Megistotherium: Not a gigantic hyaenodont creodont. More like a very basal seal.

Traditionally described as a gigantic hyaenodont creodont,
Megistotherium (Savage 1973; Miocene; Fig. 1) nests in the large reptile tree (LRT, 1544 taxa) at the base of the clade within Carnivora that ultimately produced extant seals and extinct Palaeosinopa (Figs. 1,2). In the LRT creodonts are large marsupial predators, convergent with members of the clade Carnivora.

Therefore 
Megistotherium is also a sister to the Machaeroides clade (which gave rise to Stylinodon) both derived from the Kerbos and Gulo (wolverine) clades (which gave rise to terrifying short face bears, like Arctodus). So, several gigantic, fearless bear-like taxa arise from this branch within Carnivora.

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.

Megistotherium osteothlastes (Savage 1973; Miocene, 23mya; 66cm skull length) was originally considered a giant hyaenodontid creodont based on tooth data. Here, because it nests basal to Palaeosinopa and seals it was probably semi-aquatic. Premaxillary teeth were weak and disappearing. The jaw muscles were enormous judging by the widespread cheek arches and tall cranial crest. The large diameter canines were housed in large, laterally expanded maxillae. The braincase was narrow. Note the prefrontal and lacrimal are no longer fused to one another.

Earlier mistakes in nesting Megistotherium
may be assigned to an over reliance on dental traits, which tend to converge more often than traditionally realized, and to taxon exclusion, something the LRT tends to minimize due to its wide gamut, getting bigger every week.

Figure y. Palaeosinopa in situ with tail reconstructed from disturbed elements.

Figure 2. Palaeosinopa in situ with tail reconstructed from disturbed elements. This taxon provides clues to the post-crania of Megistotherium by way of phylogenetic bracketing.

Side note:
In overall size and general features, the skull of Megistotherium was similar, by convergence, to the giant Eocene elephant shrew, the mighty 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? Again, no.

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

Unfortunately
they still hold to the outdated hypothesis of a monophyletic Pinnipedia. Adding more seals and more sea lions won’t help. You have to add taxa that still have paws, like Hyopsodus and Palaeosinopa, as in the large reptile tree (LRT, 1308 taxa; subset Fig. 1).

Figure 1. Subset of the LRT focusing on the Carnivora with tan tones on the bears newly added.

Figure 1. Subset of the LRT focusing on the Carnivora with tan tones on the bears newly added. Desmatophoca nests not with Phoca but with Enaliarctos.

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.”

Add these taxa,
(Fig. 1) then get back to us.

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.

Seals are diphyletic. Goodbye Pinnipedia!

This split has been suspected or imagined for quite some time…
…but never documented with fossil taxa in phylogenetic analysis until today in the large reptile tree (LRT 1040 taxa; subset Fig. 1).

Figure 3. Subset of the LRT focusing on Carnivora, a basal placental mammal clade. Note cats and dogs in derived nodes.

Figure 3. Subset of the LRT focusing on Carnivora, a basal placental mammal clade. Note cats and dogs in derived nodes.

Seals are diphyletic.
These marine Carnivora had two terrestrial origins. Phoca is an earless seal (family: Phocidae) derived from a sister to Paleaosinopa. On the other hand, Zalophus is an eared seal (family: Otariidae) derived from a sister to Hyopsodus and Miacis. Thus the clade Pinnipedia is no longer monophyletic. The last common ancestor of seals in the LRT is the extant common raccoon, Procyon.  Doubtless a similar form that lived closer to the Cretaceous was the actual last common ancestor. Both clades of seals (that’s what we’ll still call them forever) adapted to water in similar, but not identical ways (see below).

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.

Molecular evidence
According to Wikipedia: “While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyleticlineage (descended from one ancestral line). Pinnipeds belong to the order Carnivora and their closest living relatives are bears and musteloids (weasels, raccoons, skunks, and red pandas), having diverged about 50 million years ago.”  Of course, if the sister taxa that split seals are extinct (fossil taxa) as they are here, then molecular studies cannot service this issue or answer this question.

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

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

Earlier hypotheses
imagined otariids descended from bears and phocids descended from mustelids (weasels). Below are some of those earlier, mostly molecular studies and their intrinsic problems.

  1. Arnason et al. 2006. Molecular study. Outgroups include bears, minks other Carnivora and no fossil taxa.
  2. Flynn et al. 2005. Molecular study of the Carnivora. Outgroups include no fossil taxa.
  3. Higdon et al. 2007. Molecular study of the Pinnipedia. Outgroups include bears and dogs and no fossil taxa.
  4. Hunt and Barnes 1994. Skull base comparisons link seals to bears, not otters and ferrets.
  5. Lento GM et al. 1995. Molecular study found pinnipeds derived from the bear/raccoon/panda radiation. Outgroups include no fossil taxa.
  6. Sato JJ, et al. 2006. Molecular study allies pinnipeds with otters and ferrets. Outgroups include no fossil taxa.

