Rosa Rubicondior: Refuting Creationism - A Fossil Mammal From 62-Million-Years Before 'Creation Week'

Mixodectes pungens (foreground), small mammals that inhabited western North America 62 million years ago, weighed about 3 pounds, dwelled in trees, and largely dined on leaves. They inhabited the same forests as early primates like Torrejonia wilsoni (background).

Illustration by Andrey Atuchin.

A 62-million-year-old skeleton sheds light on an enigmatic mammal | Yale News

Sometimes, I almost feel sorry for creationists struggling to cling to their childish counter-factual superstition in the face of this constant deluge of contrary scientific evidence. Then I realise they are architects of their own defeat by consciously and conscientiously ignoring this contrary evidence. Remaining ignorant and so remaining wrong is a choice that they don't have to choose just to maintain the self-delusion that they are more expert than the experts.

So, I have no conscience about reporting yet another example of incontrovertible evidence of evolution on an Earth which is much older than creationists prefer to believe. It comes in the form of a reassessment of a 65-million-year-old fossil of an early mammal from the same branch of the evolutionary tree as primate, so is a distant cousin on humans.

What can you tell me about the New Mexico’s San Juan Basin, how a fossil of Mixodectes pungens found there was dated. and how this species fits into the primate evolutionary tree? The San Juan Basin in New Mexico is a geologically significant area known for its rich fossil record, particularly from the late Cretaceous and early Paleogene periods. The region is famous for yielding fossils that provide insight into the post-dinosaur extinction recovery of mammals, including early primate relatives. The basin contains rock formations such as the Nacimiento Formation, which is key for studying early Palaeocene mammals.

Dating the Fossil of Mixodectes pungens

The fossil of Mixodectes pungens found in the San Juan Basin was likely dated using a combination of biostratigraphy, radiometric dating, and paleomagnetic data.

Biostratigraphy: The fossil was associated with other mammalian species from the same layer, helping correlate its age with known fossil assemblages.
Radiometric Dating: The basin's sediments contain volcanic ash layers, which allow for argon-argon (⁴⁰Ar/³⁹Ar) dating or potassium-argon (K-Ar) dating of the tuffs to establish an absolute date.
Paleomagentism: Magnetic reversals recorded in the rock layers were compared with the global geomagnetic polarity time scale to refine the dating.
Through these methods, fossils from the San Juan Basin, including Mixodectes pungens, are placed in the early Palaeocene epoch, approximately 62-65 million years ago.

How Mixodectes pungens Fits into Primate Evolution

Mixodectes pungens belongs to the extinct order Mixodectidae, which is part of the larger group Euarchonta-a clade that also includes primates, tree shrews (Scandentia), and colugos (Dermoptera). While mixodectids are not direct ancestors of primates, they share several key characteristics that link them to early primate evolution:

Specialized Teeth: Mixodectids had enlarged incisors, similar to rodents, and were likely adapted for omnivory or frugivory—suggesting a dietary shift seen in some early primates.
Arboreal Adaptations: Their skeletal remains suggest they were at least semi-arboreal, an important trait in primate evolution.
Close Relationship to Plesiadapiforms: Mixodectids are often considered close relatives of plesiadapiforms, which are widely accepted as early primate relatives. Plesiadapiforms, in turn, are considered to be on the evolutionary path toward true primates (Euprimates).
Thus, while Mixodectes pungens itself is not a primate, it represents an early side branch of the Euarchontan radiation, showcasing traits that foreshadow primate evolution. It highlights the diversity of mammals that thrived after the Cretaceous–Paleogene (K-Pg) extinction and provides insight into the ecological and anatomical precursors that would eventually give rise to primates.

The reassessment of this fossil by a team of scientists including co-author Professor Eric J. Sargis, a Yale anthropologist, is published, open access, in Scientific Reports and explained in an article in Yale News by Mike Cummings.

A 62-million-year-old skeleton sheds light on an enigmatic mammal
A remarkably well-preserved skeleton of Mixodectes pungens offers insights into mammals’ evolutionary trajectory after non-avian dinosaur extinction.
For more than 140 years, Mixodectes pungens, a species of small mammal that inhabited western North America in the early Paleocene, was a mystery. What little was known about them had been mostly gleaned from analyzing fossilized teeth and jawbone fragments.

