SCIURINI INDET.

Maguire, Kaitlin Clare & Schmitz, Joshua X. Samuels and Mark D., 2018, The fauna and chronostratigraphy of the middle Miocene Mascall type area, John Day Basin, Oregon, USA, PaleoBios 35, pp. 1-51 : 20-21

SCIURINI INDET.

FIG. 11

Referred specimens —From JDNM-4: unprepared cranial and postcranial skeleton, JODA 6725; right, astragalus, JODA 15793.

Occurrence —JDNM-4

Description —JODA 6725 represents a potentially complete cranial and partial postcranial skeleton, presently embedded in a block of the Mascall Tuff. A few isolated fragments are currently free from the matrix, including the proximal end of the right humerus. The humerus of JODA 6725 is slightly smaller than that of extant Sciurus , and generally similar in morphology. The infraspinatus muscle insertion on the greater tuberosity is within a shallow depression, with the posterior margin flush with the articular surface of the head of the humerus. The deltoid ridge is narrow, but with distinct margins.

In JODA 15793 the lateral ridge of the trochlea is much larger than the medial ridge, the neck of the astragalus is long and medially directed, the head of the astragalus is somewhat saddle-shaped, the astragalo-navicular facet and sustentacular facets are joined, the sustentacular facet is well-separated from the ectal facet, and there is a well-developed medial plantar tuberosity.

Remarks —There is a substantial gap between the latest confirmed records of Protosciurus in the Hemingfordian and the appearance of Sciurus in the Clarendonian ( Emry et al. 2005, Goodwin 2008). Besides the specimens noted here, the only other early Barstovian records of tree squirrels in North America are specimens questionably assigned to Protosciurus from the Hidalgo Bluff and Trinity River local faunas on the Gulf Coast of Texas ( Lezak 1979, Albright 1996). These specimens confirm the presence of tree squirrels in the Mascall fauna and their persistence in the region from the Oligocene through the mid Miocene. There is a definite similarity of the Mascall Formation specimens to both recent Sciurus and to Oligocene and early Miocene Protosciurus .

JODA 6725 and 15793 have a number of features that are characteristic of tree squirrels. In the proximal humerus of JODA 6725 the infraspinatus insertion and narrow deltoid ridge are characteristic of tree squirrels ( Sciurini ) ( Emry and Thorington 1982, Emry et al. 2005). In contrast, ground squirrels ( Marmotini ) have a broad deltoid ridge and insertion of the infraspinatus into a pit on the greater tuberosity ( Emry and Thorington 1982). As in extant Sciurinae (both tree squirrels— Sciurini , and flying squirrels— Pteromyini ), the neck of the astragalus (JODA 15793) is long and medially directed, while the head of the astragalus is somewhat saddle-shaped ( Emry and Thorington 1982, Ginot et al. 2016). As in extinct and extant Sciurini , the lateral ridge of the trochlea in JODA 15793 is much larger than the medial ridge, the astragalo-navicular facet and sustentacular facets are joined, while the ectal facet is well-separated, and there is a well-developed medial plantar tuberosity that forms a characteristic “hook” ( Emry and Thorington 1982, Ginot et al. 2016). The two trochlear ridges are more similar in size and the astragalo-navicular facet and sustentacular facets are separated in ground squirrels ( Marmotini ) ( Ginot et al. 2016). The sustentacular facet is not well-separated from the ectal facet in Pteromyini ( Ginot et al. 2016) . Overall, the morphology of the astragalus, JODA 15793, is most similar to Protosciurus , which is well-known from the John Day Formation ( Korth and Samuels 2015). This is particularly true of the head of the astragalus, which is somewhat saddle-shaped in JODA 15793 and Protosciurus , but not to the extent observed in Sciurus ( Emry and Thorington 1982, Ginot et al. 2016).

JODA 6725 is embedded in a soft tuffaceous matrix and particularly fragile, as a result it has not been prepared. Once extracted from the surrounding matrix, this specimen has the potential to fill an important gap in the record of tree squirrel evolution in North America.

Figures

Figure 11. Sciurini right astragalus, JODA 15793. Scale bar =1mm.