Revisiting Dissacus: now a sister to Meniscotherium
According to Wikipedia,
“Dissacus is a genus of extinct carnivorous jackal to coyote-sized mammals within the family Mesonychidae, an early group of hoofed mammals that evolved into hunters and omnivores.”
Yes. It looks like a mesonychid… but it looks a little more like something else.
Figure 1.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg?w=135″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg?w=462″ class=”size-full wp-image-85909″ style=”color: #ff6600″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg?w=584&h=1294″ alt=”Figure 1. ” width=”584″ height=”1294″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg?w=584&h=1294 584w, https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg?w=68&h=150 68w, https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg?w=135&h=300 135w, https://pterosaurheresies.files.wordpress.com/2024/05/dissacus-skull588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Dissacus in several views. Colors added here.
According to the LRT
Dissacus (Fig 1) nests within the clade Condylarthra, convergent with mesonychids. Dissacus descendants evolved into sheep-like herbivores, like Meniscotherium (Fig 2), followed by larger elephant-like herbivores, like Astrapotherium (Fig 3). This clade has no extant representatives.
Figure 2. Meniscotherium skull. In this is a smaller predecessor to Astrapotherium note the genesis of maxillary tusks here and then longer dentary when the teeth are matched to occlude correctly. Note the overlapping lacrimal.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg?w=219″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg?w=584″ class=”size-full wp-image-61406″ src=”https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg?w=584&h=799″ alt=”Figure 2. Meniscotherium skull. In this is a smaller predecessor to Astrapotherium note the genesis of maxillary tusks here and then longer dentary when the teeth are matched to occlude correctly. Note the overlapping lacrimal.” width=”584″ height=”799″ srcset=”https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg?w=584&h=799 584w, https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg?w=110&h=150 110w, https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg?w=219&h=300 219w, https://pterosaurheresies.files.wordpress.com/2016/09/meniscotherium588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Meniscotherium skull. In this is a smaller predecessor to Astrapotherium note the genesis of maxillary tusks here and then longer dentary when the teeth are matched to occlude correctly. Note the overlapping lacrimal.
Dissacus zanabazari
(Cope 1881; Geisler and McKenna 2007; Early Eocene) was traditionally considered a mesonychid, but here nests basal to Meniscotherium, without molarized premolars and a reduced lacrimal overlap of the jugal. Note the separation of the lateral premaxillae as in Meniscotherium. Yes, this could be taphonomy. More specimens will tell us.
Figure 3. Meniscotherium enlarged and to scale with Astrapotherium.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg?w=216″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg?w=584″ class=”size-full wp-image-85954″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg?w=584&h=810″ alt=”Figure 3. Meniscotherium enlarged and to scale with Astrapotherium.” width=”584″ height=”810″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg?w=584&h=810 584w, https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg?w=108&h=150 108w, https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg?w=216&h=300 216w, https://pterosaurheresies.files.wordpress.com/2024/05/astrapotherium588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. Meniscotherium enlarged and to scale with Astrapotherium.
Meniscotherium tapiacitum
(Cope 1874; Williamson and Lucas 1992; Middle Eocene 54-38 mya; 25-50 cm long) nests with Astrapotherium (below) in a clade near the base of the Condylarthra. Considering the Paleocene date for Astrapotherium, Meniscotherium probably originated earlier, perhaps in the Cretaceous. The incisor canines were reduced here. That happens because fossils are so rare. Look for trends in evolution. Exceptions will occur.
If you’re wondering about the outgroup sister clade
to Dissacus and kin, here they are (Fig 4), a rather obscure clade of herbivores.
Figure 3. Dissacus compared to Conoryctes and Psittacotherium.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif?w=102″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif?w=347″ class=”size-full wp-image-85943″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif?w=584&h=1724″ alt=”Figure 3. Dissacus compared to Conoryctes and Psittacotherium.” width=”584″ height=”1724″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif?w=584&h=1724 584w, https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif?w=51&h=150 51w, https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif?w=102&h=300 102w, https://pterosaurheresies.files.wordpress.com/2024/05/conoryctes-_comma_skull588.gif 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 4. Dissacus compared to Conoryctes and Psittacotherium.
The outgroup for the Dissacus clade
included Conoryctes and Psittacotherium (Fig 4), both from the Paleocene. None of the above taxa are carnivores.
This hypothesis of interrelationships
now requires a test with a similar taxon list and your own characters for confirmation, refutation or modification.
References
Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, Hussain ST, Thewissen JGM 2014-10-08. Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls. PLoS ONE. 9 (10): e109232. doi:10.1371/journal.pone.0109232. PMID 25295875.
Cope ED 1872. Descriptions of some new Vertebrata from the Bridger Group of the Eocene. Proceedings of the American Philosophical Society 12:460-465.
Cope ED 1874. Report of the Chief of Engineers for. 1874 Vol. II, Pt. II, Appendix FFa of Appendix FF, pp. 115-130.
Cope ED 1881. Notes on Creodonta. American Naturalist 15: 1018–1020.
Geisler J and McKenna MC 2007. A new species of mesonychian mammal from the lower Eocene of Mongolia and its phylogenetic relationships. Acta Palaeontologica Polonica 52, 189-212.
wiki/Meniscotherium
wiki/Dissacus
Source: https://pterosaurheresies.wordpress.com/2024/05/13/revisiting-dissacus-now-a-sister-to-meniscotherium/
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