Eochaeotodipus, Korth, 2008Eochaetodipus asulcatus, Korth, 2008

Korth, William W., 2008, Two new pocket mice (Mammalia, Rodentia, Heteromyidae) from the Miocene of Nebraska and New Mexico and the early evolution of the subfamily Perognathinae, Geodiversitas 30 (3), pp. 593-609 : 596-603

Genus Eochaeotodipus n. gen.

TYPE SPECIES. — Eochaetodipus asulcatus n. sp.

RANGE. — Late Arikareean of Nebraska (earliest Miocene).

DIAGNOSIS. — Primitive heteromyid features: premolars (upper and lower) simple, four-cusped, with very weak development of lophodonty; cheek teeth brachydont;upper incisor asulcate;rostral perforation anterior to infraorbital foramen.Primitive characters shared with mioheteromyines not present in heteromyines,perognathines and dipodomyines: no bony flange dorsal to orbit; large temporal foramen on skull; accessory foramen ovale retained (enclosed posterior border); premaxillary-maxillary suture crosses midline of palate anterior to the posterior margin of the incisive foramina. Derived characters shared with heteromyines, perognathines, and dipodomyines, not present in mioheteromyines: length of incisive foramina less than 15% total length of the upper diastema; interparietal bone oval (transversely elongated); parapterygoid fossae shallower than in extant perognathines, but deeper than in mioheteromyines; limb bones thinner and skull less robust.Derived features shared with perognathines and dipodomyines: rudimentary inflation of the bulla and mastoid; masticatory and buccinator foramina separate on alisphenoid. Apomorphic features: posterior palatine foramina multiple.

ETYMOLOGY. — Greek, eos, dawn and Chaetodipus , extant genus of perognathine.

COMPARISONS

Eochaetodipus n. gen. is clearly referable to the Heteromyidae because of its generally small skull and skeleton and the presence of a rostral perforation anterior to the infraorbital foramen (diagnostic for heteromyids). It is separable from geomyids, entoptychines, and floreniamyids by thinner more delicate bones of the skull. It also lacks the cranial specializations of these groups (see Wahlert 1983, 1985; Wahlert & Sousa 1988). Florentiamyids also lack the lingual stylar cusps on the upper molars present on Eochaetodipus n. gen., and have teeth that are generally more robust.

Eochaetodipus n. gen. differs from the contemporaneous geomyoid Tenudomys Rensberger, 1973 , again, in having a more gracile skull that is not as deep. The foramina of the medial orbital wall of Tenudomys are positioned more posteriorly than in Eochaetodipus n. gen. ( Korth 1993) and the cheek teeth of the former are generally more hypsodont and lophate ( Rensberger 1973; Korth 1993). The upper molars of Eochaetodipus n. gen. differ from those of Tenudomys by having non-continuous lingual cingula. In Tenudomys , the central transverse valley of the upper molars is blocked lingually by the lingual cingulum, whereas in Eochaetodipus n. gen. the transverse valley is continuous and two distinct cusps are present along the cingulum (protostyle and hypostyle).

Eochaetodipus n. gen. differs from the late Arikareean or early Hemingfordian Trogomys Reeder, 1960 from southern California by its larger size, slightly lower crowned and less lophate cheek teeth, and morphology of the lower premolar. In Trogomys, the cusps of the metalophid of p4 are joined anteriorly by a loph connecting their apices, giving the loph an anteriorly curved shape. In Eochaetodipus n. gen., the metaconid cusps are not joined by a loph and will probably not fuse until a much later stage of wear when the bases of the cusps fuse. Trogomys also has a distinct supraorbital flange ( Reeder 1960) as in extant perognathines that is lacking in Eochaetodipus n. gen.

Eochaetodipus n. gen. differs from harrymyines in lacking the specialized occlusal pattern of the cheek teeth of the latter (V-shaped hypolophid that unites with the metalophid via anterior ridge on lower cheek teeth) and the nature of the inflation of the auditory bullae (Wahlert 1991).

Among the other subfamilies of heteromyids (Mioheteromyinae Korth, 1997, Perognathinae , Heteromyinae, Dipodomyinae), Eochaetodipus n. gen. has a mosaic of derived and primitive morphologies. The skull of Eochaetodipus n. gen. shares the following primitive features of the skull with mioheteromyines: 1) presence of a large temporal foramen; 2) position of the maxillary-premaxillary suture relative to the incisive foramina (crosses foramina anterior to posterior margin); 3) presence of an accessory foramen ovale on the alisphenoid that is enclosed posteriorly; 4) lack of a bony flange dorsal to the orbits; and 5) morphology of the temporalis scar on the skull (extends posteriorly to occipital).

