Alphagaulus vetus, (MATTHEW, 1924)

Calede, Jonathan J. M. & Hopkins, Samantha S. B., 2012, Intraspecific versus interspecific variation in Miocene Great Basin mylagaulids: implications for systematics and evolutionary history, Zoological Journal of the Linnean Society 164 (2), pp. 427-450 : 431-435

ALPHAGAULUS VETUS ( MATTHEW, 1924)

FIGURES 4–6

Synonymy: Mylagaulus pristinus (in part) Kellogg, 1910; Mylagaulus vetus Matthew, 1924 ; Mesogaulus vetus Cook & Gregory, 1941 ; Mylagaulus sp. (in part) Downs, 1956; Mesogaulus cf. pristinus Shotwell, 1958 ; Mesogaulus vetus Morea, 1981 ; Mesogaulus vetus Munthe, 1988 ; Mylagaulus vetus Sutton & Korth, 1995 ; Alphagaulus vetus Korth, 2000 .

Type specimen: AMNH 18905 , right mandible with incisor and p4, m2 ( Sutton & Korth, 1995: fig. 3H) from Thomson (Thompson) Quarry, Sheep Creek Formation , Nebraska .

Referred material: From type locality [Thomson (also spelled Thompson) Quarry, Sheep Creek Formation] by Korth (2000): AMNH 20504 , 20507 , 90734 ; F: AM 65515 , 65517–65520 , 65523 , 65526 , 65527 . From Observation Quarry by Korth (2000): F: AM 65532 , 65534–65536 , 65538–65551 , 65556 , 65558 , 65559 , 65561 . From Split Rock Local Fauna by Munthe (1988): UCMP 121693 , an almost complete skull and mandibles of an adult specimen ( V 69190 , Third Bench ) ; UCMP 121694 , a partial adult skull ( V 77144 , Split Rock Microsite ) ; UCMP 121679 , a P4 ( V 69190 , Third Bench ) ; CM 14268 and CM 14269 , M1; UCMP 121680 , UCMP 121685 , M2 ( V 69191 , Second Bench ) ; UCMP 121688 , UCMP 121689 , M2 ( V 69192 , First Bench ) ; CM 13998 , dp4; UCMP 121686 , dp4 ( V 69191 , Second Bench ) ; UCMP 121690 , dp4 ( V 69192 , First Bench ) ; MCZ 7318 , CM 14700 , p4; UCMP 121681 , p4 ( V 69190 , Third Bench ) ; MCZ 6224 , m1; UCMP 121692 , m1 ( V 77147 , Split Rock Eagle’s Nest ) ; CM 14695 , CM 15825 , m2; UCMP 121682 , m2 ( V 69190 , Third Bench ) ; UCMP 121687 , m2 ( V 69191 , Second Bench ) ; UCMP 121691 , m3 ( V 77144 , Split Rock Microsite). From Railroad Canyon Sequence (by Barnosky et al., 2007) : UM 4077 , right dentary with p4-m2 ( MV7316 , Cottom Lane ) ; UM 4079 , right P4 ( MV7320 , Big Camel ) ; UM 4236 , left ramus with p4, m2-m3 ( MV7323 , Turtle 2) ; UM 6443 , left