Recently discovered and described
Alcidedorbignya (Muizon and Marshall 1992 Muizon et al 2015; MHNC 8372, Figs 1, 2) as “one of the oldest and most primitive of the pantodonts” (= edentate ancestors) and “the only pantodont genus known from South America” (excluding all edentate taxa [Fig 5] known from and still living in South America).
Alcidedorbignya is also much smaller
than ancestral and descendant taxa (Fig 1). That makes it a phylogenetically miniaturized taxon. As we’ve seen many times before, such taxa often develop novel structures and niches retained by their larger descendants.
According to Muizon et al. 2015,
“In the discussion on possible scansorial habits of Alcidedorbignya, comparison with the arboreal Dendrohyrax [a tree hyrax] may appear surprising given the fact that the postcranial anatomy of this genus shows many traits suggesting a long cursorial ancestry (Fischer 1986). Nevertheless, we regard this comparison as highly relevant since tree hyraxes climb very well without claws (also absent in Alcidedorbignya) and without the great capacity of prehension of arboreal primates (also absent in Alcidedorbignya). Their ability to climb is related to the association of digital friction pads associated to unusual carpal mobility that allows wrist (instead of elbow) supination, two features that will be discussed below. Therefore, we regard Dendrohyrax as a good potential functional analogue of Alcidedorbignya on some aspects.”
Analogs can be good. Homologs are always better.
Several direct descendants of Early Paleocene Alcidedorbignya (e.g. Pantolambda, Barylambda, glyptodonts) were large, terrestrial herbivores with broad unguals on plantigrade feet.
Closely related basal phenacodonts,
like Phenacodus (Fig. 4), had digitigrade hands and feet with broad unguals starting evolve into hooves. Among those phenacodont descendants were hyraxes, like Procavia and Dendrohyrax. The cladograms presented by Muizon et al. 2015 suffer from taxon exclusion, so that team was unable to completely resolve phenacodonts from pantodonts and unable to resolve placentals from marsupials.
Let’s not forget
many edentates had long-clawed feet and hands, distinct from the broad, dull unguals of basal edentates. This return to an arboreal niche (e.g. in sloths) or to a digging niche (e.g. armadillos and anteaters) aided by trenchant claws is a reversal. That happens when the genes that initiate ungual tip spreading do not turn on. Taking advantage of this reversal, the pantodonts Ectoconus and Titanoides did not retain or develop wide, hoof-like additions to their relatively sharp unguals. When the hoof-gene was not well established in basal taxa, it was more easily lost.
What PILs reveal.
Parallel interphalangeal lines (PILs) are applied here (Fig. 3) to the manus of Alcidedorbignya. Typically, on joints able to flex and extend in concert the PILs are continuous. Here, exceptionally, on digits 1 and 5 the PILs are not continuous, so flexion and extension are both restricted and prevented at those joints. Medial and lateral PIL sets appear to be essentially useless for extension and flexion revealing that the ability to grasp a branch inside a circle defined by the flexion of digits 1 and 5 coming into contact (as in primates including humans) is not present here.
This is how a digitigrade manus with hooves on digits 2–4 begins.
The need for digits 1 and 5 is eliminated except for using them to help stiffen the proximal phalanges, aiding the elevation of all five proximal phalanges above the substrate during quadrupedal locomotion. Elevating short digits 1 and 5 above the substrate often eliminates any need for them, so in descendant ungulates (= phenacodonts) there are only one, two, three or four working digits plus vestiges. Sometimes digits 1 or 5 become useful again (e.g. Elephas), representing yet another reversal.
Alcidedorbignya inopinata
(Muizon and Marshall 1992 Muizon et al 2015; MHNC 8372, Early Paleocene, 64 mya; scale bar is 2cm) was originally and is here considered a basal pantodont close to Pantolambda. The rostrum is convex with a smaller naris. Claw tips develop wide, snowshoe-like ‘hooves’ here.
Ectoconus ditrigonus – E majusculus
(Cope 1884 Early Paleocene, 66-63mya, sheep-sized) is a basal pantodont known from a rare complete skeleton. Pantolambda is a sister. Onychodectes is basal. Though canines were present, Ectoconus was an herbivore with five digits on all four limbs. Often shown as a digitigrade, this taxon was likely plantigrade based on phylogenetic bracketing.
Pantolambda bathmodon
(Cope 1882, Middle Paleocene, 66-63 mya, sheep-sized; AMNH 3956) is another basal condylarth, and a coeval sister to Ectoconus with a shorter skull, shorter lumbar region and shorter limbs. Pantolambda is a pantodont.
References
Cope ED 1882. On the condylarthra. Academy of Natural Sciences of Philadelphia Proceedings 1882:95-97.
Cope ED 1882. Synopsis of the Vertebrata of the Puerco Eocene epoch. Proceedings of the American Philosophical Society 20
Cope ED 1884. The Amblypoda. The American Naturalist 18 (112):6=461-471.
Simons EL 1960. The Paleocene Pantodonta. Transactions of the American Philosophical Society, New Series 50(6):1-8.
de Muizon C de and Marshall LG 1992. Alcidedorbignya inopinata (Mammalia: Pantodonta) from the early Paleocene of Bolivia: phylogenetic and paleobiogeographic implications. Journal of Paleontology 66 (3): 499-520.
deMuizon C, Billet G, Argot C, Ladeveze S and Goussard F 2015. Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene
of Bolivia: anatomy, phylogeny and palaeobiology. Geodiversitas 87(4):397-634.
The Pterosaur Heresies 