The origin of hooves and digging ‘hooves’ in the LRT
Earlier we looked at
the close match between the manus of the basal primate Notharctus, and the basal ungulate (= condylarth), Phenacodus. Broadly that’s how hooves evolved in increasingly terrestrial taxa derived from arboreal taxa following the asteroid impact.
Here Alcidedorbignya,
Ernanodon and several coeval Paleocene taxa (Figs 1, 2) demonstrate the transition to hooves in a little more detail – plus a detour to digging ‘hooves’ close to the origin of the clade Ungulata.
Figure 1. Taxa at the origin of hooves in placental mammals include Vulpavus, Ernanodon Alcidedorbignya, Metacheiromys and Phenacodus.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/origin_of_hooves588.jpg?w=203″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/origin_of_hooves588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87325″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/origin_of_hooves588.jpg” alt=”Figure 1. Taxa at the origin of hooves in placental mammals include Vulpavus, Ernanodon Alcidedorbignya, Metacheiromys and Phenacodus. ” width=”584″ height=”863″ />
Figure 1. Taxa at the origin of hooves in placental mammals include Vulpavus, Ernanodon Alcidedorbignya, Metacheiromys and Phenacodus.
Alcidedorbignya is particularly interesting
because just by slightly spreading its fingers it turns from an arboreal climber with a flexible manus, as originally envisioned, to a terrestrial digger, like Ernanodon (Fig 2), with an inflexible manus. PILs (parallel interphalangeal lines, Peters 2000, 2010) show flexion and extension hinges whenever continuous and a restriction to those hinges when not continuous.
By contrast, discontinuous PILs indicate a reduction-to-blockage of flexion, turning the formerly flexible manus into a relatively inflexible shovel.
Aquatic tetrapod flippers also show discontinuous PILs for the same reasons.
Figure 2. The manus (left column) and pes (right column) of Phenacodus, Alcidedorbignya and Ernanodon are broadly comparable. Note the continuity of the PILs (parallel interphalangeal lines) in the Phenacodus and sometimes in Alcidedorbignya. Continuous lines mark hinges that permit flexion. By contrast, discontinuous lines hinder flexion, ideal when the manus is used as a shovel or flipper.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/ernanodon-phenacodus_manus.pes588.gif?w=150″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/ernanodon-phenacodus_manus.pes588.gif?w=512″ tabindex=”0″ role=”button” class=”size-full wp-image-87327″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/ernanodon-phenacodus_manus.pes588.gif” alt=”Figure 2. The manus (left column) and pes (right column) of Phenacodus, Alcidedorbignya and Ernanodon are broadly comparable. Note the continuity of the PILs (parallel interphalangeal lines) in the Phenacodus and sometimes in Alcidedorbignya. Continuous lines mark hinges that permit flexion. By contrast, discontinuous lines hinder flexion, ideal when the manus is used as a shovel or flipper. ” width=”584″ height=”1168″ />
Figure 2. The manus (left column) and pes (right column) of Phenacodus, Alcidedorbignya and Ernanodon are broadly comparable. Note the continuity of the PILs (parallel interphalangeal lines) in the Phenacodus and sometimes in Alcidedorbignya. Continuous lines mark hinges that permit flexion. By contrast, discontinuous lines hinder flexion, ideal when the manus is used as a shovel or flipper.
Recent housekeeping moved odd Ernanodon
(Figs 1, 2) from the marsupials to the base of the digging placentals (= aardvarks, anteaters, pangolins). That shift became the first domino to fall in creating this post.
Alcidedorbignya inopinata
(Muizon and Marshall 1992 Muizon et al 2015; MHNC 8372, Early Paleocene, 64 mya; scale bar is 2cm) was originally considered a basal pantodont (Condylartha) close to Pantolambda and the origin of ungulates and digging placentals. The rostrum is curved with a small naris. Long hooves are former claws on a semiplantigrade manus.
