Saturday, October 15, 2016

Xenungulates: The Strange Ungulates

The Xenungulata, whose name means “strange ungulates”, is an archaic and poorly known group of South American ungulates that were temporally restricted to the Paleocene. The order, which was coined by Paula Couto in 1958, contains a single family (Carodniidae) with five currently recognized species.

Mounted skeleton of Carodnia vierai. Photo by Lily P. Bergqvist from
General Characteristics
As with most Paleocene animals, xenungulate fossils are very scarce. By far the best known member of the group is the species Carodina vierai, which is known from reasonably complete skeletal material. C. vieirai was somewhat tapir-like in its size, build, and presumably in its ecology. It had a complete dentition with procumbunt, laterally flattened incisors and particularly large canines which may have been used as combat weapons against rivals predators. The cheek teeth were low-crowned and well-suited to a browsing diet of mostly leaves, twigs, and fruits. The foot bones are unique among South American ungulates in that they are short and robust, and the digits terminate in broad, flat, unfissured hoof-like unguals*. The limbs are short and somewhat slender, and their anatomy seems to suggest that the animal had a gait similar to that of an African Elephant (Loxodonta africana).

Xenungulates have been considered to be close relatives to both the South American pyrotheres and North American uintatheres based mainly on dental similarities with the two groups; the first and second molars are bilophodont* as they are in advanced pyrotheres, and the third molar is more complex and lophate* as in uintatheres. However, these characteristics are likely the result of convergent evolution rather than a shared heritage. The discovery of the more basal Etayoa in 1987 confirmed this; unlike uinatheres or its later relative Carodnia, it lacked a lophate third molar, thus confirming that this trait evolved separately from uintatheres. Furthermore, since bilophodonty is not present in basal pyrotheres from the early Eocene, we can also conclude that this trait evolved separately in the Xenungulata and the Pyrotheria.

Genera & Species
Etayoa (Villarroel, 1987)
Known only from upper Paleocene Colombia, Etayoa bacatensis is the most basal xenungulate yet discovered. It lacks the distinctive lophate molars observed in the more well-known genus Carodnia.

Notoetayoa (Gelfo, Lopez, Bond, 2008)
Notoetayoa gargantuai is the most recently discovered xenungulate from the middle Paleocene of Colombia. In body size it seems to have been smaller than Carodnia but larger than Etayoa.

Carodnia (Simpson, 1935)
Three species have been described within this genus; C. feruglioi (Paula Couto, 1952), C. cabrerai (Simpson, 1935), and C. vieirai (Simpson, 1935). The former two species are known only from dental remains which differ only in size, which raises the possibility of the two representing different growth stages or perhaps different genders of a single species. The third species, C. vieirai, is known from much more complete dental, cranial, and postcranial material. In fact the skeleton of this animal is among the most complete for any Paleocene mammal. The now invalid genus Ctalecarodnia is a synonym of Carodnia.

A parsimonious tree showing the proposed relationships between members of Xenungulata.
Cropped from Figure 3 in Gelfo et al., 2008.
Bilophodont: when two separate crests form transverse, often ring-shaped ridges on the tooth.
Lophate: describes a cheek tooth with heightened ridges or crests.
Ungual: the distal phalanx from which the claw or hoof grows.

References & Further Reading
Farina, Richard A; Vizcaino, Sergio F; Iuliis, Gerry De. “Megafauna: Giant Beasts of Pleistocene South America”. New York, New York: Columbia University Press, 2012. <Book>

Gelfo JN, Lopez GM, Bond M (2008). “A New Xenungulata (Mammalia) from the Paleocene of Patagonia, Argentina”. Journal of Paleontology 82(2): 329-335 <Full Article>

Rose, Kenneth D. “The Beginning of the Age of Mammals”. John Hopkins University Press, 2006. Simpson GG (1935). “Descriptions of the Oldest Known South American Mammals, from the Rio Chico Formation”. American Museum Novitates 793: 1-25 <Full Article>

