Tuesday, November 3, 2015

Steulett's Terror Bird (Andalgalornis steuletti)

The dorsal (A), ventral (B), and lateral (C)
view of the skull of Andalgalornis steuletti.
Figure 1 from Degrange et al., 2010. (Wiki)
Steulett’s Terror Bird (Andalgalornis steuletti) is a mid-sized terror bird (Phorusrhacidae) from the late Miocene and early Pliocene. It ranks as one of the deadliest predators of its time, armed with speed, agility, and a guillotine-like beak.

Habitat & Distribution
Steulett’s Terror Bird lived during the upper Miocene to lower Pliocene of South America. Its remains are currently known only from Argentina, which was covered primarily by grassland and open woodland at the time.

Physical Attributes
This species is known from a partial skeleton as well as isolated bones. Steulett’s Terror Bird was roughly the size of a Greater Rhea (Rhea americana) but was more robust, weighing perhaps 40 to 50kg compared to 35kg for the average adult male rhea. It stood 90 to 100cm high at the level of the back and could raise its head to about 140cm above the ground, or about the height of the average 12 year-old. Overall, Steulett’s Terror Bird was more powerfully built in proportion to other terror birds. Its 37cm skull in particular had a viciously hooked beak which was very tall, somewhat blade-like, and capable of withstanding considerable forces. This suggests that these animals were able to handle relatively large prey items, perhaps even larger than themselves. Detailed studies of the neck vertebrae have been performed which suggest that the neck of Steulett’s Terror Bird would have been held in an S-shaped position while at rest and was adapted for rapid and powerful movements in the sagittal plane*, an ideal motion for striking prey.
Ecology & Behavior
Although it has traditionally been thought of as a hunter of small prey, the reinforced skull and blade-like beak of Steulett’s Terror Bird was built to withstand the considerable stresses involved in bringing down animals of considerable sizes. Potential prey items included a broad range of small to relatively large mammalian herbivores like hegetotheres* and cavimorph rodents, as well as many of the smaller toxodonts*, machraucheniids*, and ground sloths with which it coexisted. Whether it hunted singly or in small social groups is unknown, but regardless of its hunting method Steulett’s Terror Bird was undoubtedly one of the top predators of its time. Competing predators included several sparassodonts*, including the Marsupial Sabertooth (Thylacosmilus atrox), and at least four other species of terror bird, including smaller Scaglia’s Terror Bird (Llallawavis scagliai).
Hegetothere: an extinct family of small, rabbit-like ungulates endemic to South America.
Macrauucheniid: an extinct family of small to large, long-necked ungulates endemic to South America.
Sagittal plane: the vertical plane which separates the body into right and left halves.
Sparassodonta: an extinct order of predatory mammals endemic to South America from the Paleocene to the Pliocene.
Toxodont: an extinct family of sheep to bison-sized ungulates endemic to South America until the Pleistocene.

References & Further Reading
Tambussi CP, de Mendoza R, Degrange FJ, Picasso MB (2012). “Flexibility along the neck of the Neogene terror bird Andalgalornis steuletti (Aves Phorusrhacidae)”. PLoS ONE 7(5): e37701 <Full Article>

Degrange FJ, Tambussi CP, Moreno K, Witmer LM, Wroe S (2010). “Mechanical analysis of feeding behavior in the extinct “terror bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae)”. PLoS ONE 5(8): e11856 <Full Article>

Alvarenga HMF, Höfling E (2003). "Systematic revision of the Phorusrhacidae (Aves: Ralliformes)". Papéis Avulsos de Zoologia 43(4): 55–91 <Full Article>

Monday, November 2, 2015

Dire Wolf (Canis dirus)

The Dire Wolf (Canis dirus) is one of the most well-known predators of Pleistocene North America and recently made famous by the television series Game of Thrones (although the animals depicted in the series are much larger than their real-life counterparts). These were mid-sized predators closely related to the modern Coyote (C. latrans) and Gray Wolf (C. lupus), both of which coexisted with it during the Pleistocene.
Reconstructed and restored Dire Wolf skeleton from Rancho La Brea,
on display at the Perot Museum, Texas. Wiki.
The genus name Canis is the Greek word for “dog”. The species name dirus is derived from the Latin word dira, which means “ominous”, “fearful”, or “dreadful”.

