Pelagiarctos thomasi

[Infinity Blade]

Pelagiarctos thomasi


A reconstruction of the head of Pelagiarctos thomasi. © @ Robert Boessenecker

Temporal range: Middle Miocene (Burdigalian-Serravallian: ~17.5-13Ma)

Scientific classification:

Life
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Cephalochordata
Clade: Craniata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Clade: Eutheriodonta
Suborder: Cynodontia
Clade: Epicynodontia
Infraorder: Eucynodontia
Parvorder: Probainognathia
Superfamily: Chiniquodontoidea
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
Clade: Holotheria
Superlegion: Trechnotheria
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Clade: Eutheria
Infraclass: Placentalia
Subcohort: Exafroplacentalia
Magnorder: Boreoeutheria
Superorder: Laurasiatheria
(unranked): Ferae
(unranked): Carnivoramorpha
Order: Carnivora
Suborder: Caniformia
Infraorder: Arctoidea
(unranked): Pinnipedimorpha
Superfamily: Pinnipedia
Family: Odobenidae
Subfamily: Odobeninae
Genus: †Pelagiarctos
Species: †P. thomasi

Pelagiarctos is an extinct genus of walrus (Odobenidae) that lived in what is now the Sharktooth Hill Bonebed (Kern County, California) and "Topanga" Formation of southern California during the middle Miocene epoch ~17.5-13 million years ago.[1][2]

Body size:
Boessenecker & Churchill (2013) estimated the body weight of P. thomasi to have been ~350 kilograms.[2][3]

Phylogenetic relationships:
Pelagiarctos was found to have been a stem "imagotariine" walrus by Boessenecker & Churchill (2013); this is consistent with the conclusions of Barnes (1988). Likewise, Pelagiarctos and Imagotaria were found to have formed a sister relationship.[2]

Diet:
When Pelagiarctos was first described, it was concluded to have been a large, macrophagous, durophagous apex predator given its dental and mandibular morphology, its supposedly large size (although it was, in any case, a large animal, see above), and its rarity in its respective locality. In particular, the mandible of the holotype specimen was robust, symphyseal fusion was present in said mandible, and the posterior teeth were supposedly similar to those of the bone-cracking hyenas and borophagines.[1][2][3]

The specimen described by Boessenecker & Churchill lacked a fused symphysis in its mandible, which incidentally raised the possibilities that the two specimens were either of different species or that this was evidence of jaw morphology variation.[2][3] A fused symphysis does not necessarily indicate certain dietary preferences (case in point, it could fulfill multiple functions ranging from strengthening the jaw for suction feeding to resisting torsional stresses created by struggling prey). Nonetheless, the durophagous carnivorans that were considered analogous to Pelagiarctos by Barnes (1988) do or did not possess particularly well-fused mandibular symphyses; rather, they have or had rather loose ones that grant enough flexibility for jaw movements that prevent dental damage when practicing durophagy.[2][3] There were even further dissimilarities between durophagous carnivorans and Pelagiarctos. The post-canine teeth are proportionately small compared to the mandible and although Barnes described sharp cusps on these teeth in the holotype specimen, Boessenecker & Churchill could not find them in the specimen they described. In fact, they are actually blunt in Pelagiarctos.[2]

In reality, Pelagiarctos' dental morphology suggests a generalist piscivore.[2] The animal was still likely capable of predating and killing other pinnipeds, but it was not particularly specialized for macrophagy.[3] Boessenecker & Churchill describe Pelagiarctos as "a generalist predator, feeding on fish, invertebrates, and the occasional warm-blooded prey item."[2][3]

Rarity:
Given how Pelagiarctos is rare in the fossil record, it has been suggested that it only occasionally lived in where its remains were recovered. The Sharktooth Hill Bonebed was formed on the order of 700,000 years, which could mean that these animals only came to and inhabited the place when the climate was suitable for them.[2][3]

References:

[1] "A new fossil pinniped (Mammalia: Otariidae) from the middle Miocene Sharktooth Hill Bonebed, California" (Barnes, 1988).
[2] "A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos" (Boessenecker & Churchill, 2013).
[3] Dissecting the "Killer" Walrus by Brian Switek

[Infinity Blade]

Further evidence found against ancient 'killer walrus' theory

An Otago-led team of scientists using techniques from the field of dentistry is shedding new light on the evolution of walruses, fur seals and sea lions. The researchers have cast further doubt on previous claims that an ancient "killer walrus" was a marine mammal eater.

