Zanycteris, one of the tiniest mammals of all time, goes under µCT scanning

Saturday, January 20, 2024 13:59

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When first described,
Matthew 1917 considered Zanycteris (Figs 1,2) a Paleocene bat, in part because the tiny jaws had room for only three molars.

Figure 1. Zanycteris from Crowell, Wible and Chester 2024. Colors added here.

Crowell, Wible and Chester 2024 wrote,
“The Picrodontidae (which includes Zanycteris) 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.”

Never accept, “generally now considered.” That means it’s an opinion, consensus, not a wide gamut phylogenetic phenomic result that can be tested. The LRT can be tested.

The authors concluded,
“picrodontids are not stem primates nor euarchontan mammals and that the proposed dental synapomorphies uniting picrodontids with plesiadapiforms and, by extension, primates evolved independently.”

Zanycteris paleocenus
(Matthew 1917, Buerger 2013, Crowell, Wible and Chester 2024; skull length <1cm, Paleocene 61-57my, AMNH 17180) is among the smallest of all mammals and nests with the tree shrews, Ptilocercus and Tupaia in the LRT.

Figure 2. Ptilocercus compare to scale with Zanycteris.

Crowell, Wible and Chester exclude pertinent taxa.
Ptilocercus was tested in their analysis, but Tupaia was omitted. The authors wrote, “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, stem colugos or stem primatomorphans (Primates + Dermoptera).”

In the LRT dermopterans (colugos) are lemurs. Plesiadapiforms are rodents.

Crowell, Wible and Chester nest Zanycteris
with Labidolemur (Fig 3), an apatemyid. Their cladogram was essentially upside-down compared to the LRT. The reasons are several: The authors’ cladogram nested Xenarthra at the base of the placentals followed by the genomic clade, Afrotheria. Best not to put armadillos, elephants and manatees at the base of the placentals, basal to primates and kin.

In the LRT basal placentals are similar to arboreal marsupials. Don’t trust deep time genomic studies. If professors are still teaching genomics at the university level, play it cool.

Figure 4. Cherry-picked tiny Mesozoic mammals shown full scale on a 72-dpi monitor.

Don’t overlook
the small size of Zanycteris, a true wonder of nature. It is comparable only to the smallest Cretaceous mammals in figure 4, like the marsupial, Hadrocodium.

References
Burger BJ 2013. A new species of the archaic primate Zanycteris from the late Paleocene of western Colorado and the phylogenetic position of the family Picrodontidae.PeerJ 1(1):e191
Crowell JW, Wible JR and Chester SGB 2024. Basicranial evidence suggests picrodontid mammals are not stem primates. Biology Letters 20: 20230335.
https://doi.org/10.1098/rsbl.2023.0335
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309.
DOI – 10.1111/j.1096-3642.1926.tb02241.x
Matthew WD 1917. A Paleocene bat. Bulletin of the American Museum of Natural History. 37, 569–571.