Back when placental mammals were first diversifying in the Jurassic
they all looked like small arboreal marsupial didelophids, like Caluromys, and small arboreal placental tree shrews, like the extant Ptilocercus and Tupaia. Two distinct specimens, both given the genus name Hapalodectes (Fig. 1), are among these basal placental taxa in the large reptile tree (LRT, 1378 taxa).
The slightly smaller
IVPP V5235 specimen attributed to Hapalodectes (Ting and Li 1987) nests at the base of the primate clade. It had already taken on the appearance of a little basal lemur or adapid (Fig. 1).
The slightly larger
IVPP V12385 specimen attributed to Hapalodectes (Ting et al. 2004; Fig. 1) nests at the base of the anagalid-tenrec-odontocete clade and it had already taken on the appearance of a little anagalid or elephant shrew.
Other than size, the differences are subtle:
- The basal primate has a postorbital ring. The basal anagalid does not.
- The basal primate has three upper molars. The basal anagalid has four.
- The basal primate cranium has no crest. The basal anagalid has a nuchal and parasagittal crest.
- The basal primate anchors the squamosal further back, with a smaller ectotympanic (middle ear container bones below the cranium). The basal anagalid anchors the squamosal further forward, with a larger ectotympanic (for better hearing).
Hapalodectes hetangensis (Ting and Li 1987; 4.5cm skull length; Paleocene, 55 mya; IVPP V 5235) This skull was originally wrongly applied to the Mesonychidae, but here nests at the base of the primates, including Notharctus. Note the transverse premaxilla, the large canine, and the encircled orbits rotated anteriorly.
?Hapalodectes ?hetangensis (Ting et al. 2004; 7 cm skull length; Early Eocene 50 mya; IVPP V 12385) was originally considered a tiny mesonychid. This species nests at the base of the anagale-tenrec-odontocete clade, between Ptilocercus and Onychodectes. The large nuchal crest is a key trait found in later taxa. The premaxilla is largely missing, but likely was transverse in orientation.
Ting S and Li C 1987. The skull of Hapalodectes (?Acreodi, Mammalia), with notes on some Chinese Paleocene mesonychids.
Ting SY, Wang Y, Schiebout JA, Koch PL, Clyde WC, Bowen GJ and Wang Y 2004. New Early Eocene mammalian fossils from the Hengyang Basin, Hunan China. Bulletin of Carnegie Museum of Natural History 36: 291-301.