The derived characters of Eochaetodipus n. gen. that are shared with heteromyines, perognathines and dipodomyines include shorter incisive foramina (less than 15% the length of the diastema), deeper parapterygoid fossa (not as deep as in extant heteromyids, but deeper than in mioheteromyines); and more gracile limb bones. Eochaetodipus n. gen. also shares some derived characters of the skull with perognathines (inflation of bulla and mastoid). The arrangement of the foramina on the lateral alisphenoid in Eochaetodipus n. gen. is unique among heteromyids. In Eochaetodipus n. gen. the accessory foramen ovale is large, and completely surrounded by bone, and the buccinator and masticatory foramina are separate from one another. This condition is known only elsewhere within the geomyoids in the entoptychines, a group clearly not closely allied with Eochaetodipus n. gen. Eochaetodipus asulcatus n. sp.

( Figs 1-3 ; Tables 1 ; 2)

HOLOTYPE AND ONLY SPECIMEN. — UNSM 130000 , skull with associated mandibles and several postcranial bones.

HORIZON AND LOCALITY. — UNSM locality Sh-112, Antelope Creek Formation, Sheridan County, Nebraska.

AGE. — Late Arikareean (earliest Miocene).

DIAGNOSIS. — Only species of the genus.

ETYMOLOGY. — Latin , a- prefix meaning without; and sulcatus, furrow; in reference to the ungrooved upper incisor of this species.

DESCRIPTION

The skull is nearly complete, lacking only the zygomatic arches and the anterior end of the rostrum ( Fig.1 ). Some damage has removed most of the bone from the orbital wall. The skull is similar in overall proportions and size to the extant perognathine species Chaetodipus hispidus Baird, 1858 , but lacks the degree of inflation of the mastoids present in the latter. In dorsal view, the snout broadens anteriorly rather than tapering as in Chaetodipus Merriam , 1889. This may be affected by the breakage and dorsoventral crushing of the anterior part of the rostrum. The nasal bones end posteriorly anterior to the anterior margin of the orbits. The premaxillaries extend slightly farther posteriorly, level with the anterior margin of the orbits. There is no flange on the frontals dorsal to the orbits, which is a characteristic of extant heteromyids ( Wahlert 1985). The crest formed by the temporalis is similar to that of extant Heteromys Desmarest, 1817 ( Wahlert 1985: fig. 2), mioheteromyines ( Korth 1997: fig. 8), and Harrymys Munthe, 1988 , originating above the orbits and extending posterodorsally to the nuchal crest (Wahlert 1991: fig. 1). The two ridges marking the attachment of the temporalis do not meet, and are separated by the width of the interparietal (6.35 mm). The interparietal is oval in shape and wider than the postorbital constriction, a condition in extant Chaetodipus and other heteromyids, that differs from extant Perognathus Wied-Neuwied, 1839 and dipodomyines where the interparietal is greatly reduced in size to accommodate bullar inflation. A large temporal foramen (0.75 mm in diameter) is present along the squamosal-temporal suture, level with the base of the zygomatic root. The mastoids are slightly swollen, not as much as in extant perognathines, but are inflated enough to project posteriorly, producing low, broad ridges along the lateral sides of the occiput. Although there is breakage on the mastoids, matrix has filled the broken areas, and it is not possible to determine if there is any cancellous bone present as in extant perognathines.

