Sinclairella dakotensis, Jepsen, 1934

Samuels, Joshua X., 2021, The first records of Sinclairella (Apatemyidae) from the Pacific Northwest, USA, PaleoBios 38, pp. 1-7 : 3-5

SINCLAIRELLA DAKOTENSIS JEPSEN, 1934

FIG. 3, TABLE 1

Referred specimens — JODA 15846, lower right incisor; JODA 15850, upper right second molar.

Occurrence — JODA 15846:locality JDNM 9, Blue Basin (Turtle Cove), Grant County, OR, Unit C, Turtle Cove Member, John Day Formation; JODA 15850: locality JDNM 9, Blue Basin (Turtle Cove), Grant County, OR, Unit E1, Turtle Cove Member, John Day Formation.

Age —Early early Arikareean (Early Oligocene, Ar1), between A-B Tuff dated 29.75 ± 0.02 Ma and Blue Basin Tuff dated 28.8 Ma ( Albright et al. 2008).

Description

M2 —JODA 15850 ( Fig. 3A, B) is an upper right second molar, trapezoidal in shape and slightly wider than it is long ( Table 1). It bears four cusps, with a large paracone and slightly smaller metacone on the labial side, and a similarly sized protocone and hypocone on the lingual side. The cusps are all brachydont and show relatively little wear, though the paracone and hypocone are more worn than the metacone and protocone. The paracone is oval and the metacone is relatively triangular in outline, and both cusps are somewhat mediolaterally compressed. The paracone and metacone are connected by a short, continuous, and relatively straight crest (centrocrista). Small crests extend from the anterior margin of the paracone (preparacrista) and posterior margin of the metacone (postmetacrista), these are continuous with the centrocrista and form a somewhat zig-zag line. A distinct, irregular stylar shelf runs from anterior margin of the paracone, along the labial surface of the tooth (at the preparacrista), and extends to the posterior surface of the metacone, ending at the posterolabial corner of the hypocone. A small, but distinct projection of the stylar shelf extends from the anterolabial surface of the paracone, and a similar projection lies on the labial surface of the metacone. While the stylar shelf bears several minute cuspules, none represent a clear parastyle or metastyle. The protocone is large and relatively anteriorly placed, with the apex slightly anterior to the midpoint of the labial cusps, in line with the posterior part of the paracone. A deep trigon basin separates the protocone from the paracone and metacone. A distinct anterior arm (preprotocrista) extends from the anterolabial corner of the protocone and ends along the anterior margin of the paracone. Posterior to the trigon is a large talon,bounded lingually by a prominent, crescentic hypocone, which lies posterior and lingual to the protocone. A relatively large crest (prehypocrista) runs anteriorly from the hypocone, extending all along the lingual surface of the protocone. Another crest (posthypocrista) runs posterolingually from the hypocone, ending along the posterior margin of the metacone. The posterior and lingual margins of the molar are straight, while the anterior margin is convex adjacent to the paracone, and the labial margin is slightly concave between the stylar projections with a shallow ectoflexus.

i1— JODA 15846 ( Fig. 3C) is a lower right incisor that is missing its anterior end. It is mediolaterally narrow with a maximum mediolateral width of 2.39 mm and dorsoventrally deep, tapering anteriorly in lateral view with a maximum depth of 4.87 mm at the posterior end and maximum depth of 2.91 mm at the anterior end. Enamel is present on the ventral and lateral surfaces, but missing from the medial and dorsal surfaces. There is an elongate and curved wear-facet on the dorsal surface. A large pulp cavity is evident within the broken posterior end of the tooth.

Comparisons

Both the John Day lower incisor (JODA 15846) and M2 (JODA 15850) show features typical of the Apatemyidae . The incisor bears the distinctive shape of apatemyids: elongate, anteriorly tapered, and dorsoventrally deep ( Jepsen 1934). As in other described apatemyids, the upper molar is low crowned, bears a hypocone, lacks conules, and has a prominent labial stylar shelf.

The M2 falls within the size range of Sinclairella dakotensis and is clearly larger than any known specimens of S. simplicidens ( Czaplewski and Morgan 2015, Tornow and Arbor 2017) ( Table 1). As is typical of S. dakotensis , the M2 trigon cusps are subequal in size and the hypocone is large, similar in size to the protocone ( Silcox et al. 2010, Czaplewski and Morgan 2015, Tornow and Arbor 2017). The crests between cusps in JODA 15850 are well developed and valleys between cusps are deep, as in other described specimens of S. dakotensis ( Czaplewski and Morgan 2015, Tornow and Arbor 2017). Addition- ally, the labial stylar shelf of JODA 15850 has distinct projections anterolabial to the paracone and labial to the metacone, but no distinct parastyle or metastyle, as has previously been observed in S. dakotensis and scale into the Arikareean of North America.