Issues: Palaeosinopa
was considered a non-Eutherian placental mammal. Here it nests within Carnivora without a priori assumptions clouding the selection of the inclusion group.

Figure 1. Palaeosinopa, complete and largely articulated. Body length about 50 cm. Tail adds 35 cm. From Rose and Koenigswald 2005.

Figure 4. Palaeosinopa, complete and largely articulated. Body length about 50 cm. Tail adds 35 cm. From Rose and Koenigswald 2005. This taxon nests with Phoca in the LRT.

Issues: Hyopsodus
was considered an odd-toed ungulate that was swift and lived in burrows. Here it nests within Carnivora without a priori assumptions clouding the selection of the inclusion group.

Figure 1. Hyopsodus as originally reconstructed (below) and as reconstructed here above in two views. This former condylarth now nests with dogs.

Figure 5. Hyopsodus as originally reconstructed (below) and as reconstructed here above in two views. This former condylarth now nests with dogs.

What about Enaliarctos?
Found in Late Oligocene strata, this earliest otariid (Fig. 6; Mitchell and Tedford 1973) nests between Zalophus and Hyopsodus in the LRT.

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

Figure 6. Enaliarctos nests between Zalophus and Hyopsodus in the LRT. The long bone around the knees is a baculum, or penis bone, found only in males.

What about Puijila?
Puijila darwini (Rybczynski, Dawson and Tedford 2009; Late Oligocene 23 mya;1m in length) was originally considered an extinct species of seal based chiefly on skull and tooth traits. Here in the LRT it nests at the base of the clade that produced phocid seals, not otarid seals. It was derived from a sister to Mustela the river otter and lived in and near high Arctic lakes.

Figure 6. Pujilia was considered a basalmost pinniped, but here nests at the base on only the phocids, not the otarids.

Figure 6. Puijila was considered a basalmost pinniped, but here nests at the base on only the phocids, not the otarids.

Differences
According to Wikipedia“Otariids use their front limbs primarily to propel themselves through the water, while phocids and walruses use their hind limbs. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while phocids and walruses lack these.”

By the way,
moving one seal next to the other in the LRT adds 14 steps.

You might remember
the LRT for all of its faults (the list grows shorter every day) was able to similarly separate toothed whales (Odontoceti) from baleen whales (Mysticeti) and document they each had separate terrestrial ancestors, tenrecs and desmostylians respectively. Given the overall similarity of Otariids to Phocids, their separation in the LRT is another demonstration of the acuity and authority of large gamut phylogenetic analyses.

By the way, since this is science…
this is something anyone can do. Repeat the experiment if you have doubts, and let me know what you get. Apparently earlier workers were excluding pertinent outgroup taxa from their analyses, and this is something we’ve seen over and over again. That’s what set the stage for ReptileEvolution.com and this blog.

References
Arnason U, et al. (6 other authors) 2006. Pinniped phylogeny and a new hypothesis for their origin and dispersal. Molecular Phylogenetics and Evolution. 41 (2): 345–54.
Flynn JJ, Finarelli JA, Zehr S, Hsu J and Nedbal MA 2005. Molecular phylogeny of the Carnivora (Mammalia): Assessing the impact of increased sampling on resolving enigmatic relationships. Systematic Biology. 54 (2): 317–37.
Higdon JW, Bininda-Emonds OR, Beck RM and Ferguson SH 2007. Phylogeny and divergence of the pinnipeds (Carnivora: Mammalia) assessed using a multigene dataset. BMC Evolutionary Biology. 7: 216.
Hunt RM Jr and Barnes LG 1994. Basicranial evidence for ursid affinity of the oldest pinnipeds. Proceedings of the San Diego Society of Natural History. 29: 57–67.
Lento GM, Hickson RE, Chambers GK and Penny D 1995. Use of spectral analysis to test hypotheses on the origin of pinnipeds. Molecular Biology and Evolution. 12(1): 28–52.
Mitchell E and Tedford RH 1973. The enaliarctinae a new group of extinct aquatic carnivora and a consideration of the origin of the otariidae. Bulletin of the American Museum of Natural History 151:284 pp.
Orlov YA 1933. Semantor macrurus (ordo Pinnipedia, Fam. Semantoridae Fam. nova) aus den Neogen-Ablagerungen Westsibiriens. Trudy Paleontologicheskii Institut Akademiia Nauk SSSR 2, 249-253.
Rybczynski N, Dawson MR. and Tedford RH 2009. A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia. Nature 458, 1021–1024.
Sato JJ, et al. (7 other authors) 2006. Evidence from nuclear DNA sequences sheds light on the phylogenetic relationships of Pinnipedia: Single origin with affinity to Musteloidea. Zoological Science. 23 (2): 125–46.

wiki/Pinniped
wiki/Enaliarctos
wiki/Phoca
wiki/Zalophus
wiki/Hyopsodus
wiki/Palaeosinopa

/tetrapod-zoology/pinnipeds-descended-from-one-ancestral-line-not-two/