Ernanodon anteilos
(Ting [= Ding] 1979; Ting and Tong 2005; Paleocene; 50 cm in length) was originally considered a placental mammal, perhaps a primitive anteater, then regarded as a primitive pangolin, like Manis. Here Ernanodon nests with Metacheiromys. The skull was robust with a jaw joint nearly as far back as the occiput. The claws = hooves were broad and long, ideal for digging. The tail was long, but very slender.
Figure 1. Barylambda looks like a large ground sloth for good reason. It is a sister to the direct ancestor and nests at the base of the Xenarthra along with Orycteropus, the aardvark.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/09/barylambda_faberi588.jpg?w=221″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/09/barylambda_faberi588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-24107″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/09/barylambda_faberi588.jpg” alt=”Figure 1. Barylambda looks like a large ground sloth for good reason. It is a sister to the direct ancestor and nests at the base of the Xenarthra along with Orycteropus, the aardvark.” width=”584″ height=”792″ />
Figure 3. Barylambda looks like a large ground sloth for good reason. It is a sister to the direct ancestor and nests at the base of the Xenarthra along with Orycteropus, the aardvark.
Which brings up an interesting question…
Late Paleocene Barylambda(Fig 3) also has hooves. In the LRT Barylambda is basal to glyptodonts and sloths, famous for their huge manual claws (Fig 4).
Figure 4. Megatherium the giant sloth. Inserts show the manus and pes. These are former hooves in the ancestral taxon, Barylambda.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/megatherium-giant-sloth588.jpg?w=298″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/megatherium-giant-sloth588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87332″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/megatherium-giant-sloth588.jpg” alt=”Figure 4. Megatherium the giant sloth. Inserts show the manus and pes. These are former hooves in the ancestral taxon, Barylambda.” width=”584″ height=”588″ />
Figure 4. Megatherium the giant sloth. Inserts show the manus and pes. These are former hooves in the ancestral taxon, Barylambda.
A transition from hooves back to claws in this clade
( a reversal?) appears to be happening in Glyptodon (Fig 5).
Figure 5. Glyptodon skeleton with closeups of the manus and pes.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/glyptodon_skeleton588.gif?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/glyptodon_skeleton588.gif?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87335″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/glyptodon_skeleton588.gif” alt=”Figure 5. Glyptodon skeleton with closeups of the manus and pes. ” width=”584″ height=”363″ />
Figure 5. Glyptodon skeleton with closeups of the manus and pes.
In 2015 deMuizon et al wrote,
“the ungual phalanges of Alcidedorbignya did not bear true hooves (and certainly not claws), but elongated, primate-like nails.”
Perhaps if they are not true hooves and certainly not claws, then perhaps the ungual phalanges can be considered transitional morphologies, since Alcidedorbignya is not related to primates.
“Hyraxes, have small hooves and short fingers and lack a thumb. Because this condition
does not prevent Dendrohyrax from being extremely efficient climber it is reasonable
to hypothesize the same for Alcidedorbignya.”
“Congruent with grasping ability is also the remarkable development of the flexor tubercle on the ventral face of the ungual phalanges even if such structure is present in diggers as well as in climbers.”
Neither Ernanodon nor Metacheiromys wer mentioned in the text
of deMuizon et al 2015.
Considering the proximity of Alcidedorbignya to Ernanodon in the LRT, both at the base of their related clades, perhaps Alcidedorbignya just missed being a digger. Descendant taxa are generally much larger, herbivorous and terrestrial. However, among the ancestor list one can find the extant pink fairy armadillo, Chlamyphorus, a known digger and the two extant tree sloths, further down the line.
References
de Muizon C de & 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.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods
Ichnos, 7: 11-41.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Ting S, Wang B and Tong Y-S 2005. The type specimen of Ernanodon antelios. Journal of Vertebrate Paleontology 25(3):729-731.
wiki/Metacheiromys
wiki/Ernanodon
wiki/Alcidedorbignya
Source: https://pterosaurheresies.wordpress.com/2024/07/08/the-origin-of-hooves-and-digging-hooves-in-the-lrt/
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