Megistotherium: Not a gigantic hyaenodont creodont, a bear-dog marsupial

Updated April 5, 2022
with more taxa and revised scores.

Traditionally described as a gigantic hyaenodont creodont,
Megistotherium (Savage 1973; Miocene; Fig. 1) nests in the large reptile tree (LRT, 1544 taxa, then, 2070 taxa now) with the unnamed marsupial bear-dog, ‘not Amphicyon‘ (Fig. 2).

Figure 1. Megistotherium skull in several views. It is 2/3 of a meter in length. Don't overlook the skull of tiny relative, Palaeosinopa with a 10cm skull length.

Figure 1. Megistotherium skull in several views. It is 2/3 of a meter in length. Don’t overlook the skull of tiny relative, Palaeosinopa with a 10cm skull length.

Figure 2. The unnamed marsupial bear-dog here labeled, 'not Amphicyon'.

Figure 2. The unnamed marsupial bear-dog here labeled, ‘not Amphicyon’.

Megistotherium osteothlastes (Savage 1973; Miocene, 23mya; 66cm skull length) was originally considered a giant hyaenodontid creodont. Here it nests with the marsupial beardog ‘not Amphicyon‘. The jaw muscles were enormous. The large diameter canines were housed in large, laterally expanded maxillae. The braincase was narrow. In overall size and general features, Megistotherium is similar, by convergence, to the giant elephant shrew, Andrewsarchus.

Figure 1. Harpagolestes macrocephalus compared to sisters Sinonyx and Andrewsarchus to scale.

Figure 1. Harpagolestes macrocephalus compared to sisters Sinonyx and Andrewsarchus to scale. Compare these elephant shrews to Megistotherium (Fig. 1)/

In the past,
several mammal taxa achieved gigantic proportions not found in today’s relatives.


References
Savage RJ 1973. Megistotherium, gigantic hyaeonodont from Miocene of Gebel Zelten, Libya. Bulletin of the British Museum (Natural History) Geology 22(7):483–511.

wiiki/Megistotherium

SVP 2018: Pinniped monophyly

Updated August 1, 2022
with a new tree topology for Carnivora.

Tate-Jones, Hopkins and Davis 2018
discuss current thinking on seal and sea lion interrelations.

Here’s what they wrote:
“Despite several decades of morphological and molecular study, a consensus on the
phylogeny of the carnivoran suborder Pinnipedia has remained elusive. The majority of
studies within the last 10 years have supported a monophyletic origin for pinnipeds.
Questions remain about whether pinnipeds are more closely related to ursids or
musteloids and about the positioning of early-branching clades, such as
Desmatophocidae.” 

Figure 2. Desmatophoca skull. This extinct sea lion nests with Enaliarctos, not Phoca, the seal.

Figure 2. Desmatophoca skull. This extinct sea lion nests with Enaliarctos, not Phoca, the seal.

Adding Desmatophoca (Fig. 2) to the LRT
nests it not with Phoca, but with Enaliarctos (Fig. 3), not among the seals, but with the sea lions.

The authors report,
“Researchers have alternately placed Desmatophocidae as a sister taxon to phocids (true seals), otariids (sea lions and fur seals), and odobenids (walruses), and no clear consensus yet exists about its positioning.”

Figure 6. Enaliarctos nests between Zalophus and Hyopsodus in the LRT.

Figure 3. Enaliarctos nests between Zalophus and Hyopsodus in the LRT.

The LRT is a good fact-checking tool.
Use it to avoid future problems.

References
Tate-Jones K, Hopkins SS and Davis EB 2018. A new Middle Miocene desmatophocid pinniped (Mammalia, Carnivora) from the Oregon coast and its potential for greater resolution of pinniped phylogeny and paleoecology. SVP Abstracts.

SVP abstracts 2017: Are pinnipeds (seals/sea lions) monophyletic?

Earlier the large reptile tree (LRT, 1050 taxa) invalidated the former clade Pinnipedia (seals and kin) as it split it into two clades, each derived from separate terrestrial limbed ancestors. Now comes this well written abstract from Paterson et al. 2017 that brings up all the right questions. The question is, did it have the right outgroups? I like how they say they are going to test tradition with genes and fossils. Unfortunately, they might lack a few pertinent outgroup taxa.

