Figure 1. Three-dimensional models derived from µCT scan data of Zanycteris paleocenus (AMNH 17180) with descriptions and orientations of skeletal elements organized from left to right. (a) Cranium in dorsal and ventral views with basicranial region transparent showing original position of digitally extracted bones. Box illustrates left upper molars rotated into occlusal view with distolingual basin (asterisk) and postprotocingulum (arrow). (b) Three-dimensional model of right petrosal in ventral view, composite line drawing incorporating preserved morphology from right and left petrosal, and composite line drawing with major neurovascular structures; (c) R partial ectotympanic in oblique posterior and ventral view; (d) R partial alisphenoid/basisphenoid in lateral and ventral view. Scale bars, 1 mm. Abbreviations: C = upper canine; C a = upper canine alveolus; M = upper molar; P a = upper premolar alveolus.
The only certainty in science is that there are no certainties in science.
Unlike religions which provide unreasonable certainty in the absence of evidence and call it 'faith', science provides reasonable uncertainty and provisional opinion pending further evidence.
This was illustrated a couple of days ago by a paper published in Royal Society Biology Letters, which describes the reclassification of a middle Palaeocene placental mammal, known to science as a picrodontid, and known only from a partial cranium, some teeth and pieces of jawbone. This was discovered about 100 years ago and 50 years ago was classified as an early primate, in other words, close to the remote placental common ancestor of all the apes, monkeys, lemurs, tree shrews, etc.
Just to remind creationists: the middle Palaeocene lasted from about 66 million years before the mythical 'Creation Week' to 56 million years before the mythical creation of the Universe. It began a few million years after the non-avian dinosaurs all went extinct.
When was the Middle Palaeocene in North America and what charcterised it? The Middle Paleocene epoch lasted from approximately 66 to 56 million years ago. It followed the Paleocene-Eocene Thermal Maximum (PETM) event, a period of significant global warming. The Middle Paleocene in North America is associated with the transition from the early Paleocene to the later part of the epoch.But now, two researchers, PhD candidate Jordan Crowell and Associate Professor of Anthropology, Stephen Chester, from the Department of Anthropology, The Graduate Center, City University of New York, New York, NY, USA together with John R. Wible of the Section of Mammals, Carnegie Museum of Natural History, Pittsburgh, PA, USA, have reexamined the fossils using modern CT scan technology and found that the cranial bones that surround the ears are unlike homologous bones found in primates or close fossil relatives of primates. They have concluded therefore that picrodontid are not primates but are representative of an extinct group of early mammals. Any similarities are the result of convergent evolution in these mouse-sized mammals.
During the Middle Paleocene in North America, the climate was relatively warm, and the continents continued to shift and take on more modern configurations. The ecosystem was characterized by the recovery of plant and animal life after the disruptions caused by the PETM. Mammals, in particular, began to diversify and adapt to various ecological niches during this time. The fossil record from the Middle Paleocene in North America includes evidence of various mammalian species, including primitive forms of early primates and other mammals.
It's important to note that specific details and characteristics can vary across regions, and the information available is based on the fossil record and scientific research up to my knowledge cutoff date in January 2022. For the most recent and specific information, I recommend consulting the latest research in paleontology.
More technical details are provided in the published paper:
AbstractNothing for creationists to get over-excited by when they hear science may have made a mistake in a detail of the evolution of primates, because it makes no difference at all to the overall picture of mammalian evolution, and it all happened about 60 million years before 'Creation Week'. As I've said many times before, creationism isn't a problem for science; science is a problem for creationism, and this just adds to their problem.
