related hathliacynids,

multaneously (e.g., Arminiheringia Ameghino, 1902b, “Lycop-

including Cladosictis patagonica and other species of Acyon,

sis” longirostrus Marshall, 1977b, Prothylacynus patagonicus

but this may be due to its young age. However, the mandible

Ameghino, 1891; Forasiepi and Sánchez-Villagra, 2014).

of the adult holotype of A. myctoderos though deeper than

Indeed, UATF-V-000926 is the first known sparassodont

UATF-V-000926, is also relatively shallow, as suggested

to lack the close synchrony in the eruption of M4 and m4

by a recent morphometric analysis of dentary shape in

seen in other members of this group. Similar variation in

sparassodonts (Prevosti et al., 2012), indicating that a shallow

eruption sequences between species is present in other

dentary (compared to other large-bodied hathliacynids)

groups of carnivorous metatherians, such as didelphoids

may be characteristic of A. myctoderos. The mandibular sym-

(Tribe, 1990; Voss and Jansa, 2009; Forasiepi and Sánchez-

physis extends posteriorly to a point below the posterior

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ENGELMAN ET AL.: QUEBRADA HONDA ACYON

root of p2, making it less extensive than in the UF Acyon and

posterior to m1 (Forasiepi et al., 2006; Forasiepi, 2009).

the holotype of A. myctoderos, and is completely smooth, in

Since this area is poorly preserved in UATF-V-000926, it is

contrast to the holotype of A. myctoderos where at least a

possible that additional foramina would have been present

few ridges and grooves are present (Forasiepi et al., 2006, p.

in the complete ramus.

677). These differences are likely due to age, given that a

The only remnant of the lower incisors in UATF-V-

similar ontogenetic pattern is present in the borhyaenoid

000926 is a small root located near the base of the lower

“Lycopsis” longirostrus, where juveniles have a poorly-de-

canine that most likely represent i3, based the fact that

veloped and less rugose mandibular symphysis than older

the root is not anteroposteriorly compressed as in i1–2 of

individuals (Marshall, 1977b; Goin, 1997).

the holotype of A. myctoderos (Forasiepi et al., 2006). The

Four mental foramina are present in the lower jaw of

lower canine of this specimen is large (Tab. 5) and is im-

UATF-V-000926. This is typical for most sparassodonts,

planted at a 35° angle to the long axis of the jaw, similar to

which generally have three or four foramina but can have as

the holotype of A. myctoderos. This tooth was complete

many as six (e.g., Acrocyon riggsi [Sinclair, 1930], “Lycopsis”

when originally discovered in the field, but the middle por-

longirostrus, and possibly Arctodictis Mercerat, 1891b; Mar-

tion of this tooth was subsequently lost (Fig. S3). The tooth

shall, 1977b; Goin et al., 2007). Only three mental foramina

was apparently somewhat recurved, as in the holotype of A.

are present in the holotype of A. myctoderos (Forasiepi et

myctoderos (Forasiepi et al., 2006: fig. 5). X-ray examination

al., 2006), but the number of mental foramina often varies

of this tooth has shown that the root of the lower canine

within a single hathliacynid species (e.g., Cladosictis patago-

was most likely open in UATF-V-000926 (Fig. S4), as is the

nica; Sinclair, 1906; Forasiepi et al., 2006). The anteriormost

case for other juvenile sparassodonts (Marshall, 1976b;

mental foramen of UATF-V-000926 is greatly enlarged,

Forasiepi and Sánchez-Villagra, 2014).

as in many sparassodonts (Babot and Ortiz, 2008) and is lo-

The first lower premolar of UATF-V-000926 has a well-

cated below the anterior root of p2. The three posterior

developed, asymmetrical central cusp and a smaller poste-

mental foramina are much smaller and are located below

rior cusp that is nearly confluent with the cingulum. No

the anterior root of dp3, the dp3/m1 embrasure, and the

anterior cusp is present on p1, but the enamel on the ante-

posterior root of m2. Many hathliacynid specimens, in-

rior face of the tooth is slightly thickened, as in Cladosictis.

