Hesperogaulus gazini, Korth, 1999

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 : 435-439

HESPEROGAULUS GAZINI ( KORTH, 1999)

FIGURES 7 , 8

Synonymy: Mylagaulus cf. laevis Gazin, 1932 ; Mylagaulus cf. laevis Scharf, 1935 ; Mylagaulus cf. laevis Wallace, 1946 ; Mylagaulus sp. Downs (in part) 1956; Hesperogaulus gazini Korth, 1999 .

Type specimen: LACM ( CIT) 68, skull with left P4, M2-M3 ( Korth, 1999: figs 2.1, 2.2) from Sucker Creek Formation of Oregon (Skull Springs) .

Referred material: From Skull Springs (Sucker Creek Formation) by Korth (1999): LACM (CIT) 69, LACM (CIT) 365–367, LACM 6018, LACM 4987, partial skulls or upper dentitions; LACM (CIT) 70, LACM (CIT) 368, LACM (CIT) 369, mandibles with dentition; LACM (CIT) 516–523, p4, LACM (CIT) 524–531, P4. From Beatys Butte Fauna: UCMP 130331 ( V84106 ), UCMP 130335 ( V84100 ), UOMNH F-17650 (UO 2418), UOMNH F-21911 (UO 2418), P4; UOMNH F-17665 (UO 2418), UOMNH F-21930 (UO 2418), p4, UOMNH F-17666 (UO 2418), UCMP 188841 ( V84103 ), partial fourth premolars. From Mascall Formation JODA 3308, an almost complete skull with left partial P4 and complete M3 (V4827); JODA 2330, M2 (JDNM 4). From Simtustus Formation JODA 8678, P4 (no locality number, Coburn Well).

Distribution: Barstovian of Oregon, Sucker Creek, Mascall, and Simtustus Formations and Beatys Butte Fauna.

Emended diagnosis: Smallest species of the genus (upper tooth row length: 15.52 mm, lower tooth row length: 18.24 mm); five to six fossettes on P4, fewer than either Hesperogaulus shotwelli or H. wilsoni ; six to seven fossettids on p4, as many as H. shotwelli ; anterolingual and posterolabial fossettes do not split unlike H. shotwelli ; shape of the outline of P4 rounder than in other species of the genus; fossettids orientated obliquely like in H. shotwelli ; POI (0.31–0.35) intermediate between H. shotwelli (0.40) and H. wilsoni (0.22–0.25); occipital not as low sloping as in H. wilsoni .

Discussion: Korth (1999) described the species H. gazini with material restricted to the Sucker Creek Formation of Oregon. We present additional specimens from the Mascall and Simtustus Formations as well as the Beatys Butte Fauna of Oregon. We find that H. gazini is still restricted to the early Barstovian. A single P4 from the Simtustus Formation is the first mylagaulid specimen to be published from this Miocene unit of central Oregon. This unit is stratigraphically equivalent to the better-known Mascall Formation ( Downs, 1956) in which other H. gazini specimens have been found. The Simtustus Formation is composed of volcaniclastic sandstones and mudstones interbedded with volcanics (tuff, lava flows) of the Columbia River Basalt Group. An Ar/Ar date of 15.7 ± 0.1 Mya ( Smith, 1986) is consistent with the stratigraphical correlations with the rest of

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the well-known central Oregon units. The isolated tooth from the Simtustus is that of an adult specimen at a mid-wear stage (stage 3). The morphology of the lakes is consistent with the labiolingually compressed and anteroposteriorly extended fossettes of the specimens described by Korth (1999: fig. 1). Similarly, in concordance with the published specimens, the lakes of the tooth from the Simtustus Formation are anteroposteriorly orientated. The only major difference is in the shape and size of the posterolabial fossette. Unlike the specimens previously described, but similar to numerous specimens described herein, there is an unnamed posterolabial-most lake (see Fig. 7 ). The specimens from the Mascall Formation include an isolated M2 from the Mascall Ranch locality and an almost complete skull from Coburn Well ( Fig. 8 ). The skull, which lacks most of the dentition, is similar in cranial morphology to the type specimen (LACM (CIT) 68) of the species. Numerous isolated P4s and p4s from the Beatys Butte Fauna complete the material now available for H. gazini . The specimens from Beatys Butte include juvenile individuals. We therefore have an ontogenic series available from juveniles at Beatys Butte, young adult specimens from Beatys Butte and the Mascall Formation, to adults and late wear specimens in the Sucker Creek Formation ( Fig. 7 ). This ontogenic series indicates important changes in the shape and size of the fossettes in H. gazini through development. The changes are greater in the anterior-most fossette. The separation of the branches of the anterofossette does not consistently happen in the same direction amongst specimens of the species H. gazini . There is some inconsistency in whether the labial or the lingual branch separates first in specimens from Nebraska ( Korth, 1999) and we find similar variations in the pattern of wear in specimens from the Great Basin. There is no evidence for a more frequent separation of either branch of the anterofossette in the specimens observed in the JODA or LACM collections.