P4 ( MV7338 , Snowfence 1/2) ; UM 6444 , left unworn m2 ( MV7338 , Snowfence 1/2) ; UM 4489 , left mandible with p4, m2 ( MV7345 , Rodent Bluff ). From Virgin Valley Formation UCMP 11540 , UCMP 11683 , UCMP 11684 , UCMP 11843 , UCMP 40988 , UCMP 130241 , p4 ( UCMP 1090 , Virgin Valley 9) ; UCMP 12580 , p4 ( UCMP 1095 , Virgin Valley 14) ; UCMP 130245 , p4 ( V 73056 , Prospect 1) ; UCMP 152493 , UCMP 152495 , p4 ( V 90052 , Gooch Table NE 3) ; UCMP 130239 , UCMP 130240 , UCMP 130242 , UCMP 40955 , UCMP 40993 , P4 ( UCMP 1090 , Virgin Valley 9) ; UCMP 130243 , P4 ( V 73056 , Prospect 3) ; UCMP 130244 , UCMP 130246–130248 , P4 ( V 73056 , Prospect 1) ; UCMP 11303 , partial P4 ( UCMP 1095 , Virgin Valley 14) ; UCMP 130251 , partial P4 ( V 73062 , Prospect 1) ; UCMP 130249 , partial fourth premolars with associated partial distal humerus ( V73056 , Prospect 1); UCMP 130250 , partial right P4 with associated caudal vertebra, partial distal left humerus, and a partial edentulous maxilla ( V 73056 , Prospect 1) ; UCMP 130252 , M3 ( V 73056 , Prospect 1) ; UCMP 11686 , M3 ( UCMP 1090 , Virgin Valley 9) ; UCMP 11326 ( UCMP 1095 , Virgin Valley 14) , UCMP 152494 ( V 73062 , Prospect 3) , partial fourth premolars. From Massacre Lake Local Fauna ( RV7043 , Massacre Lake 1) : UCMP 316439 , UCMP 319224 , partial incisors; UCMP 315432 , UCMP 315433 , UCMP 315686 , UCMP 316010 , UCMP 316431 , UCMP 316433–316435 , p4; UCMP 315431 , UCMP 315685 , UCMP 315988 , UCMP 315989 , UCMP 316007–316009 , UCMP 316430 , UCMP 316432 , UCMP 319237 , P4; UCMP 318368 , partial right mandible fragment with m2; UCMP 316072 , UCMP 315687 , partial mandibles with partial incisors; UCMP 316436 , partial right mandible with incisor and p4; UCMP 316438 , partial right mandible with dp4, m1–m3; UCMP 318367 , a partial left mandible with dp4, m1–m2; UCMP 315684 , a partial skull with left P4, M2–M3 and right P4, M2; UCMP 316437 , partial skull of a juvenile with left and right P4, M1–M2 and left M3, UCMP 315730 , partial juvenile cheek tooth. From Massacre Lake Local Fauna ( V6161 , Massacre Lake 2) : UCMP 61709 , a partial left mandible with p4, m2–m3; UCMP 61710 , a partial skull of a juvenile with left and right P4, M1–M3. From Mascall Formation (Crooked

