Delotrochanter petersoni, Hunt, 2011

Hunt, Robert M., 2011, Evolution Of Large Carnivores During The Mid-Cenozoic Of North America: The Temnocyonine Radiation (Mammalia, Amphicyonidae), Bulletin of the American Museum of Natural History 2011 (358), pp. 1-153 : 74-77

Delotrochanter petersoni , new species Figure 33

Family Canidae, gen. et sp. indet: Peterson, 1907: 33–34, fig. 6. (The year of publication usually cited for this paper is 1906; however, a printed erratum in a copy in the American Museum’s Osborn Library establishes March 21, 1907, as the publication date.)

TYPE: CM 1603, associated lower jaws, with only the left canine, p1, and m1 preserved. Also included under the same catalog number are the M1 protocone, the anterolabial corner of left P4, a metastylar fragment of right P4, a premolar, and four canine fragments. The Carnegie Museum field label records the holotype from the ‘‘middle Monroe Creek beds,’’ head of Warbonnet Creek, Sioux County, Nebraska, collected by O.A. Peterson and party, May 1904, but was reported in Peterson’s publication (1907: 24) as ‘‘from the upper Monroe Creek horizon.’’

DISTRIBUTION:?Mid-Arikareean, Arikaree Group, near head of Warbonnet Creek, Sioux County, Nebraska.

ETYMOLOGY: The species name recognizes the paleontologist Olaf Peterson who collected the holotype in northwest Nebraska in 1904.

DIAGNOSIS: Smallest recognized species of Delotrochanter with m1 length of 22.3 mm; differs in size from the much larger D. oryktes and D. major . Distinguished from Temnocyon and Mammacyon by nearly straight (not sinuous) labial cingulum on m1 and by short, posteriorly broad p2–3. Differs from Temnocyon by absence of m1 metaconid; by its m1 paraconid-protoconid-hypoconid triad low, blunt, not specialized for shear and in direct anteroposterior alignment; and from Rudiocyon by much smaller size.

REFERRED SPECIMENS: None.

DESCRIPTION: The canine measures 33.3 mm in height from tip to enamel base on the labial face, 12.0 mm in width, and 16.0 mm in length at the enamel base. A wear groove is developed on the

posterolabial surface by the action of the upper canine.

The p1 measures 7.7 mm in length, 5.6 mm in width. It is a simple, conical tooth with an anteriorly placed main cusp and no accessory cusps.

Only roots of p2–4 are present, however they demonstrate that the premolars were closely spaced but not crowded. Labial placement of the anterior root of p2 suggests that the front of the tooth was rotated slightly outward. An important dental trait of Delotrochanter petersoni is that p2–3 are short with small-diameter anterior roots, a specialization of the anterior premolars also found in Delotrochanter oryktes and D. major , thus a defining characteristic of the genus. In Mammacyon and Temnocyon , p2–3 are elongate teeth with anterior roots of larger diameter.

The m1 measures 22.2 mm in length, 11.3 mm in greatest width (at protoconid), and 10 mm in transverse width across the hypoconid. There is no metaconid; the paraconid is short and robust, much lower than the protoconid, with a paraconid blade 3.7 mm in length. The wide, robust protoconid is 10.1 mm in length measured between the incisures it forms with the paraconid and hypoconid. Its talonid is dominated by a massive, blunt, centrally placed hypoconid that functions as a crushing cusp when applied to the protocone basin of M1. From the hypoconid-protoconid incisure to the rear of the talonid is 7.6 mm. An indistinct cingulum surrounds the base of m1, but this cingulum on the labial side is nearly straight, not sinuous, and is a key feature linking D. petersoni to D. oryktes .

The m2 was not preserved and its size cannot be accurately determined since both mandibles have been damaged in this area. Peterson (1907: 34, fig. 6) in his description of the specimen stated that ‘‘judging from the specimen, m2 was of considerable size.’’ Peterson figured two alveoli for the roots of m2: the anterior alveolus is represented by a root, and the posterior alveolus by a circular depression where a root tip once existed. Estimating the length of m2 from these two alveolar remnants gives an m2 length of, 13 mm, and an m1/m2 ratio of,1.7, similar to that of D. oryktes (ratio E/F, 1.69, table 6).

Little can be said concerning the four isolated canine fragments that accompany CM 1603. However, these fragments and the intact canine in the mandible show that the canines are large teeth relative to the dimensions of the mandible.

The fragment of the anterolabial corner of the left P4 indicates that the upper carnassial had a slightly swollen basal cingulum, and that a fine enamel ridge ran from the tip of the paracone to a point just medial to the anterolabial corner, a typical feature of temnocyonines. In addition, the paracone was a low, blunt cusp as are the m1 cusps. This tooth fragment is configured as in Delotrochanter where the anterolabial corner is not anteriorly extended as it is in large Mammacyon . A fragment of the metastylar blade of the right P4 shows that the blade was short and broad, measuring 9 mm in length, about 8.2 mm in width. It compares well with the metastylar blade of D. oryktes (ACM 4804). An indistinct cingulum surrounds the blade. The fragment is similar in size to the fragmentary P4 metastylar blade of T. ferox (YPM-PU 10787) but is more robust. In fact, the narrow blade of T. ferox represents a more sectorial carnassial.