Gingerich PD (1985). “South American Mammals in the Paleocene of North America”. pp 123-137 in FG Stehli (ed), The Great American Biotic Interchange <Full Article>

Paula Couto (1952). “Fossil Mammals from the Beginning of the Cenozoic in Brazil”. Bulletin of the American Museum of Natural History 99(6): 355-394 <Full Article>

Tuesday, September 27, 2016

Gaston's Giant Bird (Gastornis gigantea)

Gaston’s Giant Bird (Gastornis gigantea) was a large, flightless bird that inhabited North America during the early Eocene. It was the largest and one of the last members of its genus and the largest terrestrial vertebrate in its ecosystem. 

Mounted skeleton of Gastornis gigantea. Wiki.
Gastornis has had an interesting taxonomic history in which two generic names were used in Europe and North America; Gastornis and Diatryma respectively. Coined in 1855, the genus Gastornis was named in honor of Gaston Plante who discovered the first fossils of the type species (G. parisiensis) in Paris, France, combined with the Greek word ornis, meaning “bird”. The now defunct name Diatryma was coined by American paleontologist Edward Drinker Cope in 1876 based on fragmentary material recovered from the Wasatch Formation of New Mexico. Diatryma was derived from the Greek word diatreme, meaning “through the hole”, referring to the large foramina that penetrated some of the foot bones.

The similarity between the European and North American specimens was acknowledged as early as 1884 by American ornithologist Elliott Coues, but researchers debated the relationship between the two for nearly a century. Starting from the early 1980s, several authors began to recognize the great degree of similarity between the European and North American specimens and began to place both within the family Gastornithidae. Thereafter, scientists tentatively began to accept the synonymy between the genera pending a comprehensive review of the anatomy of these birds in 1997. Diatryma is now recognized as invalid, with Gastornis now taking priority. The scientific name of the North American species Gastornis gigantea, formerly Diatryma gigantea, may therefore be translated as “Gaston’s Giant Bird”.

Habitat & Distribution
Fossils of Gastornis have been found in Paleogene deposits from the northern European countries of France, Germany, Belgium, England, as well as the United States, Canada, and China. During the late Paleocene, Gastornis seems to have been mostly restricted to Europe which at that time was a tropical archipelago similar to modern day Indonesia. During the early Eocene, when Europe became connected with the other northern continents, Gastornis appears to have migrated to North America and Asia where they gave rise to G. gigantea and G. xichuanensis respectively. These birds inhabited the tropical rainforest environments which covered the Northern Hemisphere throughout much of the Paleogene.

Physical Attributes
The largest of the genus, Gaston’s Giant Bird was still smaller overall than an Ostrich (Struthio camelus) in terms of linear measurements. However, the former was much more heavily-built with an estimated body weight of 175kg (385.8lbs), compared to the maximum 145kg (320lbs) for the largest modern Ostriches. The height at the top of the hips was about 130cm (4.3ft), and the height at the top of the head with a fully outstretched neck was about 200cm (6.7ft). The skull was large with a deep, laterally-compressed beak with nostrils positioned in front of the eyes and midway up the skull. The skeleton was robust with particularly short and massive vertebrae and a compact torso. The vestigial wings were highly reduced and probably would have been indistinguishable from within the long body feathers as seen in many modern flightless birds. A 2007 study has found the Gastornithidae to be an early offshoot of the avian order Anseriformes, the group which contains all of today’s waterfowl (ducks, geese, swans, etc).

Although Gastornis fossils have been numerous and well-preserved, its ecology has been a matter of controversy. Past authors has suggested that these birds were herbivores. The traditional portrayal of Gastornis, however, has been that of a large carnivore which hunted the diverse array of smaller mammals with which it coexisted. This view of a predatory Gastornis was brought to life in the first episode of Walking With Beasts (2001), which depicts a female G. geiselensis terrorizing a forest full of bite-sized mammals. 