But a new study of the most complete skeleton of the species known to exist has answered many questions about the enigmatic critter — first described in 1883 by famed paleontologist Edward Drinker Cope — providing a better understanding of its anatomy, behavior, diet, and position in the Tree of Life.

The study, co-authored by Yale anthropologist Eric Sargis, demonstrates that the mature adult Mixodectes weighed about 3 pounds, dwelled in trees, and largely dined on leaves. It also shows that these arboreal mammals — an extinct family known as mixodectids — and humans occupy relatively close branches on the evolutionary tree.

A 62-million-year-old skeleton of this quality and completeness offers novel insights into mixodectids, including a much clearer picture of their evolutionary relationships. Our findings show that they are close relatives of primates and colugos — flying lemurs native to Southeast Asia — making them fairly close relatives of humans.

Professor Eric J. Sargis, co-author Department of Anthropology
Yale University, New Haven, CT, USA.

The study was published on March 11 in the journal Scientific Reports. Stephen Chester, associate professor of anthropology at Brooklyn College, City University of New York, is its lead author.

The skeleton was collected in New Mexico’s San Juan Basin by co-author Thomas Williamson, curator of paleontology at the New Mexico Museum of Natural History & Science, under a permit from the federal Bureau of Land Management. It includes a partial skull with teeth, spinal column, rib cage, forelimbs, and hind limbs.

The skeleton, discovered in New Mexico’s San Juan Basin, is the most complete of its kind known to exist.

The researchers determined that the skeleton belonged to a mature adult that weighed about 1.3 kilograms, or 2.9 pounds. The anatomy of the animal’s limbs and claws indicate that it was arboreal and capable of vertically clinging to tree trunks and branches. Its molar teeth had crests to break down abrasive material, suggesting it was omnivorous and primarily ate leaves, the study showed.

This fossil skeleton provides new evidence concerning how placental mammals diversified ecologically following the extinction of the dinosaurs. Characteristics such as a larger body mass and an increased reliance on leaves allowed Mixodectes to thrive in the same trees likely shared with other early primate relatives.

Associate professor, Stephen B. Chester, led author
Department of Anthropology
Brooklyn College, City University of New York, Brooklyn, NY, USA.

Mixodectes was quite large for a tree-dwelling mammal in North America during the early Paleocene — the geological epoch that followed the Cretaceous-Paleogene extinction event that killed off non-avian dinosaurs 66 million years ago, the researchers noted.

For example, the Mixodectes skeleton is significantly larger than a partial skeleton of Torrejonia wilsoni, a small arboreal mammal from an extinct group of primates called plesiadapiforms, that was discovered alongside it. While Mixodectes subsisted on leaves, Torrejonia’s diet mostly consisted of fruit. These distinctions in size and diet suggest that mixodectids occupied a unique ecological niche in the early Paleocene that distinguished them from their tree-dwelling contemporaries, the researchers said.

Two phylogenetic analyses performed to clarify the species’ evolutionary relationships confirmed that mixodectids were euarchontans, a group of mammals that consists of treeshrews, primates, and colugos. While one analysis supported that they were archaic primates, the other did not. However, the latter analysis verified that mixodectids are primatomorphans, a group within Euarchonta composed of primates and colugos, but not treeshrews, Sargis explained.

While the study doesn’t entirely resolve the debate over where mixodectids belong on the evolutionary tree, it significantly narrows it.
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Professor Eric J. Sargis.
The paper’s co-authors are Jordan Crowell of The Graduate Center at the City University of New York, Mary Silcox of the University of Toronto Scarborough, and Jonathan Bloch of the Florida Museum of Natural History at the University of Florida.