other early apatemyids ( Clemens 1964, Czaplewski and The mammalian faunas of the Oligocene of Oregon are Morgan 2015, Tornow and Arbor 2017). While there is very well-known, particularly the Turtle Cove Member a clear preprotocrista in JODA 15850 extending from of the John Day Formation (e.g., Albright et al. 2008, the anterolabial corner of the protocone, there is no Fremd 2010, Korth and Samuels 2015, Samuels et al. distinct paraconule, as was present in the M2 recently 2015). Finding an apatemyid from the John Day Basin described from the Sand Creek anthills in Nebraska demonstrates that even well-studied sites have potential ( Tornow and Arbor 2017). The ectoflexus of JODA 15850 for new discoveries. The lifestyle of apatemyids is also is also relatively shallow in comparison to some other rather different than any other known mammal from the described specimens of S. dakotensis from Colorado and region at this time, representing a new ecological niche, Nebraska ( Clemens 1964, Tornow and Arbor 2017). The ‘woodpecking’, for the Oligocene of Oregon.Several extant talon of JODA 15850 is large, as in previously described mammals, with the most well-known being the aye-aye specimens of S. dakotensis , and the hypocone is rela- ( Daubentonia ) and striped possum ( Dactylopsila ), are tively larger and more prominent than in some described known for feeding on wood-boring beetle larvae, which specimens of this species (e.g., UMPC 14862, Tornow and they acquire using percussive foraging with an elongate Arbor 2017). The other named species, S. simplicidens , digit and bark stripping with enlarged, ever-growing has a smaller hypocone, smooth bottom valleys between incisors ( Erickson 1991, Sterling 1994, Koenigswald et cusps,less prominent crests between cusps, and lacks the al. 2005, Morris et al. 2018). The similar elongate fingers, prominent labial stylar shelf and projections seen in JODA craniodental structure, and ecology of this living primate 15850 and other specimens of S. dakotensis (Czaplewski and marsupial have been compared to known skeletons and Morgan 2015, Tornow and Arbor 2017). of apatemyids and used to infer similar ecological niches for these organisms ( Koenigswald et al. 2005, Czaplewski DISCUSSION and Morgan 2015), which are sometimes called ‘mam- Both of the teeth collected from the Turtle Cove malian woodpeckers’.

Member of the John Day Formation are identified as The presence of an apatemyid in the lower part of S. dakotensis . The size and morphology of the M2 and the Turtle Cove Member (units C and E1) also supports lower incisor from Oregon ( Table 1, Fig. 3) are consistent reconstruction of the area as forested at that time. Unit with previously described specimens of S. dakotensis , C of the Turtle Cove Member also includes several other and differentiable from other known apatemyids. This forest-adapted taxa, specifically the tree squirrel Mioscidiscovery extends the geographic range of S. dakotensis , urus covensis Korth and Samuels (2015) and the clawed representing the first record of an apatemyid in North oreodont Agriochoerus antiquus Leidy (1850) (Lander America west of the Rocky Mountains. It also confirms 1998, Albright et al. 2008). Similarly, unit E1 has two tree widespread survival of this species into the early Arika- squirrel species, M. covensis and Protosciurus mengi Black reean, which was previously reported from the Duch- (1963), as well as A. antiquus . There is a clear ecological esnean to early Arikareean of the Great Plains (Gunnell transition higher the Turtle Cove Member, with some et al. 2008). As Czaplewski and Morgan (2015) recently forest-adapted taxa vanishing near the Picture Gorge described S. simplicidens from Florida, these records Ignimbrite and open-habitat adapted taxa, burrowing indicate survival of apatemyids on a broad geographic rodents and cursorial leporids, becoming more common ( Korth and Samuels 2015). Unit E1 is the stratigraphically lowest occurrence of an open-habitat adapted rodent, the burrowing beaver Palaeocastor peninsulatus Cope (1881) . The absence of S. dakotensis above unit E1 may be a simple consequence of their rarity overall, but may reflect the fact that as conditions became cooler and drier through the early Oligocene, the region’s forests gave way to more open habitats, while bunch grasses and shrubs became more common ( Retallack et al. 2000, Sheldon et al. 2002, Retallack 2007). The persistence of some wooded areas into the late Arikareean (Ar3) is supported by the presence of tree squirrels, Miosciurus ballovianus Cope (1881) , Pr. mengi , and Pr. rachelae Black (1963) , and a primate, Ekgmowechashala zancanellae ( Samuels et al. 2015) , but the most abundant mammals from that time in Oregon are burrowing entoptychine geomyids, palaeocastorine beavers, and archaeolagine leporids ( Korth and Samuels 2015). Combined, these faunal records support interpretation of the lower part of Turtle Cove (units A to D) as relatively heavily forested and the middle and upper parts (units E to K2) as a mosaic open woodland environment.