cluding the holotype of A. myctoderos, have mental foramina

This tooth is set at a slight angle to the rest of the tooth row

TABLE 5. Measurements of the dentary and lower dentition of UATF-V-000926. Tooth

L

W

Ltr

Wtr

Lta

Wta

Lpc

D

B

c

6.95

5.12

-

-

-

-

-

-

-

p1

5.93

2.74

-

-

-

-

-

13.25

6.43

p2

9.21

3.94

-

-

-

-

-

13.50

6.76

dp3

5.68

3.02

-

-

-

-

-

13.88

7.08

m1

9.83

3.73

6.91

3.73

2.92

4.72

3.93

14.51

6.23

m2

10.52

5.42

7.17

5.42

3.35

4.37

3.85

14.70

6.84

m3

11.71

5.99

8.79

5.99

2.92

5.04

5.61

-

-

m4

13.12*

4.80*

-

4.80*

-

-

-

-

-

All measurements in millimeters. Estimated measurements are demarcated by an asterisk (*). Abbreviations: L, greatest anteroposterior length; W, greatest mediolateral width; Ltr, greatest length of trigonid; Wtr, greatest width of trigonid; Lta, greatest length of talonid; Wta, greatest width of talonid; Lpc, length of paracristid; D, depth of mandibular ramus below tooth; B, breadthof mandibular ramus below tooth.

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AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

(approximately 20°), comparable to some other hathliacynids (Marshall, 1981: tab. 26) but less so than in Patene simpsoni Paula Couto, 1952 (Goin et al., 1986), Notictis (Marshall, 1981), and borhyaenoids (Marshall, 1979: tab. 9; Forasiepi, 2009). This differs from the holotype of A. myctoderos but partially resembles the condition in the holotype of A. herrerae, where the Lp1 is oriented at a slight angle to the tooth row but the Rp1 is in line with the other teeth of the jaw. The p2 of UATF-V-000926 is larger than p1 and has three cusps. The posterior cusp of p2 is larger and more distinct than in other large-bodied hathliacynids such as Acyon herrerae and Cladosictis, which is a diagnostic feature of A. myctoderos (Forasiepi et al., 2006). Like the holotype, p1–2 are less sectorial than those of A. herrerae (Tab. S1). There are no large gaps between the canine, p1, and p2 of UATFV-000926, in contrast to the condition in the holotype of A. myctoderos. This may be a result of the young age of the specimen, as a juvenile specimen of Cladosictis (MPM-PV 3646) that likely pertains to an ontogenetically younger individual than UATF-V-000926 completely lacks diastemata between its premolars (in contrast to the condition in adult individuals). Several recent marsupials, including the hathliacynid-like Thylacinus Temminck, 1824 (Forasiepi and Sánchez-Villagra: 2014, fig. 6) and the didelphoid Didelphis

Figure 6. Acyon myctoderos, UATF-V-000926, deciduous lower left premolar (dp3); 1, occlusal view; 2, labial view. Anterior is to the left in both views. Scale bar= 5 mm.

Linnaeus, 1758 (Abdala et al., 2001: fig. 2), undergo through a similar transition where juveniles have a closed premolar

Villagra, 2014). The p3 can be seen erupting from beneath

row but adults have teeth that are separated by diastemata.

the anterior root of dp3, illustrating that the dp3 was re-

In addition to DP3, UATF-V-000926 also preserves the

placed vertically, as in Cladosictis.

dp3 (Fig. 6), making it the first known sparassodont which

The m1 of UATF-V-000926 is approximately 88% the

preserves both DP3 and dp3 (Fig. S5). Aside from A. myc-

length of m3 (Tab. 5), relatively larger than that of the holo-

toderos, dp3 is known from only one other species of hath-

type of A. myctoderos (82% and 83% for left and right sides,

liacynid, Cladosictis patagonica. The dp3 of Acyon is highly

respectively), but comparable to A. herrerae (89% and 95%

elongate and sectorial, similar to that of Cladosictis but

for left and right sides, respectively), and within the range

unlike the more robust dp3 of borhyaenoids (Forasiepi and

of variation seen in Cladosictis patagonica (80–87%) and

Sánchez-Villagra, 2014). The dp3 is the smallest post-

Sipalocyon gracilis (86–95%). The trigonid of UATF-V-000926

canine tooth in UATF-V-000926 (Tab. 5), being only 58%

is bicuspid, with a paraconid that is anterolingual of the

the length of m1, and is proportionately smaller than that of

protoconid. The lower molars of UATF-V-000926 lack a

C. patagonica (which is about 66% the size of m1; Forasiepi

metaconid, as is true of all other known hathliacynids (Fo-

and Sánchez-Villagra, 2014). The dp3 of UATF-V-000926 has

rasiepi et al., 2006). The paracristid is nearly straight in m1,

a relatively tall, symmetrical protoconid that is connected

whereas it is oriented at an oblique angle to the long axis of

anteriorly to a small paraconid that is almost confluent

the tooth row in the posterior molars. The paracristid notch

with the precingulum. This tooth also has a distinct poste-

is shallow in m1 (Fig. S6) and is well-developed on m2–3.