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Damage to the rostrum has obliterated the pattern of the premaxillary-maxillary suture on the lateral sides. Along the ventral margin of the rostrum, the premaxillary-maxillary suture appears anterior to the rostral perforation and extends posteriorly along the ventral boundary of the infraorbital foramen before crossing the palatal surface. The infraorbital foramen is large with an associated rostral perforation (typical of heteromyids) that is situated low on the rostrum, near the ventral border. A small swelling of bone is ventral to the infraorbital foramen. As stated above, little of the morphology of the orbital wall is preserved, but there are three recognizable foramina preserved on the right side of the skull. A small, crescentic ethmoid foramen is present dorsal to M2, just above the center of the orbital wall. A small frontal foramen is also present along the dorsal border of the orbit, well posterior to the tooth row. The opening for the sphenopalatine vacuity is just dorsal to the tooth row and just posterior to M3. Although there is damage to the orbital wall, the contact between the orbitosphenoid and frontal is present on one side of the skull. In species of perognathines and dipodomyines there is an unossified area ( Wahlert 1985; Korth 1998). This unossified area is clearly not present on UNSM 130000. The alisphenoid extends dorsally only as high as the glenoid fossa. There is no distinct muscle attachment on the dorsal alisphenoid as in geomyids, but ventral to the anterior margin of the zygomatic root on the squamosal is a bony knob. This knob of bone is slightly enlarged by distortion of the skull, but is similar to a recurved ridge present in perognathines along the anterior margin of the glenoid fossa. On the posterolateral portion of the alisphenoid are three foramina. The accessory foramen ovale is the largest (0.5 mm), most ventral, and most posterior. It is circular in shape. The other two foramina are the masticatory and buccinator. The masticatory foramen is dorsal and slightly posterior to the buccinator. It is crescentic in shape and opens anterodorsally. The buccinator is a small, nearly horizontal slit, just posterior to the tooth row.

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The incisive foramen is small and tear-drop shaped. Its length (1.0 mm) is approximately 14% that of the total length of the upper diastema, well within the range of extant heteromyids ( Wahlert 1985). The premaxillary-maxillary suture crosses the midline of the palate just anterior to the posterior end of the incisive foramen as in Harrymys and mioheteromyines ( Korth 1997). The palatal surface is smooth and slightly concave ventrally with no grooves or ridges. The maxillary-palatine suture extends anteriorly on the palate to the level of the posterior margin of P4. There are three pairs of posterior palatine foramina medial to the molars. The posterior maxillary foramen is an elongated oval along the maxillary-palatine suture posteromedial to M3. The parapterygoid fossa is shallower than in Chaetodipus but deeper than in mioheteromyines with a large opening for the sphenofrontal canal in the center of the anterior margin of the fossa. Posterior and slightly lateral is the foramen ovale, along the posterolateral margin of the fossa.

None of the auditory bullae is fully preserved, but an anteroventral portion is preserved on both sides. Where exposed, the bullar wall is a single lamina of bone. The bullae are slightly inflated and end anteroventrally in a point (directed anteromedially), but do not reach the center-line of the skull. There is a small, slit-like opening along the medial border of the bulla that appears to be the same as that identified by Wahlert (1985: fig. 2) as the “fissure medial to bulla” in extant Heteromys . No other foramina are recognizable in the basicranial area due to breakage.

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No cranial foramina are recognizable on the back of the skull except the foramen magnum, again due to breakage. The nuchal crest is very low (not extending dorsal to the parietal bones) and has a small V at its center at the highest point on the skull. A low, rounded ridge runs ventrally from the apex of the occipital toward the foramen magnum. Laterally, the two ridges formed by the inflated mastoids are approximately as distinct as the central ridge of the occipital.

The mandible is slender, typical for heteromyids (depth below m1 = 4.1 mm and 4.0 mm: Fig. 2C ). The diastema is nearly as long as the tooth row (4.1 mm) and shallow. The mental foramen is just above mid-depth of the mandible and just behind the center of the diastema. The masseteric fossa is shallow with only a ventral ridge. The ventral ridge runs anteriorly from the angle along the ventral border of the mandible, then angles anterodorsally from a point below p4 and ends at the posteroventral border of the mental foramen. The ascending ramus originates lateral to m2 and blocks m3 from lateral view. The angle of the mandible has a sharp point posteriorly, similar to Chaetodipus , but does not flare laterally as in the former, it is in the same plane as the condyle. Anterior to the condyle is a rounded swelling for the base of the incisor. The coronoid process is not preserved on either of the mandibles, but the base of the process is recognizable, and very small, suggesting a minute process. There is no indication as to whether it was laterally deflected.

The upper incisors are separated from the skull, but both are present.In cross-section, they are longer (anteroposteriorly) than wide with a gently convex (nearly flat) anterior enamel surface that extends about onefourth the height of the tooth on the lateral side. The cheek teeth are brachydont as in species of perognathines. P4 is only preserved on the left side. It has a simple geomyoid pattern of four cusps ( Fig. 2A ). The protocone is the only cusp on the protoloph and is round in occlusal outline. There is a minute swelling on the buccal slope of the protocone (?paracone). The protocone is isolated, not connected to any of the cusps of the metaloph. The three cusps of the metaloph (hypostyle, hypocone, and metacone) are separated from one another by narrow valleys. The valley separating the hypocone and metacone is shallower, suggesting that the two cusps will fuse after moderate wear. The cusps of the metaloph form a curved loph that is concave anteriorly in occlusal outline.

