Miacis cognitus

Miacis cognitus Temporal range: 39–33 Ma PreЄ Є O S D C P T J K Pg N Eocene to Oligocene
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Carnivora
Superfamily:	†Miacoidea
Genus:	†Miacis Gustafson, 1986
Species:	Miacis cognitus

Miacis cognitus is the only species of the diverse genus Miacis that is regarded as belonging to the crown-group Carnivora, within the Caniformia^[1]. The type specimen or holotype was discovered in Reeve's bonebed, western Texas, in the Chambers Tuff Formation in 1986.^[2] The University of Texas holds this specimen. It is the only confirmed fossil of this species.

Morphology

Fossil

The holotype is missing the mandible, upper canines, and zygomatic arch. The remainder of the skull is damaged, but relatively intact.

Teeth

It preserves the old style of many teeth, probably having forty two as compared to a modern carnivoran in the low thirties. With the later species of Miacis, the size of the certain teeth were decreasing, namely the foremost premolars. These teeth would eventually be lost all together, resulting in the fewer number of teeth seen in most modern carnivorans, especially feliforms, including extant hyenas, viverrids, herpestid, and the famously few-toothed felids. Most member of Miacoidea have forty four teeth, so this advanced species has already lost two bottom premolars. Though the upper canines are missing, these teeth can be reconstructed due to the foramen for the tooth root remaining intact. These canines were not particularly long or short, though they were not stout or shaped for great stress. The molars of this species were small and not suited for grinding large amounts of material. The premolars show carnassial form that makes carnivorans unique and were good for slicing rather than crushing or grinding.

drawing of the original holotype

Cryptoprocta ferox, a modern species resembling Miacis cognitus in lifestyle and low, long appearance.

Comparison to modern species

The skull of M. cognitus is long and low. Their modern-day relatives have often changed greatly, but there are several species whose osteology still appears much like Oligocene M. cognitus. In skull morphology alone, the African palm civet, Nandinia binotata takes the prize of looking most like its distant relative. Considering the morphology of the entire body, various species of civet, and herpetids are pretty close to what the living animal would have resembled. The most similar in body type would be between Viverridae's binturong (without the shag of hair) and the fossa of Eupleridae.

Digital morphology and CT scanning

The information core for the Digital Morphology library is generated using a state-of-the-art high-resolution X-ray computed tomographic (X-ray CT) scanner. This instrument is comparable to a conventional medical diagnostic CAT scanner, but with greater resolution and penetrating power. The CT scanner was custom built and optimally designed to explore the internal structure of natural objects and materials at mega- and microscopic levels. This instrument is at the center of The University of Texas High-Resolution X-ray Computed Tomography Facility (UTCT), a designated NSF-supported Multi-User Facility. Now in its seventh year, UTCT has scanned hundreds of rocks, meteorites, fossils, and modern organisms, providing unique data and visualizations for a wide range of interests in education and research.

The holotype of Miacis cognitus was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Timothy Rowe of The University of Texas at Austin, Department of Geological Sciences. The specimen was scanned by Richard Ketcham on 3 December 2007 along the coronal axis for a total of 1010 slices. Each 1024×1024-pixel slice is 0.08551 mm thick, with an interslice spacing of 0.08551 mm and a field of reconstruction of 40 mm.^[3]

Surface views allow one to roll, pitch, and yaw the specimen to see the fossil as though you were holding it in your hand. A second series is much more in depth slice movies, with coronal, transverse, and sagittal slices of the fossil. The last series is a dynamic cutaway from coronal, transverse, and sagittal angles as well.

a facial profile of Miacis cognitus

Lifestyle

Though only the skull of Miacis cognitus is known, there is a great deal of inference that can be applied to the species based on the knowledge of its relatives, whose fossil records are more intact. M. cognitus was most likely an arboreal. It was an omnivore that ate anything from amphibians to birds to eggs to small mammals. It would have been lean and slender, with a tail to aid it in balancing.^[4] It was likely very agile and claws that were probably retractable aided in its climbing. Because of the configuration of its legs, it was not well suited for sprints, instead it would have ambushed. Still a small animal hiding the wake of creodonts, it would have preferred hiding to being out in the open and it may have been nocturnal or crepuscular as many small carnivorans are today.