River): UOMNH F-16722, UOMNH F-16898, p4. From Mascall Formation YPM 14311, P4; YPM 14310, p4.

Distribution: Late Hemingfordian of Sheep Creek Formation of Nebraska, Split Rock Fauna of Wyoming, and Massacre Lake Fauna of Nevada. Early Barstovian of Sand Canyon Beds Formation (Observation Quarry) of Nebraska, Railroad Canyon (even into stage 4); fossettes anteroposteriorly orientated; postcranial skeleton relatively gracile unlike Al. tedfordi .

Sequence of Montana and Idaho, Virgin Valley Formation of Nevada , and Mascall Formation of Oregon.

Emended diagnosis: Larger than Al. pristinus ; postorbital index (POI) on the larger end of the genus; skull morphology and dentition most similar to the smaller Al. pristinus ; skull not as deep (dorsoventrally) or robust as in Alphagaulus douglassi and Alphagaulus tedfordi ; five to nine fossettids on the premolars unlike Al. pristinus ; fossettids orientated obliquely running from the posterolingual corner of the tooth to the anterolabial one unlike in Al. pristinus ; fossettes more parallel to one another in adult individuals than in Al. douglassi and Al. tedfordi ; some may run more labiolingually in young individuals; hypofossettid and mesofossettid split with wear; between four and seven fossettes on the premolars unlike Al. pristinus ; anterofossette remains branched until late in wear Description of dentition: Figure 4 shows the occlusal surface of the upper and lower fourth premolars of specimens from Nevada. There are five to six fossettids on the premolars, with the exception of UCMP 16435 on which there are seven fossettids. There are up to nine fossettids in specimens from Nebraska ( Korth, 2000). The fossettids are not orientated anteroposteriorly but obliquely run from the posterolingual corner of the tooth to the anterolabial one. There are two fossettids in the anterolingual corner of the tooth (as noted by Korth, 2000), a third one in the posterolabial corner, and two others running in between them. The fossettids that split include the posterolabial one (hypofossettid, Shotwell, 1958) and the mesofossettid ( Shotwell, 1958). Often, the mesofossettid and more occasionally the metafossettid and hypofossettid run more labiolingually in young individuals (e.g. UCMP 318367, UCMP 315686). Throughout ontogeny, the anterofossettid is the most conservative fossettid. Its shape, complexity, and size do not vary much. The mesofossettid, by contrast, becomes larger, may split into different lakes, and exhibits increasing complexity in its shape from a simple elongate single branch in most specimens at wear stage 2 to two joined but diverging branches in some older specimens (e.g. UCMP 316436, late stage 2; UCMP 315686, stage 3; UCMP 316010, stage 4). The metafossettid varies greatly in shape across individuals of comparable wear stage. The hypofossettid may split and vary somewhat in shape but its size does not change much with wear. The anterolabial fossettid may be the latest to close; it varies greatly in shape and size with wear but does not appear to split.

The upper premolars exhibit a similar number of lakes (between four and six). Unlike the specimens from Nebraska that mostly show six or seven fossettes, most of the specimens from Nevada exhibit five lakes. The anterofossette is the largest fossette and remains branched late in wear. This is similar to the Great Plains specimens ( Korth, 2000). The fossettes are more or less anteroposteriorly orientated. In late stages of wear, the anterofossette is the only lake to occupy the anterior lobe of the P4 and is still forked (e.g. UCMP 316007, a stage 4 tooth). In the DP4, the anterolingual fossette runs more labiolingually (e.g. UCMP 316437). The anterofossette is joined to the anterolingual fossette early on in wear (early stage 1, e.g. UCMP 316008). It becomes larger with ontogeny but does not split. The parafossette does not appear to split, but varies greatly in size and shape across individuals of the same wear stage. The posterolabial fossette also shows different sizes and shapes and may be large in some specimens. The posterolingual fossette is one of the earliest to close and does not change much in size or shape relative to the other lakes. The anterolingual fossette does not split in any specimen observed but comes close to it in one mid-wear stage (stage 3) specimen (UCMP 130247). There is an additional lake located near the former position of the worn-off protocone. This lake is present in four different specimens ranging from wear stages 1 to 4.

Description of the postcranial elements: Postcranial elements catalogued under the same specimen numbers as dental material confidently identifiable as Al. vetus are shown in Figure 6 . The single anterior caudal vertebra of Al. vetus belongs to a juvenile specimen (UCMP 130250) from the Virgin Valley Formation. It is anteroposteriorly elongated, more so than in a specimen of Ap. rufa of similar size and age (UOMNH R-8453).The zygapophyses are small. The transverse processes are broadened posteriorly. The epiphyses of the vertebra are missing from this specimen. The neural spine is small.

The partial distal left humerus of UCMP 130250 only includes the distal part of the bone from the supracondylar ridges to the trochlea. Relative to length, the distal humerus of Al. vetus is broader than that of Ap. rufa or even than that of Al. pristinus . The medial epicondyle bends posteriorly, as in Pterogaulus laevis and Al. pristinus ( Fagan, 1960) . The fossa between the medial epicondyle and the posterior surface of the trochlea is comparable in depth to that of Al. pristinus and deeper than that of Ap. rufa . This seems to be a characteristic of Mylagaulidae because it is also true of P. laevis ( Fagan, 1960) . The capitulum is slightly broader than the trochlea, which is in turn sharper and more elongated anteroposteriorly. The distal humeral articular surface of Al. vetus is, in this aspect, larger than that of Al. pristinus . The olecranon fossa is shallow, as in other mylagaulids ( Fagan, 1960). This fossa is also similar to Al. pristinus in that it is narrower than the capitulum and trochlea together.