M1 and M2 are nearly identical, M2 being slightly shorter (anteroposteriorly). These molars are the typical six-cusped, bilophate molars of heteromyids. The central cusps (protocone, hypocone) are the largest and the lingual styles (protostyle, hypostyle) are the smallest cusps. There is a narrow valley separating the styles, and a deeper valley that continues buccally, separating the rest of the protoloph and metaloph cusps. The anterior cingulum on M1 and M2 originates at the protostyle and runs buccally, anterior to the protocone, ending at the anterolingual corner of the paracone. The valley between the anterior cingulum and protoloph is narrow (narrower on M2) and will probably fuse with the protoloph after moderate wear.

M3 is the smallest molar and oval in occlusal outline. The protoloph is the same as in the anterior molars with three recognizable cusps. However, the posterior half of the tooth is reduced. There is no hypostyle present. A lingual cingulum runs posteriorly from the protostyle, then fuses with the hypocone posteriorly. There is no valley along the lingual border of the tooth as in the anterior molars. The metacone is reduced to a minute swelling at the buccal end of the metaloph.

The proportions of the lower incisor are similar to those of the upper incisor, but the anterior enamel surface is more strongly convex with no indication of flattening. The lower premolar is simple. It consists of four rounded, subequal cusps, with no indication of the development of lophodonty. The cusps of the metalophid (metaconid, protostylid) are closer together than the cusps of the hypolophid (hypoconid, entoconid). This makes the tooth narrower anteriorly than posteriorly ( Fig. 2B ). There is no indication that the anterior and posterior pairs of cusps will unite at the center of the tooth as in Perognathus and Chaetodipus . Instead, it appears that each of the cusps of the metalophid will unite with the hypolophid independently. The only other cuspule present on p4 is a minute hypoconulid between the hypolophid cusps along the posterior border of the tooth.

The first and second lower molars, as with their upper counterparts, are bilophate and six-cusped. The buccal stylids (protostylid, hypostylid) are the smallest cusps; the remainder are subequal in size. A deep valley runs transversely across the occlusal surface of the teeth separating the metalophid and hypolophid cusps. The only cingulum present is one that originates buccally at the anterior margin of the protostylid and wraps around the anterobuccal corner of the tooth, ending at the center of the anterior margin of the tooth. Both the metaconid and protoconid extend small arms to unite anteriorly with the cingulum. The arm is directed anterolingually from the protoconid and anterobuccally from the metaconid. The cusps of the hypolophid join by fusing at their anterior margins.

The last lower molar is smaller than the anterior molars, but with basically the same occlusal pattern. The hypolophid is slightly reduced by the reduction in the hypostylid and entoconid (similar to M3).

Among the postcranial fragments associated with UNSM 130000 are: humerus lacking proximal end; two tibia-fibulas lacking both ends; two calcanea (one broken); several vertebral fragments and phalanges ( Fig. 3 ). Comparison of these bones with those of an equivalent-sized specimen of Chaetodipus hispidus shows very little difference in size and proportions. Korth (1997) stated that the limb bones of mioheteromyines were slightly more robust than those of extant heteromyids, but the bones associated with UNSM 130000 are equally proportioned with those of Chaetodipus .

DISCUSSION

Eochaetodipus n. gen. has several primitive characters that are shared with the mioheteromyines and extant heteromyines. The general shape of the mandible of Eochaetodipus n. gen. is similar to that of mioheteromyines and heteromyines with a deep emargination between the angle and the condyle, but the angle is not strongly deflected laterally as in perognathines and dipodomyines. On the cranium, the muscle scar for the temporalis in Eochaetodipus n. gen. extends the full length of the parietals as in mioheteromyines and extant heteromyines.In perognathines and dipodomyines, this scar is greatly shortened posteriorly, mainly due to the inflation of the bullae.The bulla in perognathines and dipodomyines is greatly inflated. The primitive condition in extant heteromyines and mioheteromyines, is a small bulla with little or no inflation. The bulla and mastoid in Eochaetodipus n. gen. is intermediate. It is only slightly enlarged and shows the beginnings of the inflation of the mastoid present in perognathines.

