Tweaking Palaechthon (basal Volitantia)

Kay and Cartmill 1977 wrote:
“The Middle Paleocene paromomyid Palaechthon nacimienti has the most primitive cranial anatomy known for any plesiadapoid. In relative size and functional morphology, its molars resemble those of primates and tree shrews known to feed largely on insects. Its orbits were small, laterally directed, and widely separated, and the relative size of its infraorbital foramen shows that it had well-developed facial vibrissae resembling those of extant erinaceids. Its anterior dentition was probably also hedgehog-like. These features suggest that it was a predominantly terrestrial insect-eater, guided largely by tactile, auditory and olfactory sensation in its pursuit of prey. Adaptations to living in trees and feeding on plants probably developed in parallel in more than one lineage descended from the ancestral plesiadapoids. A new genus and species of paromomyid, Talpohenach torrejonius, is erected for material originally identified as Palaechthon.”

This was done in the days before software phylogenetic analysis.
In the large reptile tree nests (LRT, 1413 taxa) Palaechthon as a sister to the dermopterans, like Cynocephalus, both derived from basal Carnivora, like Vulpavus (Fig. 1). All of these taxa are basal to Primates in the LRT.

Palaechthon is known from several partial specimens
combined to make the incomplete skull drawing shown here (Fig. 1).

Figure 1. Palaechthon compared to outgroup, Vulpavus, and sister, Cynocephalus using drawings from Kay and Cartmill 1974. Colors added.

Figure 1. Palaechthon compared to outgroup, Vulpavus, and sister, Cynocephalus using drawings from Kay and Cartmill 1974. Colors added.

Palaechthon nacimienti (Wilson and Szalay 1972) ~4cm skull length, middle Palaeocene, was originally and traditionally considered a basal plesiadapiform (traditionally considered a clade of basal primates). Here derived from a sister to the basal placental and carnivoran, Vulpavus, Palaechthon phylogenetically nests with Cynocephalus, the colugo. The premaxilla is missing and may have been nearly toothless, as in the colugo. Distinct from Vulpavus, but as in Cynocephalus, and the basalmost eutherian, Caluromys, four molars are present in Palaechthon.

Figure 3. Cynocephalus, the flying lemur, shares many traits with Ptilocercus and basal bats.

Figure 2. Cynocephalus, the flying lemur, shares many traits with Ptilocercus and basal bats.

Phylogenetic bracketing
makes Palaechthon an arboreal taxon, possibly with a prehensile tail. Kay and Cartmill 1974 imagined large, rodent-like teeth emerging from the missing premaxilla and missing anterior dentary (Fig. 1). Phylogenetic bracketing indicates just the opposite—tiny anterior teeth, as shown in Vulpavus and Cynocephalus. The auditory bulla was probably small, as indicated by phylogenetic bracketing. The post-dentary part of the skull was probably short, as in Cynocephalus.

Figure 1. Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

Figure 3. Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

The present ‘tweaking’
benefits from more recent additions to the LRT that provided more clues to the closest relatives of Palaechthon, cementing relationships recovered years earlier. First hand access did not give Kay and Cartmill more insight into the relationships of Palaechthon, a basal member of the clade Volitantia.They presumed from the start that it was a primate ancestor, close to Plesiadapis. Both presumptions have been refuted by the LRT, which tests both basal primates and plesiadapiformes, now nesting within Glires. Based on the appearance of descendant taxa in the Middle Jurassic, Palaechthon had its genesis in the Early Jurassic.


References
Fleagle JG 1988. Primate Adaptation and Evolution. Academic Press: New York
Kaplan M 2012. Primates were always tree-dwellers. Nature. doi:10.1038/nature.2012.11423
Kay RF and Cartmill M 1974. Skull of Palaechthon nacimienti. Nature 252:37–38.
Kay RF and Cartmill M 1977. Cranial Morphology and Adaptation of Palaechthon nacimienti and Other Paromomyidae (Plesiadapoidea, Primates), with a Description of a New Genus and Species. Journal of Human Evolution, Vol. 6, 19-53.
Sloan RE and Van Valen L 1965. Cretaceous mammals from Montana. Science 148:220–227
Van Valen L and Sloan R 1965. The earliest primates. Science. 150(3697): 743–745.
Wible JR, Rougier GW, Novacek MJ and Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary.” Nature volume 447: 1003-1006
Wilson RW and Szalay FS 1972. American Museum Novitates 2499:1.

 

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