considers Palaeosinopa (Matthew 1901, Paleocene, Fig. 1) a semi-aquatic pantolestid, one of several [purported] non-placental eutherian mammals. By contrast, the large reptile tree (LRT) nests Palaeosinopa between the raccoon, Procyon, and the seal, Phoca, both placental eutherian mammals.
Dunn and Rose 2015 report,
“Pantolestidae is a Paleogene Holarctic family of semi-aquatic mammals of uncertain phylogenetic affinity.”
So is this homology? Or homoplasy?
Do unrelated aquatic mammals converge on so many traits that they are difficult to phylogentically separate from one another? Or are workers putting too much emphasis on the teeth of mammals while overlooking the ‘big picture’? In other words, can a Carnivoran lose its carnasial tooth and still be considered a Carnivoran? At this point, Palaeosinopa has the opportunity to nest with non-placental eutherians in the LRT, but it does not do so. It nests within the Carnivora — but very few molar traits are employed. AND I’ve got nothing more primitive than non-placental Eutheria that looks anything like Palaeosinopa…yet. [Remember, Monodelphis does NOT have a pouch].
The other big question is
why are pantolestids nesting within Carnivora? Wikipedia reports, “Carnivorans all share the same arrangement of teeth in which the last upper premolar (named P4) and the first lower molar (named m1) have blade-like enamel crowns that work together as carnassial teeth to shear meat.” Pantolestids don’t have such teeth. In fact, the molars tend to flatten with wear. And we all know fish-eaters tend to have different tooth shapes than their more terrestrial kin.
Wikipedia also reports, “Traditionally, the extinct family Viverravidae (viverravids, including Nandinia) had been thought to be the earliest carnivorans, with fossil records first appearing in the Paleocene of North America about 60 mya, but recently described evidence from cranial morphology now places them outside the order Carnivora.”
Traditional paleontology sees
the origin of the clade Carnivora only after 65 mya. The large reptile tree pushes that origin back more than 60 million years with the appearance of Vincelestes. As we’ve seen with other taxa, it’s more important where the taxon nests than whether it has traits specified by paradigm for a certain clade. And this is especially true for basal members.
It gets pretty hairy.
Unlike seals AND unlike non-placental eutherians, Palaeosinopa has a long, robust tail. Like seals, the medial manual and pedal digits, especially digit 2 are longer than the others. The femur is short and robust. The scapulae are similar in shape.
Other pantolestids include
Buxolestes (Middle Eocene). If these are long-tailed pre-seals, they didn’t last long. I’ll add a sea otter within the week to see if it settles things down or mixes things up.
Dunn RH and Rose KD 2015. Evolution of early Eocene Palaeosinopa (Mammalia, Pantolestidae) in the Willwood Formation of the Bighorn Basin, Wyoming. Journal of Paleontology. DOI: 10.1017/jpa.2015.31
Gingerich PD 1980. A New Species of Palaeosinopa (Insectivora: Pantolestidae) from the Late Paleocene of Western North America. Journal of Mammalogy 61 (3): 449.
Linnaeus C von 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Matthew WD 1901. Additional observations on the Creodonta: Bulletin of the
American Museum of Natural History, v. 14, p. 1–38.
Rose KD and von Koenigswald W 2005. An exceptionally complete skeleton of Palaeosinopa (Mammalia, Cimolesta, Pantolestidae) from the Green River Formation, and other postcranial elements of the Pantolestidae from the Eocene of Wyoming (USA): Palaeontographica Abteilung A 273: 55–96.