Palaeictops Matthew, 1899

Velazco, Paúl M. & Novacek, Michael J., 2016, Systematics of the genus Palaeictops Matthew, 1899 (Mammalia: Leptictidae), with the description of two new species from the Middle Eocene of Utah and Wyoming, American Museum Novitates 2016 (3867), pp. 1-42 : 4-7

Genus Palaeictops Matthew, 1899

Stypolophus Cope, 1880: 746 .

Parictops Granger, 1910: 250–251 .

GENOTYPIC SPECIES: Palaeictops bicuspis Cope, 1880 (described as Diacodon bicuspis by Matthew, 1918).

REFERRED SPECIES: Palaeictops borealis ( Russell, 1965) , P. bridgeri ( Simpson, 1959) , P.matthewi ( Novacek, 1977) , P. multicuspis ( Granger, 1910) , P. altimontis (new, this paper), and P. robustus (new, this paper).

DIAGNOSIS: Leptictine differing from other members of this subfamily (i.e., Leptictis and Megaleptictis ) in having single sagittal crest (known in Palaeictops bicuspis , P. altimontis , and P. robustus ); shallow suprameatal fossa (known in P. altimontis and P. bicuspis ); more transversely flared basioccipital that overlaps ventrally the promontorium of the petrosal (known in P. altimontis ); shallow groove on the paraoccipital process for the digastric muscle (known in P. altimontis and P. robustus ); a paraoccipital process that is less extensive, so that the distance is shorter between stylomastoid foramen and posterior margin of basicranium (known in P. altimontis and P. robustus ); and a small postglenoid process (known in P. altimontis and P. robustus ). Expanded cochlear fossula (in P. altimontis and P. robustus ). Lingually swollen protocones on P5 and M1–M3. Similar to Leptictis but different from Prodiacodon in having more bunodont cusps on posterior premolars and molars. Similar to Megaleptictis in having a small suprameatal foramen; lacking the posterior concavity in the coronoid process; and short talonid on p5. Differs from Prodiacodon in having slightly lower trigonids; well-developed hypocone on P5; shallow ectoflexi; elongate p5 with enlarged paraconid; less transverse M2; less developed parastylar spurs; and the presence of only one paraconule on the upper molars. Differs from Myrmecoboides in having well-separated paraconids and metaconids on p5–m3; less elongate, relatively wider talonids; and more closely spaced premolars. Pes in P. matthewi differs from that in all other leptictids where known in having a distinctly pear-shaped sustentacular facet on the astragalus and a very reduced fibular facet on the calcaneum.

DISTRIBUTION: Wind River, Bridger, Tepee Trail, Wasatch, and Willwood formations, Wyoming; DeBeque and Huerfano formations, Colorado; Uinta Formation, Utah; and Cypress Hills and Swift Current Creek formations, Saskatchewan, Canada. Lower–Middle Eocene (Wasat-chian–Duchesnean NALMAs).

REMARKS: The above diagnosis documents the presence of cranial and postcranial traits that may exclude Palaeictops from a Leptictis grouping. It is noteworthy, however, that the posteriorly expanded nasals of P. bicuspis are primitive, but not universal for this genus. In P. altimontis the nasals are posteriorly constricted in a manner similar to that in Leptictis . There is, in fact, clear evidence that Palaeictops is closely tied to the history of the Late Eocene–Oli-gocene taxa. Postcranial features of P. matthewi also support this close phylogenetic relationship (e.g., distal fusion of tibia and fibula, head of femur strongly canted to long axis of shaft, and deep trochanteric fossa of femur).

Although there are distinct differences between Palaeictops and Prodiacodon , some of the criteria demarcating these taxa in Novacek (1977) have been questioned by Bown and Schankler (1982). The latter authors offered the following comparisons:

(1) The two genera are not easily separated by the length-width dimensions of the upper molars (as used by Novacek, 1977), although Bown and Schankler noted (1982: 11) that “the upper molars of Paleocene Prodiacodon are more transverse than in early Eocene species of Palaeictops . ”

(2) Some referred specimens of Palaeictops bicuspis (e.g., Guthrie, 1971) have M2 ectoflexi that are intermediately deep between the type of P. bicuspis and the type of Prodiacodon tauricinerei.