rior “heel” with at least one cusp, in contrast to Cladosictis,

The protoconid is the largest cusp of the trigonid and is

where there are two (Sinclair, 1906; Forasiepi and Sánchez-

conical in cross-section on m1, but is slightly more triangu-

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ENGELMAN ET AL.: QUEBRADA HONDA ACYON

lar in m2–3. The anterolabial cingulum is present on m1–3,

m1–2 and is taller than the hypoconid in m2–3, though

but is highly reduced and is little more than a ridge on the

this cusp is slightly shorter than the hypoconid on m1. The

anterior half of the paraconid, being nearly absent on m1

hypoconulid of UATF-V-000926 is oriented vertically rather

and most prominent on m3.

than posteriorly as in early Miocene species of Acyon, which

The paraconid is shorter than the protoconid on m1–3

is considered a distinguishing feature of A. myctoderos.

and is higher than the talonid basins on m2–3. By contrast, the paraconid is shorter in m1 but still forms a distinct cusp

Other Acyon specimens. Another specimen from Quebrada

and is not confluent with the precingulum as in the holotype

Honda, UATF-V-001400, probably also pertains to A.

(Forasiepi, 2009), among others. This is similar to other

myctoderos. This specimen is an articulated partial skele-

members of the clade Hathliacynidae + Borhyaenoidea,

ton preserving much of the vertebral column and a partial

where the paraconid is only distinct in juvenile or subadult

hind limb, among other elements (Fig. S11). Unfortunately,

specimens of Cladosictis patagonica (MPM-PV 3646), Pro-

the craniodental remains associated with this specimen are

thylacynus patagonicus (MACN-A 642), Borhyaena tuberata

very fragmentary, so UATF-V-001400 can only be tenta-

(MPM-PV 3554), and ‘Lycopsis’ longirostrus (UCMP 38061)

tively assigned to A. myctoderos on the basis of size. This

but is confluent with the cingulum in adult individuals of

specimen is not fully prepared; though a few preliminary

these species (or, in the case of ‘L.’ longirostrus, with speci-

long bone measurements are provided in Table 6. This

mens of Lycopsis torresi Cabrera, 1927, and Lycopsis viveren-

specimen belongs to a sparassodont much larger than Cla-

sis Forasiepi, Goin, and di Martino, 2003; Marshall, 1977b,

dosictis, roughly intermediate in size between Prothylacynus

1978, 1979, 1981; Forasiepi et al., 2003; Forasiepi, 2009;

patagonicus and Cladosictis patagonica (Argot, 2003a, b). The

Forasiepi and Sánchez-Villagra, 2014). The fact that the

crural index (tibia/femur length ratio) of UATF-V-001400

paraconid is distinct from the precingulum of m1 in juveniles

is much lower than Cladosictis patagonica, more similar to

and not distinct in adults, suggests that an m1 paraconid

the semi-arboreal Prothylacynus patagonicus and the living

that is “low and confluent with the precingulum”, as seen in

arboreal binturong (Arctictis binturong [Raffles, 1822]; Argot,

many members of the Sparassodonta (Forasiepi, 2009, ch.

2003a, b, 2004b). The femur of is also much longer than

174), may actually be due to wear and therefore may not be

would be expected assuming isometric similarity with Cla-

a phylogenetically informative character. The lower molars of UATF-V-000926 differ from the

dosictis (Tab. 6), but the paleobiological significance of this is uncertain.

holotype of A. myctoderos in the presence of an entoconid

twinned with the hypoconulid. By contrast, the entoconid is

DISCUSSION Identity of the new hathliacynid material from Quebrada Honda

absent on m3, and the shape of the lingual cristid suggests

The new hathliacynid specimens from Quebrada Honda

that no entoconid was ever present on this tooth (Fig. S7–

share several distinctive features with the holotype of A.

