Apart from some feathered dinosaurs that were destined to become birds, one of the few land vertebrates to survive the catastrophe that wiped out the remaining large dinosaurs and marine reptiles at the K-Pg boundary, was to go one to give rise to the entire mammalian order. They were small fury, probably nocturnal insectivores.
The fossil of one of these, from 65 million years ago (at about the time of the K-Pg mass extinction), has recently been discovered by Denver Museum of Nature scientists in Colorado, at Corral Bluffs, near Colorado Springs.
According to a press release from Denver Museum of Nature & Science: Denver Museum of Nature & Science researchers are excited to announce the fossil discovery of a species of a 65-million-year-old mammal that was collected in the Corral Bluffs area on the edge of Colorado Springs. The newly discovered species, named Militocodon lydae, is part of a group of animals that gave rise to all modern hoofed mammals, including deer, cows and pigs. The fossil skull and jaws of Militocodon lydae were uncovered from rocks dating back to just after the extinction of the dinosaurs. Roughly the size of a chinchilla, Militocodon lydae provides important clues about the explosive diversification of mammals in the wake of the dinosaur extinction.
Rocks from this interval of time have a notoriously poor fossil record and the discovery and description of a fossil mammal skull is an important step forward in documenting the earliest diversification of mammals after Earth’s last mass extinction.
Dr. Tyler Lyson, co-author
Museum Curator of Vertebrate Paleontology
Department of Earth Sciences
Denver Museum of Nature & Science, Denver, CO, USA.
Museum Curator of Vertebrate Paleontology
Department of Earth Sciences
Denver Museum of Nature & Science, Denver, CO, USA.
AbstractThe thing about these stem species is that they could fit into any of their descendent taxons, so, although science can assign appropriate generic and specific names to them, no creationist can fit them into any of their 'kinds' - the primitive taxonomy used by people who knew no better and didn't realise how all things are related and fit into nested hierarchies with species like this stem mammal at the base of a branching tree.
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.
Introduction
Archaic ungulates (i.e., ‘condylarths’) are a taxonomically contentious group that likely gave rise to extant hooved mammals (Archibald 1998) and whose initial diversification epitomizes the rise of eutherians in the early aftermath of the Cretaceous–Paleogene (K–Pg) mass extinction in North America (e.g., Simpson 1937a; Archibald 1982; Eberle and Lillegraven 1998.1; Alroy 1999; Clemens 2002; Wilson 2014; Lyson et al. 2019). Archaic ungulates first appear in the lowermost Paleocene (Danian), defining the onset of the Puercan North American Land Mammal ‘age’ (NALMA; Lofgren et al. 2004; but see Johnston and Fox 1984, Archibald et al. 2011, and Kelly 2014.1 for putative records of latest Cretaceous archaic ungulates), and within less than 300 thousand years (ka) after the K–Pg boundary (e.g., Wilson 2014; Smith et al. 2018; Wilson Mantilla et al. 2021; Claytor et al. 2022) they became the most species-rich group of eutherian mammals (Fig. 1).The rapid diversification of early archaic ungulates was driven in large part by the Periptychidae, which accounted for nearly half of archaic ungulate species richness in the Puercan (Fig. 1). Periptychids are characterized by bulbous, bunodont cheek teeth with enlarged posterior premolars, lingually sloping molar protocones and hypocones, and closely appressed molar cusps (Matthew 1937.1; Simpson 1937.2b; Archibald 1998). The phylogenetic position of periptychids among Placentalia is uncertain, but recent analyses have suggested that they are nested among Laurasiatheria (e.g., Zack 2009). Given their dental apomorphies, the monophyly and generic membership of the Periptychidae has remained reasonably stable relative to other, more dentally plesiomorphic or conserved archaic ungulate groups such as the Arctocyonidae and Hyopsodontidae that are less well-defined. Nonetheless, the generic and specific constituents of the periptychid subgroups—Periptychinae, Anisonchinae, Conacodontinae—have fluctuated substantially (e.g., Archibald et al. 1983a, 1983.1b), with some likely being paraphyletic (e.g., Anisonchinae; Archibald 1993, 1998; Shelley 2018.2). Further, recent phylogenetic hypotheses propose that Mimatuta and Maiorana, two genera traditionally considered basal periptychids (Van Valen 1978; Archibald 1998), do not belong in the Periptychidae (Shelley 2018.2; Atteberry and Eberle 2021.1), raising questions about periptychid character polarity and evolutionary trends.Fig. 1Raw species richness of eutherian mammal families from the Puercan North American Land Mammal ‘age’ (NALMA), subdivided into Puercan (Pu) 1, 2, and 3 subintervals, capturing roughly the first one million years of the Paleocene. Note the high species richness of periptychids