Hesperogaulus shotwelli, Calede & Hopkins, 2012

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 : 439-443

publication ID	https://doi.org/ 10.1111/j.1096-3642.2011.00765.x
persistent identifier	https://treatment.plazi.org/id/03C387A8-2839-FFD0-ECBB-4E296ABCFB57
treatment provided by	Marcus
scientific name	Hesperogaulus shotwelli
status	sp. nov.

Treatment

HESPEROGAULUS SHOTWELLI SP. NOV.

FIGURES 9–11 View Figure 9 View Figure 10 View Figure 11

Synonymy: Mylagaulus monodon (in part) Kellogg, 1910; Mylagaulus sp. MacDonald, 1956 ; Mylagaulus sp. (in part). Shotwell, 1958; Mylagaulus sp. Hall, 1930 ; Hesperogaulus sp. (in part) Korth, 1999.

Type specimen: UCMP 320004 View Materials , partial skull with left incisor, P4, M2–M3 and right incisor, M2–M3, and associated mandibles with left and right incisors, P4, M2–M3 from RV-8000, Thousand Creek General, Thousand Creek Formation , Nevada .

Etymology: Patronym for J. A. Shotwell in recognition of his extensive work in south-eastern Oregon, his early interest in understanding Great Basin mylagaulids, and his collection of numerous specimens, included in this study.

Referred material: From Esmeralda Formation ( V 2804) by Korth (1999): UCMP 29637 View Materials , p4. From Thousand Creek Formation by Korth (1999): F: AM 65871 , FAM 65873 , F: AM 65874 , P4s. From Truckee Formation ( V4845 ): UCMP 38665 View Materials , p4; UCMP 152496 View Materials , P4. From Juntura Formation (Black Butte Fauna): UOMNH F-5425 ( UO 2335 ), UOMNH F-5443 ( UO 2334 ), UOMNH F-5557 ( UO 2344 ), UOMNH F-5558 ( UO 2344 ), UOMNH F-5772 ( UO 2341 ), UOMNH F-6273 ( UO 2340 ), UOMNH F-10977 ( UO 2335 ), UOMNH F-17508 ( UO 2334 ), P4s; UOMNH F-5771 ( UO 2341 ), p4. From Drewsey Formation UOMNH F-6113, UOMNH F-6115, UOMNH F-15697, p4s ( UO 2347 ); UOMNH F-15691, isolated P4 ( UO 2356 ). From Thousand Creek Formation ( UCMP 1098 View Materials ): UCMP 11878 View Materials , P4.

Distribution: Middle Clarendonian through to late early Hemphillian of the Esmeralda, Truckee, and Thousand Creek formations of Nevada; Juntura and Drewsey formations of Oregon.

Diagnosis: Species intermediate in size between H. gazini and H. wilsoni (upper tooth row length: 16.86 mm, lower tooth row length: 16.97 mm); six to nine fossettes on P4; six to seven fossettids on p4; anterolingual and posterolabial fossettes split, fossette present and persistent between protocone and metaconule on P4 unlike in H. gazini ; outline of P4 not as oval as in H. wilsoni , anterior and posterior lobes of P4 not as marked as in H. gazini ; fossettids orientated obliquely; POI (0.40) large; largest infraorbital foramen of the genus (6.5 by 3.95 mm).