S10). The pre-entocristid of m2–3 is slightly notched and

myctoderos, including a relatively shallow dentary compared

runs lingual to the trigonid, whereas the corresponding

to other large-bodied hathliacynids, absence of maxillary

crest on m1 is incomplete and concave labially. A small

‘cheeks’, well-developed posterior cusp on p2, premolars

notch is also present in the cristid obliqua of m3, and may be

that are less mediolaterally compressed than those of Acyon

present in the cristid obliqua of m1–2. This crest is aligned

herrerae, large hypoconids, vertically oriented hypoconulids,

with the base of the protoconid in m1–3. The talonids of

and absence of the entoconid on m3. Several of these fea-

m2–3 are completely basined, whereas the talonid of m1 is

tures are unique to A. myctoderos among sparassodonts

slightly open near the base of the protoconid due to the in-

(see diagnosis) providing strong support for the assignment

complete entocristid. The hypoconid is relatively large in

of these new specimens to this species. However, these

m1–3, similar to the holotype of A. myctoderos (Forasiepi et

specimens also differ from the holotype in several respects

al., 2006). The hypoconulid is taller than the entoconid on

(Tab. 7). Some of these features, such as the reduced pre-

on m1–2. This cusp is labio-lingually compressed, slightly smaller than the entoconid of Cladosictis patagonica, and

217

AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

TABLE 6. Postcranial measurements of cf. Acyon myctoderos (UATF-V-001400) compared to the large-bodied hathliacynid Cladosictis patagonica and the basal borhyaenoids 'Lycopsis' longirostrus and Prothylacynus patagonicus Taxon

Specimen

LF

CF

AD

MD

RL

CI

LT

LF

LA

LC

cf. Acyon myctoderos

UATF-V- 001400

170.0*

37.0

11.0

10.5

15.8

0.829

141.0

136.0

20.3

30.0 (R), 32.0 (L) without epiphysis

Cladosictis patagonica

YPM-VPPU 15046

98.8

23.3

7.8

7.0

13.4

0.976

96.4

-

-

23.0

Cladosictis patagonica

YPM-VPPU 15170

109.3

26.7

9.0

8.0

12.9

-

-

-

-

-

Cladosictis patagonica

YPM-VPPU 15702

125.3

32.2

10.8

9.7

12.2

0.907

113.7*

110.0

-

-

'Lycopsis' longirostrus

UCMP 38061

167.0

41.0

14.3

11.8

12.8

0.976

163.0

156.0*

21.0

37.7

YPM-VPPU 15700

198.0

53.0

16.9

16.7

11.8

0.871

172.5

164.0

26.0

-

Prothylacynus patagonicus

Measurements of Cladosictis, Prothylacynus patagonicus, and 'Lycopsis' longirostrus, taken from Argot (2003a, b, 2004b), respectively. Estimated measurements are marked by an asterisk (*). All measurements in millimeters. Abbreviations: LF, length of femur; CF, circumference of femur at midshaft; AD, midshaft anteroposterior diameter; MD, midshaft mediolateral diameter; RL, relative length of femur (length of femur/average diameter at midshaft); CI, crural index (length of tibia/length of femur); LT, length of tibia; LF, length of fibula; LA, greatest length of astragalus; LC, greatest length of calcaneum.

molar diastemata and more gracile dentary of UATF-V-

presence of a metacone, the position of the protocone, and

000926, are probably age-related, based on comparisons

the orientation of this tooth relative to M3. Given that M4 is

with other juvenile sparassodonts and other groups of

the last molar to erupt in hathliacynids and had a relatively

marsupials (Hough, 1961; Clemens, 1996). However, other

minor role in food processing (as the postmetacrista is

differences cannot be attributed to age, most notably the

absent on this tooth and the trigon occludes with the re-

presence of an entoconid on m1–2. This difference may in-

duced talonid of m4 [Marshall, 1978; Forasiepi et al., 2006]),

stead be the result of individual variation, given that similar

this tooth may be less subject to selective pressure and hence

variation in talonid morphology can be seen in Cladosictis,

more likely to exhibit individual variation. Although large

where some specimens have a basined, tricuspid talonid

numbers of specimens are known for Sipalocyon gracilis and

(e.g., MACN-A 674), whereas other have a bicuspid talonid

Cladosictis patagonica (see Marshall, 1981), variation within

(i.e., there is no entoconid) that is open lingually (MACN-A 9;

these samples have not been documented in detail.

see Forasiepi et al., 2006). In Acyon, the morphology of the

The new specimens described here also resemble the

talonid can even vary between different teeth of the same

holotype of A. myctoderos in size, providing further evi-

individual: the entocristid is absent on Lm2 of the holotype

dence that they belong to the same species. Like extant car-

of A. myctoderos but present in Rm2; an entoconid is present

nivorous marsupials (Jones and Barmuta, 1998) and some

in m1–2 UATF-V-000926 but not m3; and in the holotype of

carnivorans (Dayan and Simberloff, 1994), sparassodont

A. herrerae a distinct entoconid is absent on most teeth but

predator guilds appear to be primarily partitioned by size

visible on m4 (Fig. S12).