relative to all other eutherian families. Species richness data were compiled from Lofgren et al. (2004), Silviria (2019.1), Atteberry and Eberle (2021.1), and Wilson Mantilla et al. (2021). Note that the white portion between Purgatoriidae and Taeniodonta represents the putative carnivoran Ravenictis krausei, Arctocyonidae includes taxa (e.g., Carcinodon) that were previously considered oxyclaenids (see Williamson and Carr 2007), and taxa included in the Mioclaenidae and Apheliscidae have been variously referred to the Hyopsodontidae (see Zack et al. 2005 and Williamson and Weil 2011.1). Duration of the Puercan NALMA is incompletely resolved and not taxonomically or temporally homogeneous across different basins, but see Sprain et al. (2018.1), Lyson et al. (2019), and Flynn et al. (2020) for the most recent temporal constraints in the Williston, Denver, and San Juan basins, respectively
Like for most early Paleocene mammals, the uncertainties surrounding the systematics of the Periptychidae are due in part to their fossil record, comprised mostly of isolated teeth and jaw fragments, with associated upper and lower dentitions and more complete skeletal fossils being rare. Indeed, relatively complete periptychid cranial remains are only known from the periptychines Periptychus, Ectoconus, and Carsioptychus (e.g., Matthew 1937.1; Shelley et al. 2018.3; Lyson et al. 2019), the anisonchine Haploconus (Matthew 1937.1), and the putative basal periptychid Maiorana (reported but undescribed; Van Valen 1978). Most of these taxa are morphologically derived, large-bodied, and geologically young members of the clade (e.g., Matthew 1937.1; Shelley 2018.2) and have only been cursorily described. The cranial morphology of the Conacodontinae remains unknown. As such, what cranial apomorphies characterize the Periptychidae and when in geologic time those cranial morphologies arose remains unclear.
Here we describe the partial skull and additional dentary fragment of a small-bodied periptychid (273–455 g; see ‘Materials and methods’ below) from the Puercan 2 (Pu2) of the Corral Bluffs area in the Denver Basin, Colorado (Fig. 2). To date, Corral Bluffs has yielded hundreds of exceptionally preserved fossil vertebrates, including nearly 100 mammalian fossils consisting of skulls, jaws, and postcrania (Lyson et al. 2019; Krause et al. 2021.2). Based on these new fossils we erect Militocodon lydae gen. et sp. nov., which we interpret as a basal conacodontine. The craniodental morphology of M. lydae sheds new light on the early evolutionary history of the Periptychidae and highlights the importance of the Corral Bluffs mammalian fossil record for understanding the rise of eutherian mammals in the aftermath of the K–Pg mass extinction.Institutional abbreviations: AMNH, American Museum of Natural History, New York, New York, USA. DMNH, Denver Museum of Nature & Science, Denver, Colorado, USA. MNHN.F, Fossil cranium collections at the Muséum national d’Histoire naturelle, Paris, France. NMMNH P, Paleontological collections at the New Mexico Museum of Natural History & Science, Albuquerque, New Mexico, USA. UCM, University of Colorado Museum of Natural History, Boulder, Colorado, USA. UCMP, University of California Museum of Paleontology, Berkeley, California, USA. YPM-PU, Princeton University Collection at the Yale Peabody Museum, New Haven, Connecticut, USA.Fig. 2Corral Bluffs study area and chronostratigraphic location of the fossils described here. a. North America; b. Colorado; c. Denver Basin and coarse geologic map of exposures of the D1 sequence of the Denver Formation, which spans the uppermost Cretaceous and lowermost Paleocene. Corral Bluffs is represented by the blue star just east of Colorado Springs. d. Chronostratigraphic framework and distribution of plant and vertebrate fossil localities at Corral Bluffs (see text and references therein). DMNH EPV.136181 and 103390 (yellow star) were found 97–101 m above the K–Pg boundary (denoted by red line), corresponding to an approximate age of 610 ka after the K–Pg mass extinction. Abbreviations: ka, thousands of years; km, kilometer; Ma, millions of years
Geologic setting Corral Bluffs is located east of Colorado Springs, Colorado, USA, and is situated within the Denver Basin (Fig. 2c). The south-facing arc of exposures that make up Corral Bluffs are part of the D1 sequence of the Denver Formation, which spans the uppermost Cretaceous through lower Paleocene (Fig. 2d; Raynolds 2002.1). Corral Bluffs is located near the Rocky Mountain Front Range and exposures of the D1 sequence at Corral Bluffs are much coarser grained than those in the eastern, mountain-distal portions of the Denver Basin which preserve comparably aged sediments (Barclay et al. 2003). Corral Bluffs outcrops are characterized by stratigraphically thick (~10 m), laterally persistent > 100 m), medium-to-coarse-grained trough cross-bedded fluvial-channel sandstones interbedded with siltstones or silty-to-sandy claystones (Raynolds 1997, 2002.1; Lyson et al. 2019).