Abstract
Mixodectids are poorly understood placental mammals from the Paleocene of western North America that have variably been considered close relatives of euarchontan mammals (primates, dermopterans, and scandentians) with hypothesized relationships to colugos, extinct plagiomenids, and/or microsyopid plesiadapiforms. Here we describe the most complete dentally associated skeleton yet recovered for a mixodectid, specifically Mixodectes pungens from the early Paleocene of the San Juan Basin, New Mexico. A partial skull with all the teeth erupted and associated axial skeleton, forelimbs, and hind limbs, with epiphyses fused, indicate that it was a mature adult. Results from cladistic analyses incorporating new data robustly support primatomorphan (Primates + Dermoptera) affinities of Mixodectidae, but relationships within Euarchonta are less clear, with Mixodectes recovered as a stem primatomorphan, stem dermopteran, or stem primate. Analyses of postcrania suggest that M. pungens was a relatively large (~ 1.3 kg), claw-climbing arborealist capable of frequent clinging on large diameter vertical supports. With teeth suggesting an omnivorous diet that included leaves, M. pungens occupied a unique ecological niche in the early Paleocene of North America that differed from contemporary, arboreal plesiadapiforms that were smaller and more frugivorous. Euarchontans were thus a more diverse radiation in the early Cenozoic than previously appreciated.

Introduction
Mixodectids are a poorly defined group of enigmatic placental mammals from the Paleocene of western North America whose evolutionary relationships have long been hard to resolve due to their sparse fossil record that consists mostly of dental and gnathic remains^1,2. Taxa classified as mixodectids have been linked to extant mammalian groups including Primates¹, Rodentia³, “Insectivora”^4,5, and Dermoptera⁶. The family Mixodectidae includes two species of Mixodectes from the Torrejonian North American Land Mammal Age (NALMA) of New Mexico (and possibly Wyoming); the monotypic Dracontolestes from the Torrejonian NALMA of Utah; and three species of Eudaemonema from the Torrejonian and Tiffanian NALMAs of Wyoming, Montana, and Alberta^7,8. Similarities between the dentition of mixodectids and those of other extinct Paleogene families such as the Plagiomenidae and Microsyopidae have been recognized and debated for over a century^2,3,4,5,9,10. These extinct families have also often been hypothesized to have affinities with one or more clades of euarchontan mammals (primates, colugos, treeshrews), but fossil evidence to test these hypotheses has been mostly limited to craniodental characters [e.g., ¹¹]. Although fragmentary postcrania previously attributed to early Paleocene Mixodectes show similarities to those of other arboreal euarchontans^6,12,13, more complete mixodectid specimens are needed to assess their significance regarding the early evolution of this group.

Here we describe a new partial skeleton of Mixodectes pungens (NMMNH P-54501; Fig. 1) recovered from the West Flank of Torreon Wash (locality NMMNH L-6898) within the Ojo Encino Member [sensu ¹⁴], Nacimiento Formation, San Juan Basin, New Mexico (see Supplementary Fig. S1 online). NMMNH L-6898 is stratigraphically located near the middle of the Tj6 fossil horizon [sensu ¹⁵]. The fauna from the Tj6 fossil zone is the primary source among faunas that define the early Paleocene late Torrejonian (To3) NALMA Mixodectes pungens interval zone¹⁶. The age of NMMNH L-6898 is estimated to be 62.4 ± 0.03 Ma based on average sediment accumulation rates between the upper and lower reversal boundaries of a normal polarity zone correlated with Chron C27n¹⁷. A detrital sanidine age of 62.48 ± 0.02 from within this normal polarity zone from the nearby Escavada Wash provides an additional age constraint for this chron¹⁷. See^18,19,20 for additional information on this locality and other fossil vertebrates collected at this site.

The new dentally associated adult skeleton of M. pungens (NMMNH P-54501) is the most complete specimen of a mixodectid yet recovered, preserving a partial skull and much of the axial skeleton, forelimbs, and hind limbs (Fig. 1). Partial disarticulated skeletons of the palaechthonid plesiadapiform Torrejonia^18,19 and the cimolestid Acmeodon were recovered from the same horizon, but they are easily distinguished based on their smaller size alone. No repeated tooth loci or skeletal elements have been recognized, which suggests that only one individual of M. pungens was present. Previously described fragmentary postcrania attributed to a smaller species, M. malaris, were mixed with similar-sized postcrania of the arctocyonid archaic ungulate Chriacus orthogonius from the Torrejonian “Pantolambda zone” (Tj6 fossil horizon sensu¹⁵) in the San Juan Basin⁶. The new partial skeleton of M. pungens analysed here could be distinguished from elements attributed to the smaller M. malaris^2,6 by its larger size. Here, we describe the best-preserved skeletal elements of M. pungens and discuss the functional implications of this newly documented morphology. We also compared the new M. pungens skeleton to those of other placental mammals including plesiadapiforms, euprimates, treeshrews, and colugos, and we coded it into two morphological data matrices to assess its phylogenetic relationships among eutherian mammals (Crowell et al.²¹ modified from Wible et al.²², and Chester et al.¹⁸ modified from Silcox et al.²³).

