Hesperogaulus wilsoni (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 : 443-444

HESPEROGAULUS WILSONI ( KORTH, 1999)

FIGURE 12

Synonymy: Mylagaulus cf. monodon Wilson, 1937 ; Mylagaulus sp. Shotwell, 1958 ; Hesperogaulus wilsoni Korth, 1999 .

Type specimen: LACM 142506 , complete skull with incisors and right P4 from Rome Fauna (Drewsey Formation), Oregon .

Referred material: From Drewsey Formation (Rome Fauna) by Korth (1999): LACM 142507, skull; LACM (CIT) 1952, LACM (CIT) 1953, LACM (CIT) 1955, P4s; LACM (CIT) 1954, LACM (CIT) 1956–1958, lower dentitions. From McKay Formation (UO 2222): UOMNH F-10349, P4; UOMNH F-10347, UOMNH F-10348, p4s.

LTRL, lower tooth row length; M, mean; N, sample size; SD, standard deviation; UTRL, upper tooth row length.

Distribution: Late early to early late Hemphillian of the Drewsey and McKay Formations of Oregon.

Discussion: Part of the material referred to H. wilsoni by Korth (1999) comes from the McKay Reservoir locality (UO2222). When first describing the fauna from the area, Shotwell (1956) hypothesized that this was a unit of the Shutler Formation. Since then, other authors ( Wagner, 1976; Feranec, Barnosky & Quang, 2005) have used the term Shutler Formation when referring to the McKay specimens. However, Farooqui et al. (1981) and Newcomb (1971) proposed that the Shutler Formation be broken up into the McKay Formation (containing the locality UO 2222) and the Alkali Canyon Formation ( Farooqui et al., 1981). We therefore refer to the specimens from the McKay Reservoir as being part of the McKay Formation.

When describing H. wilsoni, Korth (1999) mentioned two specimens from the UOMNH collections. These specimens were figured by Shotwell (1958) and attributed to Mylagaulus sp. There are actually three specimens of H. wilsoni from the McKay Formation (McKay Reservoir, UO 2222). Figure 12 shows the occlusal morphology of these three specimens. As mentioned by Korth (1999), the number of lakes in H. wilsoni is the greatest of the genus and in late stage of wear specimens can exhibit eight, nine, or even ten fossettids. The fossettids vary more in orientation than the fossettes, which are elongated and orientated anteroposteriorly. However, although usually the fossettids extend from the posterolingual corner of the tooth to the anterolabial one, the orientation seems to be more prone to changes with wear and amongst individuals. UOMNH F-10347, in particular, at a wear stage comparable to that of UOMNH F-10348, shows many more small smaller curved lakes with varying orientation. It is worth noting that in addition to the six main fossettids (see Shotwell, 1958: fig. 7), there are two other lakes of different homology, as both lower teeth exhibit an additional lingual lake. There is a posterolingual fossette, an anterolingual one, and a third one located in between. Furthermore, UOMNH F-10347 also shows another fossettid lingual to the anterior-most fossettid. A similar phenomenon is found in the upper tooth, UOMNH F-10349, as well as in the specimens figured by Korth (1999: figs 1.3, 1.4) in which there is an additional lake in the posterolabial corner of the tooth. Within the species H. wilsoni , there is little variation in size of the premolar. All specimens from the UOMNH as well as the specimens described by Korth (1999) are in the same size range (± 2 mm in length and width), 12–14 mm anteroposterior length for the p4 ( Table 3). Note that the toothrow length (as a proxy for body mass) of H. wilsoni is biased by the loss of the molars (the toothrow is reduced to the fourth premolar). Hesperogaulus wilsoni is actually the largest species of the genus ( Korth, 1999; this paper) as indicated by cranial material. The size of the fourth premolar by itself should not be considered as a proxy for body size because it may be affected by changes in ecology as well.

Figures

Figure 12. Occlusal morphology of the fourth premolars of Hesperogaulus wilsoni. Numbers refer to wear stages described in the text. Anterior is to the left of the figure. A, UOMNH F-10347, left p4 (mirrored); B, UOMNH F-10348, right p4; C, UOMNH F-10349, right P4. Scale bar = 1 cm.