(3) A large individual from the Lysite biostratigraphic zone of the Willwood Formation has a mosaic of characters seen in teeth of both Palaeictops and Prodiacodon .

(4) The conules of the few known upper molar specimens are positioned more lingually in Prodiacodon tauricinerei , as noted in Novacek (1977).

(5) The type specimens of three Palaeictops species ( Palaeictops bicuspis , P. matthewi , and P. multicuspis ) show heavy wear, which precludes accurate assessment of cusp height. The trigonids of Prodiacodon may therefore not be much higher than those of Palaeictops .

(6) p5 paraconids in Palaeictops are better developed than in Prodiacodon only because p5s of the former are larger and their anterior trigonids more elongate.

We address these remarks as follows:

(1) The quotation of Bown and Schankler (1982) given above supports the original demarcation of Palaeictops and Prodiacodon based on dimension of upper molars. Some variation in these dimensions among specimens and species is expected. The significant character in this context is the markedly more transverse, anteroposteriorly compressed M2 relative to M 1 in Prodiacodon , a feature definitely lacking in Palaeictops ( Novacek, 1986: fig. 4).

(2) Variation in the depth and configuration of the ectoflexi of M2 is expected. The types of Palaeictops bicuspis , P. matthewi , and P. altimontis are clearly distinguished in this character from the types of Prodiacodon tauricinerei and P. puercensis (M2s of P. crustulum were not definitely identified; see Novacek, 1977; and Clemens, 2015).

(3) New specimens with a mosaic of dental features found in both Palaeictops and Prodiacodon is expected. Perhaps the “large-toothed” specimen from Willwood is a new taxon.

(4) We concur with Bown and Schankler’s (1982) agreement with Novacek’s (1977) initial observation concerning the relative position of the upper molar conules in Palaeictops vis a vis Prodiacodon . We now emphasize, however, that the significant feature of the conules is the twinned paraconule, a feature of Prodiacodon definitely lacking in Palaeictops .

(5) We acknowledge that elevation of the trigonids is difficult to assess in the worn types of Palaeictops bicuspis , P. matthewi , and P. multicuspis . The trigonids are, however, definitely lower in the relatively less worn teeth of P. bridgeri and P. altimontis than in Prodiacodon ( Novacek, 1986: fig. 4). We agree with Bown and Schankler (1982) that the lower trigonids are possibly shared specializations developed by the Middle Eocene Palaeictops . The case is moot until better-preserved molars of Early Eocene Palaeictops are known.

(6) The greatest difference in p5 paraconid size is between the type series of Palaeictops ( P. bicuspis ) and Prodiacodon ( P. puercensis ). We agree with Bown and Schankler (1982) that Prodiacodon tauricinerei approaches Palaeictops (especially the Middle Eocene species) in development of the p5 paraconid. However, the difference in development is worthy of recognition. The size of the p5 paraconid in P. bicuspis refers to its relatively robust proportions; it is not simply a function of overall tooth size. Moreover, the relationships between a larger paraconid and a more elongate anterior portion of the trigonid are hardly surprising; this portion of tooth is occupied by little more than a paraconid.

These considerations lead us to retain the generic separation of Palaeictops and Prodiacodon . We acknowledge that Prodiacodon tauricinerei has features that resemble the dental morphology characteristic of Palaeictops more closely than other species of Prodiacodon . Prodiacodon tauricinerei is, however, clearly referable to Prodiacodon based on features of the molars (e.g., P5 and upper molars transverse, anteroposteriorly compressed with sharp cusps; precingula on P5 and molars long, nearly reaching anterolingual corner of crown; M1 and M2 paraconules twinned and paraconules lingual to metaconules; lower molars with sharp cusps, trigonids relatively high and anteroposteriorly compressed). These features, as well as a nearly contemporaneous age, exclude Prodiacodon tauricinerei as a possible ancestor of Palaeictops bicuspis or other species of this genus. This view runs contrary to the argument of Bown and Schankler (1982: 11) that P. tauricinerei , persisting into Lysitean time, may have given rise to a second lineage that ultimately led to Palaeictops .