Habitat & Distribution
Dire Wolves were geographically widespread throughout North and South America during the Pleistocene, reported from 136 localities in North America and 3 localities in South America. Whether or not this species originated in North or South America is a matter of much debate. Its range extended from Alberta, Canada to Tarija, Bolivia north-to-south. Its habitat included forested mountains to open grasslands and plains up to 2255m (7400ft). Compared to Pleistocene Gray Wolves, Dire Wolves appears to have favored somewhat wetter environments and their fossils are often found in association with ancient marshes, rivers, and lakes. Most famously, the highly productive site of Rancho La Brea, California has yielded the remains of over 4,000 individual Dire Wolves, which had become mired in the asphalt trap most likely after attempting to hunt or scavenge other animals that died there. These wolves are frequently found at the same localities as the Saber-toothed Cat (Smilodon fatalis).
Physical Attributes
Dire Wolves were comparable to Gray Wolves in terms of linear measurements. However, the Dire Wolf was physically more robust and heavily built than the Gray and could have weighed as much as 20% more than a comparably sized Gray Wolf. The skull in particular is proportionally larger and broader with a more prominent sagittal crest*. The canines and carnassials* are also larger. Measurements of the skull suggest that Dire Wolves had a bite force that was about 20% greater than that of the Gray Wolf. Dire Wolves also had sturdier limbs with shortened metapodials and a longer tail. Although undoubtedly a swift runner, Dire Wolves were not built to pursue fast-moving herbivores over great distances the way that Gray Wolves do. Instead, they may have relied more on ambushing prey, taking turns chasing it, or even by chasing animals into the water where its movements are hindered, all of which are tactics still employed by modern pack-hunting canids.
Ecology & Behavior
Because of their greater biting capacity, Dire Wolves have often been suspected of being bone-cracking specialists similar to modern hyenas of the genus Hyaena and extinct “hyena-dogs” of the genus Borophagus. However, studies of their overall dentition and bite damage of fossil bones have demonstrated that their teeth were no better at breaking bones than its modern relative the Gray Wolf, although it must be noted that these wolves can (and do) damage and ingest considerable amounts of bone during feeding. Furthermore, the crushing aspect of the Dire Wolf dentition is not any more developed than that of its modern relative while the slicing aspect is enhanced. Thus, it may be inferred that the robust skull, stronger bite, and larger teeth of the Dire Wolf, coupled with its sturdier frame, was more of an adaptation for seizing and pulling down and rapidly consuming larger herbivores rather than an adaptation for bone cracking. In this way, the hunting and feeding style of the Dire Wolf was likely more akin to that of modern canids like Dholes (Cuon alpinus) or Bush Dogs (Speothos venaticus) which frequently and efficiently hunt prey that is considerably larger than themselves.
Skeletal comparison of Gray Wolf (left) and Dire Wolf (right). Wiki
This, in turn, hints at the potential niche stratification between the Dire Wolf and the Gray Wolf during the Pleistocene. Gray Wolves were more commonly found in drier and more well-drained environments and can exist at higher altitudes than what the Dire Wolf appears to have tolerated. Pleistocene Gray Wolves would probably have focused more on ungulates weighing 50 to 300kg (110 to 660lbs) as its primary prey base. Modern Gray Wolves struggle to bring down ungulates weighing more than 500kg (1,102lbs), even in winter when such animals are easier to overwhelm, with hunts often lasting hours and with a high incidence of injury. Dire Wolves, on the other hand, were potentially hunting prey as much as 10 times their own weight, with a prey menu that included horses, tapir, large deer, camels, and bison, although they would also have opportunistically hunted smaller animals in their environment such as capybaras, giant beavers, and peccaries as encountered. Given that larger prey animals were a more prominent element of the Dire Wolf diet, it is also likely that these animals occurred in relatively larger packs numbering as many as 20 individuals, as opposed to modern Gray Wolves whose packs average 10 members in most areas.
Display of some of the thousands of Dire Wolf skulls recovered from Rancho La Brea
on display at the Page Museum, California. Wiki.
Carnassial: specialized shearing cheek teeth found in terrestrial mammalian predators.
Sagittal crest: the ridge of bone that runs down the midline of the skull in many mammals.

References & Further Reading
Pardi MI & Smith FA (2015). "Biotic responses of canids to the terminal Pleistocene megafauna extinction". Ecography 38: 1-11 <Abstract>

Anyonage W & Baker A (2006). “Craniodental morphology and feeding behavior in Canis dirus, the extinct Pleistocene dire wolf”. Journal of Zoology 269: 309-316 <Abstract>

Dundas RG (1999). “Quaternary records of the dire wolf, Canis dirus, in North and South America”. Boreas 28: 375-385 <Abstract>

Dire Wolf, Canis dirus (Mammalia; Carnivora; Canidae), from the Late Pleistocene (Rancholabrean) of East-Central Sonora, Mexico <Abstract>

Kurten B & Anderson E. “Pleistocene Mammals of North America”. New York City, New York: Columbia University Press, 1980. 171-172 <Book>