Evolution

The mass extinction of the K-T event left many ecological niches open and powerful competition to fill them. Species who previously had needed to be small and hide as a way of life suddenly had the requirement released and there was a strong pressure to get big and successful. The Paleocene was a period of struggle in which animals that had been occupied with avoiding being eaten by dominant species, namely the dinosaurs, were suddenly competing against each other to become the next dominant organism. Most mammals were small, many the size of rabbits, few exceeding the size of cats. The competition between the two groups of predatory mammals, the creodonts and the Carnivoramorpha, resulted in the better adapted creodonts occupying the position of top predators. As the Eocene continued, they were the dominant hunters and the carnivorans remained relatively small, staying out of the way. Miacis lived during this period. Miacis belongs to the stem-group Carnivoramorpha that began to diversify during the middle and late Oligocene. Two major lineages appeared, the feliformes (cats, hyenas, civets, palm civets, etc.) and the caniformes (dogs, bears, pinnipeds, skunks, ferrets, etc.). The tables began to turn and the creodonts were struggling during the late Oligocene when the carnivorans, while small, were well off. Open niches left by extinct predators were quickly filled by the small carnivorans, and they became larger and more diverse until their peak in the Miocene when the last of the creodonts had died out.

Ecology

the local, large oreodont Merycoidodon

the medium-sized equid Mesohippus

Reeves bonebed is well known for it oreodonts, especially Bathygenys. This oreodont, related to modern llamas, is very common in the formation, and being small for an oreodont at about thirteen pounds, it would have been hunted by Miacis regularly. The creodont Hyaenodon was also present in the fossil bed, and it is probably that this larger creodont hunted the larger oreodont Merycoidodon who, at over two hundred pounds, would have killed attacking Miacis. The large brontothere Menodus would have been far too large for Miacis, and if anything would have been a danger for the small carnivore. Miacis would have avoided this species completely to avoid injury from massive perissodactylid. The medium-sized herbivores Agriochoerus, Hyracodon, Mesohippus and Leptotragulus, were again probably Hyaenodon prey and Miacis would have left it alone completely. Leptomeryx inhabited the same range and was smaller. Its young would have been ample prey, and the adults might have been tackled. The rodent Ardynomys was of perfect size and would have been hunted by Miacis. Within the region, there were many smaller rodents, and with so many animals in the area that were too large for Miacis to kill, it is likely that the small animal, somewhere between the size of a cat and a begal, would have been a prolific rodent hunter and a hunter of Bathygenys.^[5]

References

1. ^ Spaulding, M.; Flynn J.J.; Stucky, R.K. (2010) Anew basal Carnivoramorphan (Mammalia) from the ‘Bridger B’ (Black’s Fork Member, Bridger Formation, Bridgerian NALMA, Middel Eocene) of Wyoming, USA. Paleontology 53: 815-832. DOI: 10.1111/j.1475-4983.2010.00963.x
2. ^ Paleobiology Database. "Miacis cognitus Gustafson 1986 (carnivoran)". http://www.paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=48481. 
3. ^ Digimorph (Digital morphology). "Miacis cognitus, extinct carnivoran". http://digimorph.org/specimens/Miacis_cognitus/. 
4. ^ "Miacis". http://www.myetymology.com/encyclopedia/Miacis.html. 
5. ^ University of Texas, Austin. Bonebed&county=&state=&specid=&geol_form=&epoch=&site_no=&showimage=&geog=&country=&others=&geol=&epoch=&period=&age=&era=&strat=&formation=&group=&horizon=&member=&sorted_by= "Reeves bonebed". http://www.npl.utexas.edu/vpl/databases/index.php?page=33&page_size=10&mode=search&action=search&searchtype=&taxon=&locality=Reeves Bonebed&county=&state=&specid=&geol_form=&epoch=&site_no=&showimage=&geog=&country=&others=&geol=&epoch=&period=&age=&era=&strat=&formation=&group=&horizon=&member=&sorted_by=. 

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