The Picrodontidae from the middle Palaeocene of North America are enigmatic placental mammals that were allied with various mammalian groups but are generally now considered to have close affinities to paromomyid and palaechthonid plesiadapiforms based on proposed dental synapomorphies. The picrodontid fossil record consists entirely of dental and gnathic remains except for one partial cranium of Zanycteris paleocenus (AMNH 17180). Here, we use µCT technology to unveil previously undocumented morphology in AMNH 17180, describe and compare the basicranial morphology of a picrodontid for the first time, and incorporate these new data into cladistic analyses. The basicranial morphology of Z. paleocenus is distinct from plesiadapiforms and shares similarities with the Palaeogene Apatemyidae and Nyctitheriidae. Results of cladistic analyses incorporating these novel data suggest picrodontids are not stem primates nor euarchontan mammals and that the proposed dental synapomorphies uniting picrodontids with plesiadapiforms and, by extension, primates evolved independently. Results highlight the need to scrutinize proposed synapomorphies of highly autapomorphic taxa with limited fossil records.
1. Background
Plesiadapiforms are a likely non-monophyletic group of placental mammals known from the Palaeocene and Eocene of North America, Europe, and Asia that have been regarded as close fossil relatives of crown clade primates based mostly on aspects of dental morphology [1–5]. Results of recent phylogenetic analyses support plesiadapiforms as euarchontan mammals and either support members of this group as stem primates [6–10], stem colugos or stem primatomorphans (Primates + Dermoptera) [11,12]. The Picrodontidae consists of three genera (Picrodus, Zanycteris, and Draconodus) known from the Torrejonian and Tiffanian North American Land Mammal Ages (NALMA) of western North America [13–15]. Due to the autapomorphic dental morphology of picrodontids and the lack of an obvious sister group, researchers have allied picrodontids with caenolestid marsupials [16], chiropterans [17], insectivorans [18], and most recently primates (sensu lato) [13]. Over the past half-century, picrodontids have often been viewed as plesiadapiforms and more recently belonging to the superfamily Paromomyoidea (Paromomyidae + Palaechthonidae + Picrodontidae) based on the shared presence of dental features such as a strong postprotocingulum and expanded distolingual basin of the upper molars (figure 1a) [5,15,19]. However, picrodontid dental morphology differs significantly from plesiadapiforms (e.g. greatly enlarged M1/m1 with a decrease in molar area to M3/m3, absence of m3 hypoconulid) [13,15]. Because the picrodontid fossil record consists only of autapomorphic dentitions, except for one partial cranium, they are usually excluded from cladistic analyses aimed at assessing primate supraordinal relationships and/or interrelationships among plesiadapiforms. Here, we use µCT scanning technology to describe and compare previously undocumented morphology from the only known partial cranium of a picrodontid and include these novel data in cladistic analyses to assess the phylogenetic affinities of the Picrodontidae.
Figure 2. Hypothesis of evolutionary relationships of Zanycteris paleocenus and other picrodontids among eutherian mammals. Simplified strict consensus resulting from cladistic analysis forcing the monophyly of Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires. See electronic supplementary material for full strict consensus tree. Three-dimensional models of petrosals illustrated to the right are, from top to bottom, Nyctitheriidae sp. (UM 85176), Labidolemur kayi (USNM 530208), Zanycteris paleocenus (AMNH 17180), Ptilocercus lowii (USNM 481107), Cynocephalus volans (FMNH 56521), Microsyops annectens (UW 12362), Dryomomys szalayi (UM 41870), Plesiadapis tricuspidens (MNRH BR 17418). Zanycteris paleocenus and other picrodontids (red) are not recovered within a monophyletic Euarchonta (blue) which includes all plesiadapiforms. Highlighted features of petrosals illustrate comparisons among taxa including a caudal tympanic process (blue), rostral tympanic process (orange), and posterior septum (red). Note that the plesiadapiforms Dryomomys and Plesiadapis have posterior septa which are absent in Z. paleocenus. Scale bars, 1 mm.Crowell Jordan W., Wible John R. and Chester Stephen G. B. 2024
Basicranial evidence suggests picrodontid mammals are not stem primates
Biol. Lett.202023033520230335 http://doi.org/10.1098/rsbl.2023.0335
Copyright: © 2024 The authors.
Published by The Royal Society. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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