Description: The type specimen, UCMP 320004, of the species provides much information about the cranial and dental morphology of the species. The skull of H. shotwelli is intermediate in depth and robustness between those of H. gazini and H. wilsoni ( Fig. 9 View Figure 9 ). The zygomatic arch is broken and mostly missing, but the zygomatic plate is tall, taller than in H. gazini . The posterior part of the skull is missing. The maxilla–premaxilla suture begins at the posterior edge of the incisive foramina (as it does in most Aplodontidae ). The diastema is long (17 mm, as in Al. pristinus ), longer than in H. gazini from the Mascall Formation (11 mm, JODA 3308). The rostrum is composed of a large premaxilla and a slightly smaller maxilla. The P4 is placed about 6 mm posterior to the premaxilla–maxilla suture, which continues dorsally up the lateral sides of the rostrum as a highly convoluted suture. The nasals are broken laterally, exposing the root of the incisor in the upper jaw. There is a small bump on the dorsal surface of the nasal bones similar to that found in the specimen of H. gazini from the Mascall Formation (JODA 3308). The interpremaxillary foramen is minute in derived mylagaulids ( Wahlert, 1974); H. shotwelli is no exception. The incisive foramina are about 8 mm long, close to half of the rostrum length. For comparison, they are only 5 mm long in Al. pristinus , which has a rostrum of similar length. There is no procumbency of the upper incisors. The anterior root of the zygomatic arch, the zygomatic plate, is thickened and almost as tall as the entire skull. This is a feature common to mylagaulids ( Hopkins, 2006). The dorsal edge of the zygomatic plate is thickened anteriorly and posteriorly above the foramen. It is high, extending from the alveolus of the P4 to the dorsal-most surface of the frontal. It is about 17.5 mm in height as compared to about 13.3 mm in JODA 3308, the Mascall H. gazini . The anterior-most part of the zygomatic arch just posterior to the zygomatic plate is thick, thicker than in H. gazini but thinner than in Al. pristinus . Therefore, it seems that the zygomatic plate is greatly enlarged relative to the jugal in H. shotwelli compared to other mylagaulids. The postorbital process of the jugal is broken and missing; that of the frontal is rounded and lobate in form and quite prominent. The POI ( Korth, 2000) of H. shotwelli is larger than those of H. gazini or H. wilsoni (0.40 versus 0.31–0.35 and 0.22–0.25, respectively, Korth, 2000). It falls within the range of the genus Pterogaulus from the Great Plains. This is primarily because of the narrow width of the frontal posterior to the postorbital process (parameter C of the POI equation of Korth, 2000). The outline of the orbit indicates a small opening for the eye. The infraorbital foramen is ovoid. It runs from the dorsolateral end of the plate (where the curvature of the zygomatic arch initiates) towards the medioventral end of it, dorsolaterally to P4. The infraorbital foramen of H. shotwelli is close in shape to that of Aplodontia . It is large with a length of 6.5 mm and a width of 3.95 mm. This is larger than for the H. gazini specimen from the Mascall Formation (JODA 3308) whose round infraorbital foramen is 4.8 mm wide. This latter measurement is at the high end of the range (1.9 to 4.8 mm) given for mylagaulids by Wahlert (1974). Hesperogaulus shotwelli ’s infraorbital foramen size is thus the largest ever described for a mylagaulid. Because of the increased thickness of the zygomatic plate in mylagaulids, the infraorbital foramen appears to be positioned more dorsally than in Aplodontia . The orbital region is well preserved, but the bones of this area are fused as a consequence of the maturity of the specimen. Although the sutures cannot be distinguished, the foramina of the orbital area can. The optic foramen is very large for a mylagaulid, approaching the size of that of Ap. rufa (see Wahlert, 1974). Aplodontia rufa has relatively smaller optic foramina than other rodents and can therefore be inferred to have poor eyesight ( Carraway & Verts, 1993; Kay & Kirk, 2000). Hesperogaulus shotwelli , with even smaller optic foramina can be expected to have had even poorer eyesight (see Hopkins 2005 for a discussion of the consequences of vision for behaviour in mylagaulids). An interorbital foramen is also present immediately anterior to the optic foramen. This feature is also observed in Allomys and in Aplodontia , in which there is a pit in front of the optic foramen where the rectus muscles of the eye originate ( Wahlert, 1974). The palate is strongly grooved as it is in Al. pristinus , Al. vetus , and other specimens of the genus Hesperogaulus ( Korth, 1999: figs 2, 3). The grooves terminate in the single pair of posterior palatine foramina. As in other mylagaulids with increased hypsodonty and unlike Meniscomys ( Hopkins, 2006) , the maxilla is deep dorsal to the cheek teeth. The posterior maxillary foramen is missing as a result of taphonomic damage. Most of the basicranium is missing in this specimen but the pterygoid fossa and part of the pterygoid flanges are present. The pterygoid fossa is deep, more so than in Al. pristinus , and is comparable to that of H. gazini from the Mascall Formation or the modern Ap. rufa . Only the anterior-most parts of the pterygoid flanges are present. They are similar to those of H. gazini . The portion of the dorsal surface of the skull available for description is flat as in all mylagaulids. Both dentaries of UCMP 320004 are preserved. The dentaries are short but extremely robust ( Fig. 10 View Figure 10 ). They are deep (i.e. 11.5 mm at the diastema and 19.5 mm at its deepest point underneath the greatly hypsodont p4). This short, deep dentary is a common feature of mylagaulids. In addition, the lower jaw is also wide in lateral dimension. In contrast with the members of the genus Alphagaulus such as Al. pristinus , in which the short, deep lower jaw is gracile, the lower jaw of H. shotwelli is widened mediolaterally. This is emphasized by the bulge on the lateral surface of the lower jaw caused by the roots of p4. The tips of the lower incisors project slightly anteriorly, as in Al. pristinus or Al. vetus . The root of the incisors can be observed curving below and around the cheek teeth to a point in the coronoid process posterolateral to the m3, forming a half-circle. The diastema is long (about 10.2 mm). The mandibular symphysis is long and extends over the whole depth of the mandible. The dentaries are strongly fused. There is a single mental foramen on the lateral side of each dentary ventral to the diastema, close to its posterior end, a few mm anterior to the root of p4. This foramen is large, almost 2 mm in diameter. It is almost round with a narrower end pointing anteriorly. The posterior end of both dentaries is missing, preventing the description of the masseteric fossa, the angular, condyloid, and coronoid processes. The mandibular foramen is present. It is ovoid and located a few mm posterior to m3. It cannot be easily measured but is larger than that of Al. pristinus . The dentition is hypsodont. The roots of the left p4 can be seen and extend as much as 1.3 cm into the bone of the dentary. The total height of the p4 of this adult specimen is 1.7 cm.