Discussion: Alphagaulus ( Korth, 2000) is a critical genus when trying to understand the evolutionary history of Mylagaulidae . This paraphyletic taxon at the base of the Mylagaulinae sheds light on the early history of the hypsodont members of the family Mylagaulidae ( Hopkins, 2008) . Alphagaulus vetus is particularly important, because it is represented by a more complete and geographically widespread fossil record than any other species of the genus Alphagaulus ( Korth, 2000) . Alphagaulus vetus is present in three different biogeographical regions (the northern Great Plains of Nebraska, northern Rockies of Wyoming, Montana, and Idaho, and the Columbia Plateau in Nevada; Barnosky, Carrasco & Davis, 2005) and two NALMAs (Hemingfordian and Barstovian), whereas the other three species in this genus ( Al. pristinus , Al. tedfordi , and Al. douglassi ) are known only from the Barstovian of south-west Montana. Moreover, all four stages of ontogeny (as indicated from wear; see Material and methods) from juveniles to adults are represented in our sample of Al. vetus . The specimens from Crooked River (Mascall Formation) constitute the first record of Al. vetus from Oregon.

Comparisons of measurements between specimens from Nevada and those from the Great Plains reported by Korth (2000) show few differences (see Table 2). The width of the teeth is greater relative to the length in the specimens from Nevada than it is in those of the Great Plains or Rockies. The difference is within the published ranges for the species. A juvenile specimen from the Massacre Lake Local Fauna (UCMP 316437, Fig. 5 ) provides information on changes within Al. vetus through ontogeny. This individual retains a juvenile dentition with the remaining deciduous P3 and P4, the retained M1, the adult M2, and the erupting M3. The skull is poorly sutured. As in Al. pristinus and H. gazini (see discussion of H. gazini in this paper), there are a number of cranial features that change drastically between juveniles and adults. The zygomatic arch of UCMP 316437 is straight and gracile. It has been crushed taphonomically and thus brought closer to the midline of the skull. However, prior to crushing, it could not have been as broad and as widely curving as it is in adult specimens prior to crushing. The infraorbital foramen is small, the partial occipital plate available for description is quite straight and does not slope posteroventrally as in adults (see Munthe, 1988: fig. 8). The parasagittal and occipital crests, which differ importantly amongst early mylagaulines ( Hopkins, 2008) are missing and therefore cannot be described.

Figures

Figure 4. Occlusal morphology of the fourth premolars of Alphagaulus vetus. Numbers refer to wear stages described in the text, and d to deciduous. Anterior is to the left of the figure. A, UCMP 318367 left deciduous p4 (mirrored); B, UCMP 11540, left p4 (mirrored); C, UCMP 61709, left p4 (mirrored); D, UCMP 316434, left p4 (mirrored); E, UCMP 316431, left p4 (mirrored); F, UCMP 11843, left p4 (mirrored); G, UCMP 315432, left p4 (mirrored); H, UCMP 316435, right p4; I, UCMP 316436, right p4; J, UCMP 316433, left p4 (mirrored); K, UCMP 11684, left p4 (mirrored); L, UCMP 315686, right p4; M, UCMP 316010, right p4; N, UCMP 316437, right deciduous P4; O, UCMP 316008, left P4 (mirrored); P, UCMP 315684, right P4; Q, UCMP 130244, left P4 (mirrored); R, UCMP 152495, left P4 (mirrored); S, UCMP 130240, right P4; T, UCMP 130247, right P4; U, UCMP 319237, right P4; V, UCMP 315431, left P4 (mirrored); W, UCMP 316007, right P4. Scale bar = 1 cm.

Figure 5. Cranial morphology of UCMP 316437, the skull of a juvenile Alphagaulus vetus. A, ventral view; B, lateral view.Scale bar = 1 cm.

Figure 6. Postcranial morphology of UCMP 130250. A, partial distal humerus; B, caudal vertebra. Scale bar = 1 cm.