Monday, March 24, 2014

Andrewsarchus mongoliensis

Andrewsarchus mongoliensis is one of the more famous mammals of the Eocene. It was long hailed as the largest mammalian predator to have ever lived until it became recognized that there were entelodonts and bears that grew just as large.

A cast of the only skull known of Andrewsarchus mongoliensis, on display
at the American Museum of Natural History in New York City. Wiki
Etymology
Andrewsarchus was named for renowned explorer and fossil hunter Roy Chapman Andrews who led an expedition to the Gobi Desert in 1932, which was sponsored by the American Museum of Natural History. The actual fossil remains (see picture above) were discovered by Kan Chuan Pao, a member of Andrews' team. The original specimen is on display at the American Museum of Natural History and remains the only known evidence of this species to this day.

This species was once believed to be part of the family Mesonychidae, a group of predatory ungulates that lived from the early Paleocene to the late Eocene, and for a time was nicknamed "the giant Mesonychid of Mongolia". However, recent research has confirmed that this animal is actually a basal Artiodactyl, an early offshoot of the branch that gave rise to whales, hippos, and entelodonts. 

Habitat & Distribution
At the time Andrewsarchus lived, the former island continent of India had collided with southern Asia, closing off the eastern part of the Tethys Sea and pushing up the Himalayan Mountain Range. The resulting rain shadow resulted in intense monsoon seasons to the south, but robbed central Asia of much of its rainfall and caused it to become drier. Forests receded, causing drier open woodland and savanna habitats to open up. In response to these new open conditions, herbivores started to diversify and many became larger and longer-legged, and this in turn led to an increase in predator diversity as well. Andrewsarchus was among these new predators. In fact, Andrewsarchus was the largest land predator known to have walked the Earth since the extinction of the dinosaurs 25 million years earlier.

Physical Attributes
Andrewsarchus is known only from a single skull that was 83cm long and 56cm wide at the widest point. From this skull we know that its eyes were very small and set rather low close to the mouth. This resulted in limited binocular vision and the animal's sense of sight was probably somewhat less important to it. The long nasal tube implies an extended surface area for olfactory receptors and an acute sense of smell. The animal has the complete placental mammal dental formula of I3/3 + C1/1 + P4/4 + M3/3 x 2 = 44 teeth. The rostrum was narrow but strong, with large canines and incisors. This suggests that the jaws were capable of seizing and dispatching prey its own size or slightly larger. The molars and premolars are large, blunt, and somewhat triangular in shape; a good structure for gripping and perhaps cracking open bones, but ill-suited for grinding plant matter.

Based on the size of the skull, Andrewsarchus would have measured around 1.8m (6ft) high at the shoulder and 3.4m (11ft) in head-and-body length, with a weight of slightly over 1,000kg. This estimate puts it in the same league with other great mammalian predators such as the Short-faced Bear (Arctodus simus), the Terminator Pig (Daeodon shoshonensis), and the giant hyaenodont Megistotherium osteothlastes

Because the body of Andrewsarchus has never been found, any attempts to reconstruct it are purely speculative. The below restoration is based on similarly-sized land predators, specifically Daeodon shoshonensis and Arctodus simus (with a little hippo reference for the feet). As a large carnivore adapted to life in open country, this animal would have benefited from long, slender legs and a short torso allowing it to cover great distances with minimal energy.The large head would have been supported by a powerful neck and shoulders. As an artiodactyl, its toes would have ended in blunt hooves, which would have protected its foot bones when running over rough terrain. In the hot climate of the late Eocene, an animal this size is likely to have had a coat of short, sparse fur similar to that of large bovines in southern Asia and Africa of the genera Bubalus and Bos. As with other carnivores, thick skin and coarse fur around the neck region would have protected it from bites during aggressive encounters between males.




Ecology & Behavior
Based on the skull and dentition alone, the picture that emerges is of a carnivore that could attack and kill its own prey, but was also an active scavenger and frequent kleptoparasite. With its well-developed sense of smell it could detect the smell of blood or decaying flesh from miles away. Potential prey animals included brontotheres and rhinos. As the largest predator in its environment it could dominate any carcass it came across. Powerful jaws and sturdy teeth enabled it to smash open very large bones to utilize the highly nutritious marrow within. Other predators that lived in the same environment as Andrewsarchus included the creodont Sarkastodon mongoliensis, and the Mesonychids Harpagolestes immanis and Mongolestes robustus

References & Further Reading
Spaulding M, O'Leary MA, Gatesy J (2009) Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution. PLoS ONE 4(9): e7062. doi:10.1371/journal.pone.0007062 <http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0007062>

Friday, March 14, 2014

Land Crocs: Archosaur Predators in the Age of Mammals

When we think of modern crocodiles and their relatives, the image of an aquatic animal that is relatively slow-moving on land typically comes to mind. However, the animals that we know today are but one small (and highly specialized) part of a much larger and more diverse group of animals, most of which were better suited to life on dry land.

