Ground sloth dermal ossicles: how ubiquitous were they?
Mar 12, 2019 21:59:02 GMT 5
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Post by Infinity Blade on Mar 12, 2019 21:59:02 GMT 5
Since last night I became interested again in ground sloth osteoderms. Dermal ossicles are definitively known from mylodontids, having been found embedded in the preserved skin of these animals. But did any other ground sloths have them?
I'll review the case for all other families containing giant ground sloths. Some have more information available than others, and so the analysis is correspondingly much more thorough.
Nothrotheres:
Pretty straightforward: skin is known from nothrotheres like Nothrotherium and Nothrotheriops, but they show no evidence of ossicles embedded in their dermis.
From McDonald (2017):
www.utep.edu/leb/pleistnm/taxaMamm/Paramylodon.htm
blogs.scientificamerican.com/tetrapod-zoology/the-anatomy-of-sloths/
Megalonychids:
There doesn't seem to be quite as much information for these guys. The second quote I have above suggests that Megalonyx lacked dermal ossicles, as does this quote that creature386 showed me.
pdfs.semanticscholar.org/ca12/6f237143098a6d4542e41b693e3169a8b9fe.pdf?_ga=2.85676112.857177708.1552396311-852069318.1552396311
I briefly discussed with him whether or not this really meant skin has been found for Megalonyx, but regardless, I think the fact that any ground sloth family lacks osteoderms (i.e. the nothrotheriids) raises a lot of questions as to whether or not megalonychids possessed dermal ossicles. The fact that we have to ask ourselves such questions and that there technically seems to be no evidence for dermal ossicles for any megalonychid is enough for us to be skeptical that they had them at all.
Megatheres:
Now wait a second, did I not just post a quote from Darren Naish suggesting that Eremotherium osteoderms have been recovered, suggesting that, by phylogenetic bracketing, all megatheres probably had them? Well, yes I did. Osteoderms have been alleged to come from both Eremotherium laurillardi and Megatherium americanum. But...
Again, from McDonald (2017):
I have seen two cladograms each from a different study that suggest that, if anything, megatheres form a clade with the unarmored nothrotheriids (Pant et al., 2014, Slater et al., 2016).
Scelidotheres:
Dermal ossicles have been attributed to Valgipes bucklandi (Miño-Boilini & Zurita, 2015).
TL;DR: the evidence for dermal ossicles does not seem to be solid for any ground sloth taxa with the exception of the mylodontine mylodontids (the lestodondines apparently show no evidence for them) and scelidotheriids. I won't completely rule their presence out in other ground sloth families (particularly for megatheres), but for now I will assume their absence in all non-mylodontids.
More knowledgable posters can correct me wherever they see fit.
I'll review the case for all other families containing giant ground sloths. Some have more information available than others, and so the analysis is correspondingly much more thorough.
Nothrotheres:
Pretty straightforward: skin is known from nothrotheres like Nothrotherium and Nothrotheriops, but they show no evidence of ossicles embedded in their dermis.
From McDonald (2017):
Ameghino (1907), referencing Lund, reported that osteoderms were present in the nothrothere sloth, Nothrotherium, from Lagoa Santa, but Cartelle and Fonseca (1983) in their description of a complete skeleton of this animal determined it did not possess osteoderms.
Kurtén and Anderson (1980) characterized Harlan's Ground Sloth as a grassland species widespread in the U.S. during the Irvingtonian and Rancholabrean. They further note that it differs from Megalonyx and Nothrotheriops by larger size, lobate teeth, and dermal ossicles (bone nodules imbedded within the skin).
www.utep.edu/leb/pleistnm/taxaMamm/Paramylodon.htm
Mummified sloth skin preserved in the arid caves of Chile, Argentina, Arizona and Nevada provides excellent information on ground sloth skin and fur. Small bony ossicles were embedded in the skin of the mylodontids Mylodon, Glossotherium and Paramylodon, and probably also in Eremotherium, but are definitely not present in the mummified skin of Nothrotheriops. The fur itself was either yellowish or reddish brown.
blogs.scientificamerican.com/tetrapod-zoology/the-anatomy-of-sloths/
Megalonychids:
There doesn't seem to be quite as much information for these guys. The second quote I have above suggests that Megalonyx lacked dermal ossicles, as does this quote that creature386 showed me.
As is well known, Megatherium and Megalonyx (as well as Nothrotherium) are without dermal osicles, but the skin of Mylodon is thickly studded with these.
I briefly discussed with him whether or not this really meant skin has been found for Megalonyx, but regardless, I think the fact that any ground sloth family lacks osteoderms (i.e. the nothrotheriids) raises a lot of questions as to whether or not megalonychids possessed dermal ossicles. The fact that we have to ask ourselves such questions and that there technically seems to be no evidence for dermal ossicles for any megalonychid is enough for us to be skeptical that they had them at all.
Megatheres:
Now wait a second, did I not just post a quote from Darren Naish suggesting that Eremotherium osteoderms have been recovered, suggesting that, by phylogenetic bracketing, all megatheres probably had them? Well, yes I did. Osteoderms have been alleged to come from both Eremotherium laurillardi and Megatherium americanum. But...
Again, from McDonald (2017):
While most of the sloth genera with osteoderms are mylodonts, Cartelle and Bohórquez (1986) described osteoderms from the late Pleistocene megathere, Eremotherium laurillardi. Eremotherium laurillardi, the Panamerican sloth, has the largest known distribution for any extinct sloth and is known from numerous skeletons (Cartelle and De Iuliis 1995). While this single record is intriguing as osteoderms have never been reported for any other specimens, the presence of a mylodont sloth has been reported from the same locality, so it is possible that this is the source of the osteoderms. Politis and Messineo (2008) reported but did not illustrate any of the 142 osteoderms they assigned to Megatherium americanum, the only sloth from the late Pleistocene (Lujanian SALMA) Campo Laborde site, Argentina, which is also represented by other parts of the skeleton. Given that two glyptodonts, Neosclerocalyptus sp. and Doedicurus sp., are also present in the fauna it cannot be ruled out that these osteoderms are possibly from the legs of either of these two taxa as they are also present in skin of the limbs of Glyptodon (Rinderknecht 2000). If it can be definitely demonstrated that osteoderms are present in these distantly related sloths, mylodonts and megatheres, it further supports the contention of separate acquisition of this feature in sloths, or if plesiomorphic for the Xenarthra it requires the loss of this feature in all other sloths, including the immediate ancestors to the Pliocene and Pleistocene genera in which osteoderms are present.
I have seen two cladograms each from a different study that suggest that, if anything, megatheres form a clade with the unarmored nothrotheriids (Pant et al., 2014, Slater et al., 2016).
Scelidotheres:
Dermal ossicles have been attributed to Valgipes bucklandi (Miño-Boilini & Zurita, 2015).
MCL 4262, skull almost complete, hemimandible with a portion of the symphysis, with the mf1-2 and dermal ossicles; MCL 4264, partial skull, partial right hemimandible, left and right astragali, right tibia and fibula; MCL 4293, partial skull, partial right hemimandible that preserves only the mf3, several vertebrae, almost complete hand, right femur, right tibia and fibula, right and left astragali.
TL;DR: the evidence for dermal ossicles does not seem to be solid for any ground sloth taxa with the exception of the mylodontine mylodontids (the lestodondines apparently show no evidence for them) and scelidotheriids. I won't completely rule their presence out in other ground sloth families (particularly for megatheres), but for now I will assume their absence in all non-mylodontids.
More knowledgable posters can correct me wherever they see fit.