(Ercoli et al., 2014). Such partitioning tends to occur even if

Many prominent differences between the UF Acyon

species greatly differ in dietary or locomotor preferences (e.g.,

and the holotype of A. myctoderos involve the M4, including

the bone-cracking, terrestrial Sarcophilus harrissii [Boitard,

the presence or absence of para- and metaconules, the

1842] versus the scansorial, cat-like Dasyurus maculatus

218

ENGELMAN ET AL.: QUEBRADA HONDA ACYON

TABLE 7. Differences between UATF-V-000926, the UF Acyon, and the holotype of A. myctoderos (MNHN-Bol-V-003668). Character

MNHN-Bol-V-003668

UF Acyon

UATF-V-000926

Absent

Absent

Present

Absent (para- and metacone are worn)

Absent (para- and metacone are worn)

Present

Absent

Present

-

Slightly basined

Basined

-

Aligned with paracone and preparacrista Present but nearly confluent with cingulum Rotated anterolabial-posterolingually

Anterolabial to (left) or aligned with paracone and preparacrista (right)

-

Absent

-

Perpendicular to tooth row

-

Orientation of p1

In line with tooth row

-

At slight angle to tooth row

Paraconid of m1

Nearly confluent with cingulum

-

Distinct cusp

Absent

-

Present

Ectocingulum on M2 Centrocrista Para- and metaconules on M4 Structure of M4 protocone Orientation of M4 protocone Metacone on M4 Orientation of long axis of M4

Entoconid on m1–2

[Kerr, 1792] in Tasmania). At other Miocene localities such

to 17.5 kg (Tab. 8). These estimates are slightly greater

as Arroyo Chasicó, La Venta, and Santa Cruz (Prevosti et al.,

than the tooth-based mass estimate of 12.03 kg for A. myc-

2013; Ercoli et al., 2014), as well as Oligocene sites such as

toderos of Prevosti et al. (2013) and exceed those calcu-

Salla (Forasiepi et al., 2014), no two species of sparas-

lated for all other hathliacynids (Argot, 2003a; Ercoli and

sodonts closely overlap in size (one possible exception could

Prevosti, 2011; Prevosti et al., 2012, 2013), supporting pre-

be the very small-bodied Pseudonotictis pusillus [Ame-

vious assertions that this taxon is the largest member of the

ghino, 1891] and Perathereutes pungens Ameghino, 1891;

Hathliacynidae (Forasiepi et al., 2006). In fact, the body mass

see Prevosti et al., 2012, 2013; Ercoli et al., 2014). The close

estimates of cf. A. myctoderos are closer to those of small

correspondence in size among Acyon specimens from Que-

borhyaenoids such as Pseudolycopsis cabrerai Marshall,

brada Honda argues against their referral to different

1976c, and ‘Lycopsis’ viverensis than to other hathliacynids,

species, as it is unlikely that two closely-related species of

supporting the idea that A. myctoderos is positioned out-

similar size would be present at the same locality, particu-

side the typical hathliacynid morphospace based on size

larly since a similar degree of niche overlap has not been

(Zimicz, 2014). In terms of living taxa, body mass estimates

documented elsewhere in South America.

for this species are roughly comparable to an African civet (Civettictis civetta [Schreber, 1776]; Ray, 1995).

Body mass of Acyon myctoderos As of this writing, the body mass of A. myctoderos has

Relationships of large-bodied hathliacynids

only been estimated using dental measurements (Prevosti

Together with the genera Cladosictis and Chasicostylus,

et al., 2013). UATF-V-001400, the specimen provisionally

Acyon has traditionally been considered part of a clade of

referred to A. myctoderos here, includes the first long bones

large-bodied hathliacynids (>3.5 kg), members of which

referred to this taxon, permitting the first postcrania-based

tend to be 2.5 to 8.5 times the size of other Neogene hath-

estimates of its body mass. Mass estimates for cf. A. myc-

liacynids (Prevosti et al., 2013). However, support for a clade

toderos based on the femur using the equations of Any-

of large-bodied hathliacynids has been equivocal, despite

onge (1993; see also Argot, 2003a, b) range from about 13

being recognized by most authors (e.g., Marshall, 1981;