Land-based crocodilians were very common throughout the Mesozoic Era and were incredibly diverse with a broad range of morphologies and several distinct families with many species between them. Despite experiencing a considerable drop in diversity at the end of the Cretaceous, crocodiles were among the first animals to take over the terrestrial predator niches that were left vacant by the theropod dinosaurs. As the Cenozoic Era progressed, competition from placental mammals has largely discouraged the long-term success of these animals on the continents of North America, Eurasia, and Africa. However, these predators did remain abundant on the island continents of South America and Australia.

Pristichampsidae
Pristichampsids appear in the fossil record immediately after the Cretaceous-Paleogene extinction event. They were a rather short-lived family with a temporal range spanning the early Paleocene to the middle Eocene. Closely related to living crocodiles, these animals inhabited Eurasia and North America. They gradually faded into extinction as mammalian carnivores were becoming more diverse. Three genera are currently recognized; Planocrania, Boverisuchus, and Pristichampsus. In their heyday though, they were major predators of the small to mid-sized mammalian herbivores with which they coexisted.
Heads of two Pristichampsids; Planocrania datangensis
(top) and Pristichampsus rollinattii (bottom).
 Mekosuchidae
Mekosuchids had a much longer run than the earlier Pristichampsids, having existed from the Eocene all the way into the Holocene where they briefly coexisted with humans as recently as 3,000ya. These crocs inhabited Australia and its nearby islands. The group's longevity may be due to the absence of placental mammals in these regions. Most of them appear to have been at least partly aquatic. Others, such as Mekosuchus and Quinkana, evolved to become fully terrestrial. The latter of the two genera was once mistakenly interpreted as a late-surviving Pristichampsid by early researchers.
Heads of two late-surviving Mekosuchids; Mekosuchus
inexpectatus (top) from Holocene New Caledonia and
Quinkana fortirostrum (bottom) from Pleistocene Australia.
Sebecidae
The sebecosuchians were part of a much more diverse, mostly Gondwanan* group of crocodilians known as the Notosuchia, which first appeared during the early Cretaceous. The sebecosuchians comprise two families; the late Cretaceous Baurusuchidae, and the Cenozoic Sebecidae which we will be focusing on in this blog. Sebecids were a successful group that lasted from the early Paleocene (66mya) to the middle Miocene (11mya). They were an exclusively South American family that, together with the sparassodont marsupials and Phorusrhacid birds, formed an important part of the predator guild on that continent.
Restored head of Sebecus icaeorhinus, a Sebecid.
Crocodylidae
Living crocodilians are renowned for being aquatic predators, more at home in the water than the land. However, the affinity these animals have to water varies between the species and some do have adaptations that make them better adapted for terrestrial locomotion. 

The Cuban Crocodile (Crocodylus rhombifer) evolved on the Caribbean islands where it had little or no competition from mammalian predators. It is one of the largest endemic carnivores in the region second only to the American Crocodile (Crocodylus acutus). Compared to others within its family, it has brighter adult coloration, larger teeth, rougher skin, longer legs, and reduced webbing between its toes. Though still amphibious by nature, it spends much more of its time on land than other crocodiles and it runs and leaps very well. Fossil evidence shows that these crocs once fed on extinct ground sloths, and the above attributes would have enabled them to hunt these animals on land. In fact, a colony of this species at Gatorland, Florida has exhibited possible pack-hunting behavior which could have helped it hunt such extinct megafauna. Other extant crocodilians are also known to hunt cooperatively, which raises implications into what hunting methods the extinct species above were capable of.
The Cuban Crocodile (Crocodylus rhombifer) spends more
time on land than any other extant crocodilian. Wiki
 Another island crocodile, the extinct Horned Crocodile (Voay robustus) of Madagascar, exhibited characteristics that would have made it well-suited to terrestrial life. Compared to the Cuban Crocodile, this species had even more robust limbs, a shorter, deeper snout, and lower, more anteriorly-facing nostrils. On an island with few mammalian predators, this crocodile was one its top predators. It only became extinct after humans arrived a few thousand years ago.
The skull of the extinct Malagasy Horned Crocodile
(Voay robustus). Wiki
Glossary*
Gondwana: the more southerly of two ancient supercontinents that consisted of South America, Africa, Antarctica, Australia, India, Arabia, and Madagascar

References & Further Reading
Ross, Charles A. “Crocodiles and Alligators”. New York, NY. Facts on File, Inc. ISBN: 0-8160-2174-0

Brochu CA (2007). “Morphology, relationships, and biogeographical significance of an extinct horned crocodile (Crocodylia, Crocodylidae) from the Quaternary of Madagascar”. Zoological Journal of the Linnean Society 150(4): 835-863.

Molnar EM (2010). “A new reconstruction of the skull of Sebecus icaeorhinus (Crocodyliformes: Sebecosuchia) from the Eocene of Argentina”. Bazilian Geographical Journal: Geosciences and Humanities research medium 1(2): 314-330

Pol D, Powell JE (2011). “A new Sebecid mesoeucrocodylian from the Rio Loro Formation (Paleocene) of north-western Argentina”. Zoological Journal of the Linnean Society 163: 7-36


Paolillo A, Linares OJ (2007). “Nuevos Cocodrilos Sebecosuchia del Cenezoico Suramericano (Mesosuchia: Crocodylia)”. Paleobiologia Neotropical 3: 1-25 <Full Article>