A fragment of the left M1 protocone shows that the protocone region was already anteroposteriorly widened. The protocone fragment measures 12.5 mm in anteroposterior width but in life the tooth was wider since the posterior part of the thickened lingual cingulum is missing. The M1 protocone of CM 1603 was knoblike and isolated in an enamel flat surrounded by an expanded lingual cingulum.

The depth of the mandible below m1 is 32.8 mm and below p2 is estimated at 30 mm. This is a rather shallow jaw relative to its thickness (12.9 mm below m1).

Note that Peterson’s figure (1907: fig. 6) gives CM 1506 for this specimen, whereas his text reads CM 1603: this last is the correct catalog number and is the only number on the specimens. The specimen labels show that the mandibles initially were given CM 1506 and the isolated teeth fragments assigned CM 1505; all this material later was united under CM 1603.

DISCUSSION: This mandible, collected in 1904, was the first record of Temnocyoninae in the North American midcontinent, and was believed by Peterson (1907) to be a canid (within the broader meaning of that term as used at that time, i.e., canids and amphicyonids). The few intact teeth present in the left mandible (canine, p1, m1) made identification difficult. However, the following traits suggest that this is the earliest North American representative of Delotrochanter :

(1) absence of m1 metaconid, straight labial cingulum on m1, and large centrally placed m1 hypoconid filling the talonid; (2) short p2–3, with small-diameter anterior roots and largerdiameter posterior roots; (3) unexpanded anterolabial corner of P4 like that of D. oryktes ; low, short P4 metastylar blade; (4) slender, shallow horizontal ramus of the mandible.

Fine-grained gray volcaniclastic sandstone adheres to the mandibles, leaving no doubt as to the sediment in which the fossil was found. Along the Pine Ridge escarpment in Sioux County near the head of Warbonnet Creek, this matrix is typical of the Arikaree Group, and indicates gray volcaniclastic sandstone exposures of the middle and upper part of the escarpment. Peterson’s confusing attribution to ‘‘middle’’ Monroe Creek (on field labels) and later on, ‘‘upper’’ (in his published article), at least indicates that CM 1603 came from beds stratigraphically between the lower Arikaree fluvial sandstones at the base of the Pine Ridge and the eolian Harrison Formation sandstones forming the upper part of the escarpment, but prevents an exact stratigraphic designation.

The Pine Ridge escarpment immediately east of Warbonnet Creek includes the stratotypes for the Monroe Creek Formation and Harrison Formation of Hatcher (1902a) at Monroe Creek Canyon. Because Peterson did not attribute the fossil to the Harrison Formation, one can be reasonably confident that CM 1603 did not come from that rock unit as understood by Peterson and Hatcher (i.e., the upper, 200 ft of the Pine Ridge Arikaree escarpment). Hatcher in 1902 described the Monroe Creek Formation as ‘‘ 300 feet of very light-colored, fine-grained, not very hard, but firm and massive sandstones.’’ Based on Peterson’s published attribution to ‘‘upper Monroe Creek,’’ one would predict that CM 1603 came from fine-grained gray tuffaceous sandstones of the Pine Ridge escarpment, 200–400 ft below the terminal paleosol of the Harrison Formation ( Hunt, 1985). These sandstones exhibit large-scale eolian cross-strata, in which CM 1603 was possibly found, that constitute much of the middle part of the Pine Ridge stratigraphic section in Sioux County from Monroe Creek west to Warbonnet Creek. Fossil mammals are rarely encountered in these beds.

Previously I attributed the temnocyonine here named Delotrochanter petersoni to an early Arikareean chronofauna ( Hunt, 1985: 192) but Peterson’s (1907: 24) ‘‘upper Monroe Creek’’ allocation, which places CM 1603 in association with the oreodonts Promerycochoerus and Phenacocoelus , cannot be ruled out and suggests a younger age. It is at least certain D. petersoni occurs in a fauna older than that typical of the Harrison Formation of Peterson. This is supported by the size relationship of D. petersoni to its presumed descendant, D. oryktes . The latter species occurs in the Harrison Formation in the Niobrara River valley at Agate National Monument, and its predecessor, D. petersoni , is of the smaller size and dental features one would predict in ‘‘middle to upper Monroe Creek’’ sediments of the Pine Ridge.

Figures

Fig. 33. Holotype mandibles of Delotrochanter petersoni (CM 1603), with left canine, p1 and m1, and alveoli for p2–p4, from the Monroe Creek beds (fide Peterson, 1907), head of Warbonnet Creek, Sioux Co., Nebraska. A, labial view and B, high oblique lingual view of left mandible and the alveoli of right mandible. The m1 lacks a metaconid.