219

AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

TABLE 8. Estimated body mass of cf. Acyon myctoderos compared to other large-bodied hathliacynids. Specimen

Taxon

Reference

Mass (kg)

Dental/Postcranial

UATF-V-001400

cf. Acyon myctoderos

Present Study (Equation 1)

17.5

Postcranial

UATF-V-001400

cf. Acyon myctoderos

Present Study (Equation 2)

13.0

Postcranial

UATF-V-001400

cf. Acyon myctoderos

Present Study

15.3

Postcranial

Acyon myctoderos

Prevosti et al., 2013

12.0

Dental

FMNH P13521

Acyon herrerae

Wroe et al., 2004

9.7

Dental

FMNH P13521

Acyon herrerae

Prevosti et al., 2013

7.0

Dental

N/A

Acyon tricuspidatus

Wroe et al., 2004

4.0

Dental

N/A

Acyon tricuspidatus

Prevosti et al., 2012

4.3

Dental

N/A

Acyon tricuspidatus

Prevosti et al., 2013

5.3

Dental

N/A

Chasicostylus castroi

Prevosti et al., 2013

6.7

Dental

N/A

Chasicostylus castroi

Wroe et al., 2004

9.8

Dental

N/A

Cladosictis centralis

Prevosti et al., 2013

3.4

Dental

N/A

Cladosictis centralis

Wroe et al., 2004

4.4

Dental

YPM-VPPU 15046

Cladosictis patagonica

Argot, 2003a

3.5

Postcranial

MPM-PV 4329

Cladosictis patagonica

Ercoli and Prevosti, 2011

4.3

Postcranial

N/A

Cladosictis patagonica

Prevosti et al., 2013

4.7

Dental

YPM-VPPU 15170

Cladosictis patagonica

Argot, 2003a

5.0

Postcranial

YPM-VPPU 15046

Cladosictis patagonica

Ercoli and Prevosti, 2011

5.1

Postcranial

YPM-VPPU 15170

Cladosictis patagonica

Ercoli and Prevosti, 2011

6.1

Postcranial

N/A

Cladosictis patagonica

Prevosti et al., 2012

6.6

Postcranial

N/A

Cladosictis patagonica

Wroe et al., 2004

8.0

Postcranial

YPM-VPPU 15702

Cladosictis patagonica

Argot, 2003a

8.0

Postcranial

MPM-PV 4326

Cladosictis patagonica

Ercoli and Prevosti, 2011

8.6

Postcranial

MACN-A 6288-6298

Cladosictis patagonica

Ercoli and Prevosti, 2011

11.9

Postcranial

YPM-VPPU 15702

Cladosictis patagonica

Ercoli and Prevosti, 2011

14.8

Postcranial

MNHN-Bol-V-003668

Forasiepi et al., 2006; Forasiepi, 2009; Engelman and Croft,

within members of this family (as noted above). Thus, their

2014), possibly due to the lack of agreement on hathlia-

phylogenetic utility is questionable.

cynid systematics in general (Forasiepi et al., 2014). Most of

In his review of the Hathliacynidae, Marshall (1981) con-

the characters used to support this clade, such as a highly

sidered the early late Miocene (Chasicoan SALMA) Chasi-

reduced to vestigial StB (Forasiepi et al., 2006) and the

costylus to be most closely related to (specifically, an

presence of a diastema between p1 and p2 (Forasiepi,

anagenetic descendant of) the large-bodied hathliacynid

2009; Engelman and Croft, 2014) are widespread within

Cladosictis patagonica. However, at the time of Marshall’s

the Sparassodonta and subject to significant variation even

study, the upper dentition of Acyon (Marshall’s Anatherium)

220

ENGELMAN ET AL.: QUEBRADA HONDA ACYON

was unknown, and so this taxon could not be easily com-

only 1% or less of the total number of specimens identified.

pared to Chasicostylus. Based on the new material described

By contrast, sparassodonts make up a greater percentage

here and by Forasiepi et al. (2006), it is clear that the M1 of

of the Papachacra local fauna, but this may be related to

A. myctoderos shares several features with Chasicostylus

the small number of specimens and/or possible tapho-

castroi that are not present in Cladosictis, including a

nomic biases against smaller specimens at this site. Even

preparacrista oriented in the same axis as the para/meta-

when non-mammalian carnivores (e.g., phorusrhacids) are

cone and stylar cusps located anterior to the paracone.