Corral Bluffs chronostratigraphy is anchored by a combination of mammalian biostratigraphy (Brown 1943; Eberle 2003.1; Lyson et al. 2019), palynostratigraphy (Lyson et al. 2019), magnetostratigraphy (Hicks et al. 2003.2; Fuentes et al. 2019.2), the Geomagnetic Polarity Time Scale (GPTS; Gradstein et al. 2012), and radiometric dating (Fuentes et al. 2019.2; Lyson et al. 2019) (Fig. 2d). The K–Pg boundary has also been identified in the eastern portions of the Denver Basin via an iridium anomaly, shocked minerals, and palynostratigraphy (Barclay et al. 2003; Nichols and Fleming 2002.2) and at Corral Bluffs via palynostratigraphy (Fuentes et al. 2019.2). This high-resolution chronostratigraphic framework at Corral Bluffs allows us to infer a reasonably precise age of the specimens described here—DMNH EPV.136181 and DMNH EPV.103990—relative to the K–Pg boundary.
DMNH EPV.136181 consists of a laterally compressed cranium with both dentaries in occlusion and was preserved in a small phosphatic concretion, a mode of preservation that is typical of the exceptional vertebrate fossil remains at Corral Bluffs (Lyson et al. 2019). This specimen was not discovered in-situ, but it was collected on a flat with only ~4 m of outcrop above it (DMNH Loc. 18898). Thus, the stratigraphic position of DMNH EPV.136181 is between 97 and 101 m above the K–Pg boundary, yielding an estimated age of ca. 65.43 Ma, approximately 610 ka after the K–Pg boundary (Fig. 2; Gradstein et al. 2012; Lyson et al. 2019). Mammalian biostratigraphy at Corral Bluffs further indicates that this specimen falls within the upper portions of the Puercan 2 (Pu2) NALMA (Eberle 2003.1; Lyson et al. 2019).
A second specimen, DMNH EPV.103990 (previously identified as Oxyacodon sp.; Lyson et al. 2019), consists of a partial right dentary preserving a portion of m1 and nearly complete m2–3. DMNH EPV.103990 was not preserved in a phosphatic concretion, but rather was recovered from a vertebrate microfossil bonebed (sensu Rogers and Brady 2010; DMNH Loc. 6284), which has also yielded other mammalian (Loxolophus sp.), reptilian (Hoplochelys, Compsemys, baenid and trionychid turtles, Champsosaurus, and crocodilians), and gar fish fossils. DMNH Loc. 6284 is 97 m above the K–Pg boundary, yielding an estimated age of ca. 65.43 Ma, approximately 610 ka after the K–Pg boundary (Fig. 2; Gradstein et al. 2012; Lyson et al. 2019). Finally, mammalian biostratigraphy at Corral Bluffs indicates that this specimen falls within the late Pu2 NALMA (Lyson et al. 2019).
Weaver, L.N., Crowell, J.W., Chester, S.G.B. et al. Skull of a new periptychid mammal from the lower Paleocene Denver Formation of Colorado (Corral Bluffs, El Paso County). J Mammal Evol 31, 16 (2024). https://doi.org/10.1007/s10914-024-09716-5
Copyright: © 2024 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
All they knew was what they saw and none of them lived long enough, or even knew what to look for, so had never seen species evolving and diversifying. Like modern creationists, they remained ignorant of the forces that had been shaping the planet for millions of years and the processes that had been creating the biodiversity around them, and assumed it was all done with magic, just a few generations earlier, because that was all the history their collective folk memories and oral traditions could recall.
The fact that this extinct species lived 65 million years before the Bible's authors thought the Universe was created out of nothing by magic, should be a big enough clue that they got it hopelessly wrong.
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