In a newly published article in the international journal The Science of Nature the multidisciplinary team of researchers report their analysis of the internal structure of tooth enamel in a fossil walrus from California, Pelagiarctos thomasi, and in teeth of modern pinnipeds the New Zealand fur seal and sea lion.

Study co-author Dr Carolina Loch says this was the first time the enamel ultrastructure of fur seals and sea lions, as well as the extinct walrus Pelagiarctos, was studied using scanning electron microscopy.

"Pelagiarctos was originally thought to have been a "killer walrus" that fed on large prey such as other marine mammals, but we found it has an enamel layer reasonably similar to that of modern New Zealand fur seals and sea lions, which are fish and squid eaters," Dr Loch says.

The enamel structure the researchers identified in Pelagiarctos meant the walrus was unlikely to be up to crunching through large bones without cracking its teeth -- suggesting that it was a dietary generalist like the modern New Zealand pinnipeds studied, she says.

Dr Loch says the study showed how using techniques and methods commonly employed in dentistry can answer questions with broader implications in the biology and evolution of animal species.

"Features and structures of the enamel layer have long been associated with differences in diet and tooth usage among animals, and can also help in the understanding the relationships among fossil and living species.

"Teeth are not only the focus of modern dentistry, but also valuable tools for biologists, archaeologists and paleontologists," Dr Loch says.

The study was conducted by Dr Loch, research fellow at the University of Otago's Sir John Walsh Research Institute, Faculty of Dentistry (and Otago alumna -- Department of Geology and Faculty of Dentistry); Dr Robert Boessenecker, College of Charleston USA (and Otago alumni -- Department of Geology); Dr Morgan Churchill (New York Institute of Technology College of Osteopathic Medicine, USA) and the late Professor Jules Kieser (Otago Faculty of Dentistry) -- who is always remembered for his prolific multidisciplinary dental research.

A 2013 paper by Drs Boessenecker and Churchill was the first to cast doubt on the "killer walrus" claims and the latest findings bolster their case, Dr Loch says.
link
-------------------------------------------------------------------------------------------------------------------------
Enamel ultrastructure of fossil and modern pinnipeds: evaluating hypotheses of feeding adaptations in the extinct walrus Pelagiarctos
Loch, C., Boessenecker, R.W., Churchill, M., Kieser, J. (2016)

"This study aimed to assess the enamel ultrastructure in modern otariid pinnipeds and in the extinct walrus Pelagiarctos. Teeth of the New Zealand fur seal (Arctocephalus forsteri), sea lion (Phocarctos hookeri), and fossil walrus Pelagiarctos thomasi were embedded, sectioned, etched, and analyzed via scanning electron microscopy. The enamel of NZ otariids and Pelagiarctos was prismatic and moderately thick, measuring 150–450 μm on average. It consisted of transversely oriented Hunter-Schreger bands (HSBs) from the enamel-dentine junction (EDJ) to near the outer surface, where it faded into prismless enamel less than 10 μm thick. The width of HSB was variable and averaged between 6 and 10 prisms, and they presented an undulating course both in longitudinal and cross sections. The overall organization of the enamel was similar in all teeth sampled; however, the enamel was thicker in canines and postcanines than in incisors. The crowns of all teeth sampled were uniformly covered by enamel; however, the grooved incisors lacked an enamel cover on the posterior side of the buccal face. Large tubules and tuft-like structures were seen at the EDJ. HSB enamel as well as tubules and tufts at the EDJ suggest increased occlusal loads during feeding, a biomechanical adaptation to avoid enamel cracking and failure. Despite overall simplification in tooth morphology and reduced mastication, the fossil and modern pinnipeds analyzed here retained the complex undulating HSB structure of other fossils and living Carnivora, while other marine mammals such as cetaceans developed simplified radial enamel."

link

[jhg]

This creature so needs a movie about it!

[Infinity Blade]

Please post more meaningful comments in the profiles. Thank you.