Korth (1999) suggested that H. gazini might be subject to sexual dimorphism expressed by differences in size and muscle development. The cranial material ( Fig. 8 ) presented here does not resolve this issue. It most closely resembles the type specimen LACM (CIT) 68 illustrated by Korth (1999). There are now, therefore, two skulls for this morphotype of H. gazini versus one for the other morphotype (LACM 4987). The skull from the Mascall Formation is slightly smaller than the type and exhibits raised areas of rugose bone on the anterior end of the nasal bones (a part of the skull not preserved in the type specimen of H. gazini ) that are similar in size and shape to those of H. wilsoni . These rugosities are also present in a third species of Hesperogaulus described later in this paper. The partial P4 available for description lacks the anterior part of the tooth but exhibits the parafossette, metafossette, and hypofossette, as well as part of the anterofossette. The anterolabial lake is unusually elongated anteroposteriorly, but the morphology of the other lakes is consistent with the referred specimens of Korth (1999). Although Gazin (1932) provided a description of the type specimen, later completed by Korth’s (1999) description of the species, numerous features should be highlighted in reference to the new skull. The Mascall specimen differs from the type specimen in a few characters. The postorbital processes of the jugal and frontal are shorter and more rounded in the Mascall specimen than in the type, and are more consistent with LACM 4987 (one of the paratypes of H. gazini ). The incisive foramina differ in shape between the two specimens with the Mascall skull exhibiting more ovate and open foramina. Finally, the infraorbital foramina are rounder in the Mascall specimen. In JODA 3308, the left foramen is essentially round, whereas the right one is oval with the long axis orientated dorsolaterally. The rest of the cranial morphology is consistent with the type where comparison is possible, although the two specimens are to some degree complementary in the areas of the skull that are preserved. The parietal region of JODA 3308 is missing, whereas the basicranium is better preserved than in the type specimen. The nasal–maxilla, and the premaxilla– maxilla sutures can be observed on JODA 3308 but cannot be seen on LACM (CIT) 68. The Mascall specimen might thus be a younger individual than the holotype; the differences between the two specimens may thus be ontogenic. There are numerous characters in Al. pristinus that change throughout ontogeny including the shape and size of the infraorbital foramen and zygomatic arch, the height of occipital and sagittal crests, and the size and position of the postorbital processes ( Calede & Hopkins, 2012). The overall similarity in these cranial features between the two specimens of H. gazini examined and the fact that both specimens have a fully adult dentition (even though some teeth are missing from the JODA specimen) suggest that the JODA specimen is a young adult, not a juvenile.

The differences amongst the specimens of H. gazini in different formations of Oregon are here interpreted as ontogenetic ones. Certainly, given the dramatic differences in the amount of wear visible on the teeth, at least some of these differences must be ontogenetic; however, some of the differences that we observed may also result from spatial variability. Spatial variability in dental morphology is common in some rodents and may be related to climatic clines or differences in feeding habits (e.g. Markova et al., 2010; McGuire, 2010). However, the presence of cranial material from the Mascall and Sucker Creek Formations allows us to verify assertions about ontogenetic stages of dental remains by examining cranial evidence of age; these data support the conclusions from the dental record. Additionally, the presence of specimens of comparable morphology (here interpreted as that of young adults) in both the Mascall and Beatys Butte assemblages indicate that the differences amongst specimens cannot be explained solely by patterns of geographical variation. Similarly, the specimens interpreted as representing different life stages are not consistent with the interpretation of this intraspecific variation as a result of temporal variability. Indeed, all specimens are from the early Barstovian and within a few hundred thousand years of each other with the late wear adult type from Sucker Creek as old (geologically) as the juveniles and young adults from Beatys Butte (Table 1). There is evidence of plasticity; for example, some of the minor fossettes are variably present within populations of the species we describe here; these differences are not structured by locality, so we interpret them as withinpopulation variation. Although it is possible that these features varied through the time span averaged by individual localities, this variation is not structured in any way that can be meaningfully interpreted from the fossil record.

The ontogenetic pattern of skull morphology seen in H. gazini is similar to that described for Al. pristinus ( Calede & Hopkins, 2012) . The angle of the occipital plate mentioned by Gazin (1932) and Korth (1999) also changes throughout the development of an individual. Therefore, ontogeny needs to be considered before using this character. The fact that early Barstovian specimens are smaller was already noted by Korth (1999) and Shotwell (1958) concerning the specimens from Sucker Creek and Virgin Valley. On the basis of the more complete cranial material available to us, we cannot discriminate two different species and retain H. gazini as the lone taxon, contrary to the possibility suggested by Korth (1999).

Figures

Figure 7. Occlusal morphology of the fourth premolars of Hesperogaulus gazini. Numbers refer to wear stages described in the text. Anterior is to the left of the figure. A, CIT 521, left p4 (mirrored); B, CIT 525, right P4; C, JODA 3308, left P4 (mirrored); D, CIT 527, right P4; E, CIT 524, right P4. Scale bar = 1 cm.

Figure 8. Cranial morphology of Hesperogaulus gazini from the Mascall Formation (JODA 3308). A, dorsal view; B, ventral view; C, lateral view. Scale bar = 1 cm.