included in this estimate, macropredatory species only

These features are more strongly developed in Chasicostylus

represent about 1% of all vertebrate specimens identified

than Acyon and suggest that Chasicostylus is more closely

from Quebrada Honda, much less than would be expected

related to Acyon than Cladosictis. Another feature that may

based on Cenozoic mammal communities from northern

link Acyon and Chasicostylus is the small size/absence of the

continents.

entoconid. In the holotype of Chasicostylus castroi (MLP 55-

Interestingly, of the seven sparassodont specimens

IV-28-59), the entoconid is far smaller and shorter than the

currently known from Quebrada Honda (including the holo-

hypoconid and hypoconulid, more similar to the condition in

type of A. myctoderos, MNHN-Bol-V-003668), the majority

Acyon than in most specimens of Cladosictis.

(five) pertain to the large-bodied hathliacynid A. myctoderos, whereas the other two pertain to much smaller species (1

Sparassodont abundance and the sparassodonts of Quebrada Honda

kg or less; Croft et al., 2013; Engelman and Croft, 2014).

One unusual feature of pre-Great American Biotic In-

smaller carnivores generally tend to be more abundant in

terchange South American mammal faunas is the rarity of

an ecosystem than larger ones (Peters and Raelson, 1984;

carnivorous mammal specimens compared to Holarctic

Carbone and Gittleman, 2002). However, this pattern re-

mammal communities (see discussion in Croft, 2006; Viz-

sembles the trend seen at other well-sampled South

caíno et al., 2010; Prevosti et al., 2012). While there has

American fossil localities. According to Prevosti et al. (2012),

been some debate as to whether the South American

eight years of collecting in the Santa Cruz Formation pro-

predator guild was “depauperate” in terms of numbers of

duced 21 sparassodont specimens with a body size ~4 kg

This is unexpected from an ecological perspective, as

species, it is generally agreed that sparassodont specimens

or greater (Ercoli and Prevosti, 2011; Cladosictis, Borhyaena,

are much rarer than would otherwise be expected based

Prothylacynus, and Lycopsis), most of which pertained to the

on fossil placental carnivore assemblages (Croft, 2006;

hathliacynid Cladosictis, but only one specimen was refer-

Prevosti et al., 2012). In most typical (attritional) placental

able to a small (~1 kg) hathliacynid (MPM-PV 4322, provi-

faunas, carnivores typically account for 4–10% of all speci-

sionally assigned to Perathereutes; Prevosti et al., 2012). The

mens (Clark et al., 1967; Badgley, 1986; Croft, 2006; Viseras

general rarity of small sparassodonts may be a result of

et al., 2006; Davis and Pyenson, 2007; White et al., 2009),

sampling bias, as techniques such as screen-washing that

whereas at South American fossil localities such as La Gran

favor the recovery of small mammal remains have only re-

Hondonada and Santa Cruz, sparassodonts only represent

cently been instituted at many fossil localities (Goin et al.,

about 1% of all identified specimens (Cladera et al., 2004;

2007, 2010). However, it is also possible that the abun-

Forasiepi et al., 2004; Prevosti et al., 2012).

dance of large-bodied hathliacynids at Quebrada Honda

This pattern of low carnivore abundance also appears to

reflects a real feature of the underlying mammal commu-

have been the case at Quebrada Honda. Of the 873 fossil

nity. In Tasmania, one of the world’s few remaining marsu-

vertebrate specimens from Quebrada Honda that can be

pial carnivore-dominated ecosystems, the large-bodied

identified to family level in the UF and UATF collections, only

dasyurid Sarcophilus harrisii is more abundant than species

six pertain to sparassodonts (Tab. 9), and no more than four

of the smaller Dasyurus (Jones and Barmuta, 1998).

sparassodont specimens have been collected from any one

Although faunal provinciality has previously been docu-

local fauna. At Quebrada Honda and Río Rosario, the two

mented in Miocene South American mammal faunas with

most heavily sampled local faunas, sparassodonts make up

regards to rodents (Croft et al., 2011), cingulates (Carlini et

221

AMEGHINIANA - 2015 - Volume 52 (2): 204 – 225

TABLE 9. Relative abundance of predators relative to number of identified specimens (NISP) at Quebrada Honda. Total NISP