The dentition is the same as in other derived mylagaulids; there is only one premolar retained, the p4 (and P4). The first molars are lost early in development because they are pushed out by the enlarged growing single premolar. As for most mylagaulids, the dental formula is, therefore, 1/1, 0/0, 1/1, 2/ 2 in adults. The total length of the cheek teeth is 16.9 mm for the upper left tooth row and 17.3 mm for the lower right one. The occlusal surface of the tooth rows is complex and characteristic of mylagaulids. The P4 of H. shotwelli converges on the double-lobed shape of Al. pristinus or Al. vetus but because the indentation that separates the two lobes is not as strong in the new species, it still appears pear-shaped with a wider posterior end and a narrowing anterior one (see Fig. 11 View Figure 11 ). There are six fossettes in the type specimen of the species that are mostly anteroposteriorly orientated. Other specimens discussed below display up to nine lakes. This is a greater number than in H. gazini and approaches that of H. wilsoni . The anterofossette is the largest fossette and is forked in the holotype. In later stages of wear, the labial branch of the anterofossette separates first, followed later by the lingual one in a specimen in very late stage of wear (UOMNH F-5443, late stage 4). Other fossettes that form multiple lakes with wear include the hypofossette, parafossette, and the anterolingual fossette (protofossette of Shotwell, 1958). The anterofossette closes later than most other lakes. It splits into two or even three different lakes in wear stage 3. The parafossette also closes later than other lakes. It may be divided into two different lakes and varies greatly in shape and size across individuals (even within the same wear category). The posterolabial fossette closes before the two previous fossettes but later than all others. It changes in shape and size, becoming more elongated with wear, and may span the entire anteroposterior length of the tooth. It also sometimes divides into two different lakes. There is a lake in the posterolabial corner of the tooth in between the metacone, and the anterolabial and posterolabial fossettes that is present in teeth of all wear stages. This diagnostic fossette usually remains small and round but sometimes extends to the midline of the tooth. The posterolingual fossette and the anterolingual one are joined early in wear, but later separate and close. Later, they do not change much in shape and size but one (or the both) of them may subsequently divide into two different lakes.

There are seven fossettids on the p4s of the holotype. This number varies between six and seven depending on the specimen considered. This is fewer than the maximum for H. gazini (eight lakes). The lakes on the p4 are elongated and obliquely orientated, running from the posterolingual corner of the tooth to the anterolabial one, as discussed above in Al. vetus . A few specimens (see Fig. 11 View Figure 11 ) exhibit rounder lakes as in Al. vetus , but are still distinguished from that species by the presence of additional fossettids. There are two lakes on the lingual edge of the tooth, two on the labial one, and three aligned in between. The anterolabial fossettid and the anterofossettid divide with wear. The anterofossettid may split into two lakes early in ontogeny in some individuals (as early as stage 1). The mesofossettid varies greatly in size and shape across individuals (including individuals of the same wear stage) and may split into two lakes. The posterolingual fossette may close late in wear (as late as stage 3) but does not vary much in size or shape. The hypofossettid never splits and remains elongated throughout ontogeny. The anterolabial lake is the latest to close (with the exception of the posterolingual fossettid closing in stage 3) in stage 1. It never splits but changes in shape and size with wear to become long in mid wear.

Discussion: The assignment of H. shotwelli to the genus Hesperogaulus is both consistent with the diagnosis of the genus ( Korth, 1999) and the presence of H. gazini and H. wilsoni in the Great Basin. Hesperogaulus gazini and H. wilsoni are the only other derived mylagaulids in Oregon. As a consequence, the decision to assign the new species to the genus Hesperogaulus is both consistent with morphology and the most parsimonious biogeographically. Hesperogaulus shotwelli exhibits important morphological differences from the two previously described species of the genus calling for the description of a new species.

The characters exhibited by H. shotwelli are often either shared with H. gazini or H. wilsoni , or intermediate between the other species of the genus Hesperogaulus . There are a few exceptions, especially in the size and shape of the foramina, the POI, or in the morphology of the palatine region. Some specimens of H. shotwelli display occlusal morphology that differs from that of the type specimen. This also occurs in other mylagaulids such as H. wilsoni (see above). We recognized in H. shotwelli an extra posterolingual lake on both the upper and the lower premolars. Differences in the shape of the lakes between specimens ( Fig. 11 View Figure 11 ) correspond to the wear pattern from juveniles to late adults. Thus, few lakes of complex shapes in juveniles will separate into more numerous fossettes or fossettids of less complex shape in adults with wear.