Fig. 2.

Modified from Chester et al.¹⁹ figs. 7, 10 and 12. See Supplementary Information for institutional abbreviations.

Photographs of (a) Mixodectes pungens (NMMNH P-54501) postcrania compared to renderings of 3D virtual models based on microCT scan data of (b) paromomyid plesiadapiform Ignacius clarkforkensis (humerus, UM 108210; innominate and femur, UM 82606), (c) euprimate Galagoides demidoff (AMNH M-269904), (d) colugo Cynocephalus volans (ANSP 24797), (e) arboreal treeshrew Ptilocercus lowii (MCZ 51736), and (f) terrestrial treeshrew Tupaia gracilis (FMNH 140928). Rows from top to bottom compare right distal humeri in ventral view scaled to width of distal end, right innominates (Mixodectes reversed) in lateral view scaled to craniocaudal length of acetabulum, left proximal femora in ventral (above) and distal femora in distal (below) views scaled to width of distal end. Scale bars = 1 mm. Some elements reversed to facilitate comparisons. asb, acetabular cranial articular surface; brf, brachioradialis flange; cp, capitulum; fh, femoral head; gt, greater trochanter; is, anterior inferior iliac spine; lc, lateral condyle; lt, lesser trochanter; mc, medial condyle; me, medial epicondyle; pg, patellar groove; rf, radial fossa; tc, trochlea; tt, third trochanter; zc, zona conoidea.

Fig. 3.

Photographs of (a) left astragalus (reversed) and right calcaneum of Mixodectes pungens (NMMNH P-54501) compared to renderings of 3D virtual models based on microCT scan data of (b) archaic ungulate cf. Protungulatum (AMNH FM-118260, FM-118060), (c) purgatoriid plesiadapiform cf. Purgatorius (UCMP 197509, 197517), (d) paromomyid plesiadapiform cf. Ignacius (USNM 442235, 442240), (e) plesiadapid plesiadapiform Plesiadapis cookei (UM 87990), (f) adapiform euprimate Notharctus tenebrosus (AMNH FM-11474) (g) colugo Cynocephalus volans (UNSM 15502, AMNH M-207001), and (h) arboreal treeshrew Ptilocercus lowii (USNM 488072). Right astragali (top three rows) and calcanei (bottom three rows) illustrated in dorsal (top), plantar (middle), and distal (bottom) views. aef, astragalar ectal facet; asf, astragalar sustentacular facet; cef, calcaneal ectal facet; cf, calcaneocuboid facet; csf, calcaneal sustentacular facet; ltf, lateral tibial facet; pt, peroneal tubercle. Specimens scaled to proximodistal length. Scale bars for astragali (top) and calcanei (bottom) all 1 mm. Some elements reversed to facilitate comparisons. Figure modified from Chester et al.¹⁹ fig. 14. See Supplementary Information for institutional abbreviations.

Fig. 4.

Hypotheses of evolutionary relationships of Mixodectes pungens and other eutherian mammals with artist’s reconstruction of Mixodectes pungens by Andrey Atuchin. (a) Simplified resulting constrained strict consensus cladogram based on modified morphological dataset of Crowell et al.²¹ with Euarchonta indicated in blue and Mixodectes supported as a stem primatomorphan and indicated in orange. (b) Simplified resulting strict consensus cladogram based on modified morphological dataset of Chester et al.¹⁸ with Euarchonta indicated in blue and Mixodectes supported as a stem primate and indicated in orange. See Supplementary Information for detailed methods, descriptions of morphological characters, specimens examined, and the taxon-character matrices in TNT format.

Creationism persists by appealing to intellectually lazy and arrogant individuals who, without bothering to learn any real biology, believe that a 15-minute crash course from 'YouTube University' makes them more knowledgeable than actual experts. They then perpetuate their superstition by flaunting their ignorance on social media and retreating into controlled echo chambers, where they desperately avoid engaging with counterarguments or factual rebuttals—such as this piece of research.

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