MYLAGAULIDAE INDET.

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 : 446-447

MYLAGAULIDAE INDET.

FIGURES 14 , 15

Synonymy: Mylagaulus sp. (in part) Shotwell, 1963; Mylagaulus monodon (in part) Kellogg, 1910; Mylagaulus sp. (in part) Shotwell, 1958.

Referred material: From Juntura Formation (Black Butte Fauna): UOMNH F-5451 (UO 2334), UOMNH F-15694 (UO 2339), p4s; UOMNH F-15695 (UO 2339), fragmentary fourth premolar; UOMNH F5770 (UO 2341), partial right mandible with m1. From Virgin Valley Formation UCMP 41026 (UCMP 1090, Virgin Valley 9), left and right mandibles with partial left incisor, left m2-m3, partial right incisor and right p4; UCMP 11572 (UCMP 1065, Virgin Valley 2), partial mandible with p4; UCMP 11662 (UCMP 1065, Virgin Valley 2), isolated p4. From Mascall Formation (JDNM 4): JODA 2329, partial left mandible with dp4–m2 and erupting p4.

Distribution: Early Hemingfordian John Day Formation, Oregon; Early Barstovian Virgin Valley Formation, and Mascall Formation of Oregon; Late Clarendonian the Juntura Formation, Oregon.

Discussion: Specimen UOMNH F-5451 is a p 4 in very early stage of wear (stage 1). The tooth is damaged and no description or identification of it is possible. A single juvenile specimen (JODA 2329, Fig. 15 ) from the Mascall Formation could not be confidently identified. It is most likely either Al. vetus or H. gazini , both known to be present in the Mascall Formation. The lack of an adult dentition precludes definitive identification to a mylagaulid species. The m1 is close to being pushed out of the tooth row by the erupting absent in all specimens of Al. vetus . However, these Virgin Valley specimens share with Al. vetus a common general tooth shape. The shape of the fossettids of the three specimens is intermediate between Al. vetus and H. gazini . The orientation of the lakes is closer to that of Al. vetus . In UOMNH F-41026, we can also recognize an additional fossette located in the posterolabial corner of the tooth, labially to the metafossette and posteriorly to the parafossette of Shotwell (1958: fig. 7). This character varies intraspecifically in other taxa (e.g. H. shotwelli ). These three specimens might represent evidence for a new taxon but the scarce material currently available prevents further description.

p4. The absence of well-preserved lower second molars early in wear (in order to be comparable to that erupting in JODA 2329) and published deciduous lower fourth premolars for Al. vetus and H. gazini make impossible a definitive assignment of this specimen to either one of the mylagaulid species found in the Mascall Formation. The partial mandible from the Juntura Formation most likely belongs to H. shotwelli . Owing to the lack of diagnostic dental material, this specimen cannot be identified with confidence.

Three specimens (UCMP 41026, UCMP 11572, UCMP 11662) of mylagaulids from the early Barstovian Virgin Valley Formation exhibit a morphology intermediate between that of Al. vetus found in the Virgin Valley Formation and the contemporaneous H. gazini from the early Barstovian of Oregon. The tooth size of these specimens is close to that of H. gazini . Similarly, the number of lakes in these specimens is similar to that in H. gazini . In particular, they both exhibit an anterolingual fossettid

Figures

Figure 14. Occlusal morphology of the fourth premolars of the Virgin Valley specimens of indeterminate mylagaulids. Numbers refer to wear stages described in the text. Anterior is to the left of the figure. A, UCMP 11572, left p4 (mirrored); B, UCMP 11662, right p4; C, UCMP 41026, right P4. Scale bar = 1 cm.

Figure 15. Occlusal morphology of the dp4–m2 of an indeterminate mylagaulid from the Mascall Formation of Oregon. Scale bar = 1 cm.