# of Sparassodont Specimens

% Abundance of Sparassodonts

# of Predator Specimens

% Abundance of Predators

Quebrada Honda

385

4

1.04

6

1.56

Río Rosario

416

1

0.24

4

0.96

Papachacra

41

1

2.44

1

2.44

Total

873

6

0.69

11

1.26

Local Fauna

Both the total number of sparassodont specimens and the total number of specimens of non-mammalian macropredators (e.g., phorusrhachids) are included in the count of predator specimens. The UF Acyon material was treated as a single specimen for the purposes of this study. Several specimens from the UF collections that do not have precise locality data were included in the total specimen count, but were not considered in any of the local faunas.

al., 1997, 2008) and endemic ungulates (Croft, 2007), as of

sodonts comparable to Sipalocyon, Perathereutes, or Pseudo-

this writing no study has provided direct evidence of similar

notictis are absent. Although a didelphine opossum may

faunal provinciality among carnivorous mammals such as

have been present at La Venta (Goin, 1997), this species

sparassodonts. Goin (1997) noted significant differences

appears to have only been about the size of living species

between the sparassodont fauna of La Venta, Colombia and

of Philander Brisson, 1762 (~450 g based on the equations

other Miocene sites in Argentina and Chile but could not di-

of Gordon [2003]; see Flores et al., 2008), and therefore

rectly compare these localities because they were not con-

probably did not occupy a similar ecological role to small

temporaneous. In contrast, the locality of Quebrada Honda

sparassodonts (which are all generally ≥1 kg; Prevosti et

has been well-dated to between 12 and 13 Ma (MacFadden

al., 2013). These differences between the low-latitude site

et al., 1990; Croft et al., 2013), making the mammal fauna of

of La Venta and mid-to-high latitude sites such as Que-

this locality coeval with that of La Venta (Flynn et al., 1997).

brada Honda mirror those already documented for rodents

At least six different species of sparassodonts have been

and endemic ungulates. Testing whether such patterns in

identified at La Venta (Goin, 1997), none of which are pres-

sparassodont diversity reflect real faunal differences

ent at Quebrada Honda. In fact, no genera or even families

rather than sampling problems due to the general rarity of

of sparassodonts are shared between these two localities,

sparassodont remains will rely on documenting new re-

despite their similar age. Hathliacynids, which are normally

mains of these mammals from other middle or low latitude

the most abundant group of sparassodonts at most South

fossil sites.

American fossil sites (Villarroel and Marshall, 1982; Prevosti et al., 2012), are absent from La Venta (Marshall, 1977a), whereas borhyaenoids have yet to be found at Quebrada Honda. It is particularly unusual that no hathliacynids have been found at La Venta, as one might expect the lowland forest or mixed forest environment of this locality (Kay and Madden 1997a, b) to be quite suitable for these scansorial to arboreal sparassodonts (Argot, 2003a; Ercoli et al., 2012; Prevosti et al., 2012). The ecological niche occupied by large-bodied hathliacynids such Cladosictis or Acyon appears to have been filled by the basal sparassodont Hondadelphys (Marshall, 1976a) and an unnamed species of basal thylacosmilid (Goin, 1997), whereas smaller sparas-

222

ACKNOWLEDGMENTS We thank B. MacFadden and R. Hulbert for permitting us to study the UF specimens described in this manuscript; M. Takai for providing locality information for the holotype of A. myctoderos; R. Wherley and G. Svenson (Cleveland Museum of Natural History) for generating high-resolution photographs of the specimens; A. M. Forasiepi for providing us with additional information and pictures of MPM-PV 3646 and the holotype of A. myctoderos and for helpful discussions regarding sparassodont morphology; R. Chávez and K. Bamba for helping to collect UATF-V-000926 and UATF-V001400, respectively; A Shinya for preparation of UATF-V-000926; R. McCord (Arizona Museum of Natural History) for loans from the Larry Marshall Marsupial Dentition Collection; A. M. Forasiepi, N. Zimicz, and an anonymous reviewer for comments and criticisms of earlier drafts of this paper; and the Facultad de Ingeniería Geológica of the Universidad Autónoma Tomás Frías for supporting our fieldwork at Quebrada Honda. This research was supported by the National Geographic Society and NSF EAR 0958733 to D. Croft.

ENGELMAN ET AL.: QUEBRADA HONDA ACYON

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doi: 10.5710/AMGH.19.11.2014.2803 Submitted: July 15th, 2014 Accepted: November 19th, 2014

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