Eochaetodipus n. gen. has some derived features that appear to ally it with the perognathines and dipodomyines, but not the heteromyines. The primitive condition in geomyoids is that the buccinator and masticatory foramina are fused; this is the case in heteromyines and mioheteromyines ( Korth 1997: fig. 11). In perognathines and dipodomyines, the two foramina are separated. The latter condition is present in Eochaetodipus n. gen. However, the primitive condition of the accessory foramen ovale is a large, round foramen on the posteroventral corner of the alisphenoid ( Korth 1997). In the derived condition, the accessory foramen ovale is lost completely (perognathines and dipodomyines), or at least the posterior border is lost (heteromyines). The condition in mioheteromyines and harrymyines is a complete accessory foramen ovale and fused buccinator and masticatory foramina, believed to be the primitive condition for heteromyids. In Eochaetodipus n. gen., the entire accessory foramen ovale is retained as in mioheteromyines and harrymyines (primitive) but the buccinator and masticatory foramina are no longer fused similar to the perognathines and dipodomyines (derived). The condition in Eochaetodipus n. gen. appears to be transitional between the mioheteromyine condition and that of perognathines and dipodomyines but not with heteromyines.

Korth (1997) noted that the postcranial bones of mioheteromyines were more robust than those of extant heteromyids. The postcranial bones of Eochaetodipus n. gen. are more gracile than those of mioheteromyines and similar to those of extant perognathines and heteromyines.

The cheek teeth of Eochaetodipus n. gen. are brachydont and have a primitive heteromyid occlusal morphology. This bars Eochaetodipus n. gen. from inclusion in the Dipodomyinae, which are distinguished by much higher crowned cheek teeth (at least mesodont).

Wood (1935) separated fossil species into the Heteromyinae and Perognathinae , based on the union of the lophs of p4. In fossil perognathines, the union was central and in heteromyines, the union was either buccal or lingual of the center of the tooth, leaving a central basin. However, this distinction appears to break down in the more primitive species. In early heteromyids like Proheteromys and Mookomys Wood 1931 , the cheek teeth are fairly low-crowned and the cusps of the metalophid of p4 do not form a distinct loph, but rather are isolated and circular in occlusal outline. The union of either of these metalophid cusps to the hypolophid is usually at the greatest anteroposterior diameter of the cusp (even with the center), so the ultimate union with the hypolophid is never central, but neither is it distinctly buccally or lingually placed to form a central basin. This suggests that the central union of the lophs in the premolar of extant perognathines is a derived condition, but so is the development of a central basin due to the buccal and lingual fusion of the lophs of the premolars in extant heteromyines. The premolars of Eochaetodipus n. gen. are brachydont and the cusps are simple and rounded, as would be expected in the primitive condition for heteromyids. The fusion of the lophs of the lower premolar lacks the derived condition of either the perognathines or heteromyines. Similarly, the lower premolars of several of the genera included in the Mioheteromyinae are not necessarily derived in the direction of extant heteromyines, but reflect their primitive nature (see Korth 1997 for figures).

It has also been the practice of authors to include fossil taxa with low-crowned cheek teeth in the Perognathinae because that is the condition in extant Perognathus and Chaetodipus . However, it is just as likely that these fossil taxa are simply earlier, primitive members of other subfamilies that have not yet modified their dentition towards hypsodonty. It is easy to exclude fossil species from the Perognathinae by the presence of mesodont or hypsodont cheek teeth, but inclusion for the primitive morphology (brachydonty) is not justified.

It appears that Eochaetodipus n. gen. represents a primitive perognathine based on skeletal and dental features, differing only from extant genera of the subfamily by the retention of several primitive heteromyid characters.

Three Tertiary species previously referred to Perognathus may be referable to Eochaetodipus n. gen.: Perognathus trojectioansrum Korth, 1979 from the late Barstovian of Nebraska, P. ancenensis Sutton & Korth, 1995 from the early Barstovian of Montana, and specimens referred to P. minutus, James, 1963 from the Barstovian of southern California (Lindsay 1972). There are several features that these species share with Eochaetodipus n. gen. that are distinct from all other Tertiary and Recent species of Perognathus (including Recent Chaetodipus ).

