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| Militocodon lydae Weaver, Crowell, Chester & Lyson, 2024 Artwork by Andrey Atuchin |
Abstract
The Periptychidae, an extinct group of archaic ungulates (‘condylarths’), were the most speciose eutherian mammals in the earliest Paleocene of North America, epitomizing mammalian ascendency after the Cretaceous–Paleogene (K–Pg) mass extinction. Although periptychids are mostly known from fragmentary gnathic remains, the Corral Bluffs area within the Denver Basin, Colorado, has yielded numerous exceptionally well-preserved mammalian fossils, including periptychids, from the earliest Paleocene. Here we describe a partial cranium and articulated dentaries plus an additional unassociated dentary fragment of a small-bodied (~273–455 g) periptychid from ca. 610 thousand years after the K–Pg mass extinction (Puercan 2 North American Land Mammal ‘age’) at Corral Bluffs. Based on these new fossils we erect Militocodon lydae gen. et sp. nov. The dentition of M. lydae exhibits synapomorphies that diagnose the Conacodontinae, but it is plesiomorphic relative to Oxyacodon, resembling putatively basal periptychids like Mimatuta and Maiorana in several dental traits. As such, we interpret M. lydae as a basal conacodontine. Its skull anatomy does not reveal clear periptychid synapomorphies and instead resembles that of arctocyonids and other primitive eutherians. M. lydae falls along a dental morphocline from basal periptychids to derived conacodontines, which we hypothesize reflects a progressive, novel modification of the hypocone to enhance orthal shearing and crushing rather than grinding mastication. The discovery and thorough descriptions and comparisons of the partial M. lydae skull represent an important step toward unraveling the complex evolutionary history of periptychid mammals.
Keywords: Archaic ungulates, Condylarths, Eutherians, Periptychidae, Puercan
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| Holotype of Militocodon lydae gen. et sp. nov. (DMNH EPV.136181) in right (a) and left (b) lateral views. Scale bar equals 2 cm |
MAMMALIA Linnaeus, 1758
EUTHERIA Gill, 1872
?UNGULATA Linnaeus, 1758
PERIPTYCHIDAE Cope, 1882
CONACODONTINAE Archibald, Schoch, and Rigby, 1983
Militocodon gen. nov.
Etymology. In honor of Sharon Milito, for her dedication to paleontology and education in the Denver Basin and for finding referred specimen DMNH EPV.103390.
Distribution. Puercan 2 of the Denver Formation, D1 Sequence at Corral Bluffs (El Paso County, Colorado).
Diagnosis. Resembling the Conacodontinae (sensu Archibald et al. 1983b) in: M1–3 hypocone large and lingually expanded; protocone absent on P3. Resembling Oxyacodon (sensu Archibald et al. 1983a) in: premolars slightly inflated but P/p4 shorter or subequal in length to M/m1; para- and metastylar lobes on M1–3 more buccally expanded than in Alticonus, Ampliconus, Miniconus, Tinuviel, Anisonchus, Conacodon, Haploconus, and Hemithlaeus; metacingulum continuous with metastylar region; paraconule absent with postparaconule wing meeting preprotocrista near apex of protocone; hypocone expanded lingually beyond the protocone; paraconid on p4 small, situated near the base of the crown; narrow buccal cingulid present on lower molars and lingual cingulid absent on lower premolars and molars. Differing from Oxyacodon in: greater expansion of para- and metastylar lobes on P4, with the former also projecting farther mesially; hypocone lingual face more vertically oriented on M2–3, not sloping prominently lingually; hypocone apex distinctly distal to protocone apex, especially on M2, resulting in a more rectangular (rather than triangular) occlusal outline; paraconid on m1–3 not closely appressed to metaconid, resulting in a mesiolingually open trigonid basin; trigonid taller relative to talonid, due in part to a taller protoconid.
Militocodon lydae sp. nov.
Etymology. In honor of Lyda Hill, a longtime champion of Colorado Springs and key supporter of post-K–Pg recovery research at the Denver Museum of Nature & Science.
Holotype. DMNH EPV.136181, partial skull, including posterior maxillae and dentaries, RP/p4–M/m3 and LP/p3–M/m3, and partial neuro- and basicranium.
Lucas N. Weaver, Jordan W. Crowell, Stephen G. B. Chester and Tyler R. Lyson. 2024. Skull of A New periptychid Mammal from the lower Paleocene Denver Formation of Colorado (Corral Bluffs, El Paso County). Journal of Mammalian Evolution. 31, 16. DOI: 10.1007/s10914-024-09716-5