Figure 1. Phoca the phocid seal is most closely related to Palaeosinopa of all tested taxa.

Figure 1. Phoca the phocid seal is most closely related to Palaeosinopa of all tested taxa.

From the Paterson et al. abstract:
“Monophyly of pinnipeds is well-established. However, it is difficult to reconcile a monophyletic origin of pinnipeds with the disparate locomotor modes and associated skeletal morphologies observed between the extant families. Furthermore, the fossil record suggests many of the conventional pinniped synapomorphies arose independently, as many are not present in fossil taxa (Eotaria, Prototaria, Devinophoca) that have been firmly established as early-diverging crown members of the three extant families (e.g., homodont dentition, loss of fossa muscularis, reduction of nasolabialis fossa, loss of M2/m2, fusion of tibia and fibula, reduction of fossa for teres femoris).

“Herein, we test the hypothesis that otarioids (otariids + odobenids) and phocids share a common ancestor that was not yet fully aquatic. In the present analysis, a total evidence approach was employed to investigate the relationships of 19 extant and 37 fossil caniforme genera. Our analysis sampled five genes totalling 5490 bp and 184 morphological characters, sampled relatively evenly across morphological partitions (cranial, dental, postcranial). With Canis as an outgroup, Bayesian inference produced strong support for a monophyletic origin of pinnipeds, and recovered Puijila and Potamotherium as early-diverging pinnipedimorphs

(Ursidae(Musteloidea(Potamotherium(Puijila(Enaliarctos, (Desmatophocidae(Phocidae,(Odobenidae, Otariidae)))))))). Similar results were obtained from Bayesian and parsimony analyses of a morphology-only data set, a cranial-only data set, a craniodental-only data set, and a post-cranial-only data set. Bayesian inference of morphology-only partitions recovered Mustelavus and a sister grouping of Allocyon + Kolponomos along the stem to later-diverging pinnipedimorphs. The parsimony analysis recovered 20 synapomorphies of Potamotherium + Puijila + Pinnipedimorphs, and nine synapomorphies for a crown group Pinnipedia, to the exclusion of the pinnipedimorphs. In spite of a reinterpretation of the plesiomorphic state of many previously proposed pinniped synapomorphies, there remain more than enough pinniped synapomorphies to exclude the semi-aquatic  pinnipedimorphs, thereby challenging our hypothesis of a dual origin of flippers. However, this may be an artifact of a Bayesian model of morphological inference which, among other limitations, cannot model direction evolution, and thus, may be incapable of capturing parallel evolution in such a context.”

Figure 3. Zalophus, an otariid seal, is most closely related to Hyoposodus among tested taxa in the LRT

Figure 2. Zalophus, an otariid seal lion, is most closely related to Hyoposodus among tested taxa in the LRT. Seals and sea lions are incredibly alike. It’s a tribute to the authority of the LRT that it was able to separate these two clades, both derived from distinct and different terrestrial ancestors.

I don’t have their complete taxon list. 
If it includes the pertinent taxa that split the Pinnipedia, then we’ll have to reexamine the data. If not, it’s still worth comparing.

Their choice of
Canis (the wolf/dog) as an outgroup is a weakness. They should have let a large gamut of mammals decide the outgroup(s) for pinnipeds. I don’t see Palaeosinopa in their published taxon list, but it might be in there somewhere. In the LRT it nests with phocids, like Phoca and the limbed carnivore, PuijilaI don’t see Miacis and Hyopsodus in their taxon list. In the Paterson et al. study Enaliarctos nests basal to all extant pinnipeds, but in the LRT, Enaliarctos nests with Miacis, Hyopsodus and Zalophus, the California sea lion, an otariid. So, it looks like taxon exclusion is present here, yet again. Paterson et al. appear to be missing some pertinent outgroups. The last common ancestor of seals and sea lions goes back to something like Herpestes, the mongoose, and/or Procyon, the raccoon.

Seals and sea lions are incredibly alike.
It’s a tribute to the authority of the LRT that it was able to separate these two clades, both derived from distinct and different terrestrial ancestors.

References
Paterson RS et al. 2017. The evolution of pinnipeds from a terrestrial ancestor: the possibility of parallel evolution within a monophyletic framework. SVP abstracts 2017.