The first morphology is the occlusal pattern of p4. On the holotype of P. minutus, UCMP 54575 from the Clarendonian ( James 1963: fig. 45), the anterior cusps (metaconid and protostylid) unite posteriorly and form a short loph that runs posteriorly and joins the center of the hypolophid. This loph is not present on the stratigraphically lower referred specimens of P.minutus from the Barstovian of California ( Lindsay 1972: fig. 22i). It is also lacking on the referred specimens of P. trojectioansrum (Korth 1979: fig. 3B) and P. ancenensis (Sutton & Korth 1995: fig. 9C, D) which is the same morphology of the p4 of E. asulcatus n. sp. ( Fig. 2B ). This character does not appear to be variable. All Recent specimens of Chaetodipus and Perognathus examined have this feature. In all other described Tertiary species of Perognathus , the loph connecting the anterior cusps of p4 to the hypolophid is present. It appears likely that the specimens identified by Lindsay (1972) as P. minutus are referable to a different species based on this character.

Another feature that unites these species to Eochaetodipus n. gen. is the morphology of the anterior cingulum of M2. On all other figured Tertiary species of Perognathus , the anterior cingulum is either absent or reduced to a minute cusp between the paracone and protocone along the anterior margin of the tooth ( Sutton & Korth 1995). In the holotype of Eochaetodipus asulcatus n. sp., the anterior cingulum of M2 is nearly as complete as in M1, running nearly the entire width of the tooth anteriorly, joining the protostyle lingually. The only other species noted with this morphology was the specimens referred to “ P.minutus ” from the Barstow Formation ( Lindsay 1972: fig. 22c) and P.ancenensis from Montana ( Sutton & Korth 1995: fig. 9A, B). This feature was used to diagnose the latter species. Unfortunately, M2 is unknown for P. trojectioansrum (Korth, 1979) .

The final distinctive feature of these species is the masseteric scar on the mandible. In all heteromyids, the anterior extent of the masseteric scar is a small shelf on the lateral side of the mandible that extends anterior to p4. In all Tertiary and Recent specimens of Perognathus and Chaetodipus , the anterior end of this shelf is dorsal to the mental foramen and extends anteriorly to at least the anterior margin of the foramen. On the mandible of E. asulcatus n. sp. this scar ends posterior to the mental foramen. This same condition is true for P. trojectioansrum (Korth 1979: fig. 3C). The mandible of P. ancenensis and the referred Barstovian specimens of “ P. minutus ” from Barstow are not known.

The similarities of these three species (“ P. ” ancenensis, “ P. ” trojectioansrum , and “ P. minutus ”) to E. asulcatus n. sp. suggest that they may be referable to Eochaetodipus n. gen. However, all of these shared features appear to be primitive, so their presence in these species may exclude them from Perognathus or Chaetodipus , but it does not automatically include them in Eochaetodipus n. gen. Since all the characters are not known for all of the species, it is impossible to include them in Eochaetodipus n. gen. at this time.

Figures

FIG. 1. — Photographs (left) and line diagrams (right) of the skull of Eochaetodipus asulcatus n. sp. based on the holotype UNSM 130000: A, dorsal view; B, lateral view (zygomatic arch removed); C, ventral view. Dashed lines indicate outlines of broken or covered areas; patterned area indicates missing bones. Abbreviations: bu, buccinator foramen; eth, ethmoid foramen; fo, foramen ovale; foa, accessory foramen ovale; fr, nutritive foramen in frontal; in, incisive foramen;mbf, fissure medial to bulla;msc, masticatory foramen; spv, sphenofrontal vacuity; pom, posterior maxillary foramen; ppl, posterior palatine foramen; t, temporal foramen; rp, rostral perforation (including infraorbital foramen); vf, venous foramen. Scale bar: 1 cm.

FIG. 2. — Mandible and cheek teeth of Eochaetodipus asulcatus n. sp. (UNSM 130000): A, lateral view of mandible; B, right p4- m3 (anterior to right); C, left P4-M3 (anterior to left). Scale bars: A, 5 mm; B, C, 1 mm.

FIG. 3. — Postcranial elements and mandible of Eochaetodipus asulcatus n. sp. (UNSM 130000): A, ulna (distal end missing); B, tibiofibula (proximal end and fibula broken); C, humerus (proximal end missing); D, calcanium; E, lateral view of right mandible. Scale bar: 5 mm.