Those who favor the predator hypothesis site the birds’ massive skull, powerful jaw muscles, and large size. However, as I alluded to in an older blog post (here), the blunt and straight-edged beak of Gastornis was ill-suited for stripping flesh from carcasses. The eyes are set more to the sides of its head, an adaptation most commonly seen in prey animals that provides a wider field of vision while limiting the visual field overlap which predators use to judge the distance between themselves and their prey. Furthermore, each of Gastormis’ toes terminated in a blunt hoof-like claw, useful for gaining traction on almost any surface but not useful for pinning down its prey.

Close-up of the skull of Gastornis gigantea. Although large and  undoubtedly 
powerful, the beak lacks the hooked tip which modern predatory birds use to 
tear the flesh of prey. The small eyes are also set far apart, limiting the depth 
perception which predators rely on when seizing prey. Wiki.
Several recent studies have revealed further details about the diet of Gastornis. The jaw musculature has been found to be similar to that of modern herbivorous birds. Studies of the calcium isotopes in the bones carried out in 2013 yielded no evidence of animal matter in these birds’ diets, instead showing similar biochemical profiles to those known fossil dinosaurian and mammalian herbivores.

Ecology & Behavior
Rather than being giant predators, Gastornis have been confirmed instead to have been a large browsing herbivores; more like giant land-parrots than terror birds. With their powerful, crushing beaks, they likely fed on a variety of hard-shelled fruits and seeds, as well as the woody branches of trees and bushes. Furthermore, adult Gastornis were the tallest animals in their ecosystems and thus would have had competitive advantage over other herbivores in their environments in being able to access plants that were higher off the ground. The predator niches of the late Paleocene-early Eocene in the Northern Hemisphere were instead occupied by terrestrial crocodilians (Pristichampsidae), creodonts (Oxyaenidae and Hyaenodontidae), mesonychians (Mesonychidae), and early carnivorans (Miacidae).

Gastornis were the largest terrestrial vertebrates in Europe during the Paleocene, a time when the largest land mammals were no larger than tapirs. The structure of the Paleocene terrestrial ecosystem in Europe may have been similar to that of Madagascar prior to human occupation, which had been home to a host of small to moderately-large mammals with the largest terrestrial vertebrate being the giant herbivorous Elephant Bird (Aepyornis maximus). After Europe became connected to the other northern continents, Gastornis was successful in colonizing these new regions where, for a time, they remained the largest herbivores on the landscape. However, these birds increasingly had to compete with ever larger mammalian herbivores, some of which having attained the size of modern rhinos by the middle Eocene when these birds seem to have disappeared from the fossil record.

Eggs attributed to Gastornis have been identified from parts of Europe, including southern France. These eggs were slightly larger than those of a modern Ostrich, with a maximum length of 17.8cm and a diameter of 12cm, an estimated volume of 1330.4cm3, and an estimated weight of 1.4kg when fresh. These were, in fact, the second largest bird eggs ever known, surpassed only by the Elephant Bird of Madagascar. On the basis of egg mass, the parent birds have been estimated to have been between 135.4 to 156.4kg. These estimates match the mass estimates for Gastornis and no other bird alive at the time were large enough to have laid these eggs.

References & Further Reading
Angst D, Lecuyer C, Amiot R, Buffetaut E, Fourel F, Martineau F, Legendre S, Abourachid A, Herrel A (2014). “Isotopic and anatomical evidence of an herbivorous diet in the early Tertiary giant bird Gastornis. Implications for the structure of Paleocene terrestrial ecosystems”. Naturwissenschaften 101(4): 313-322 <Full Article>

Angst D, Buffetaut E, Lecuyer C, Amiot R, Smektala F, Giner S, Mechin A, Mechin P, Amoros A, Leroy L, Guiomar M, Tong H, Martinez A (2014). "Fossil avian eggs from the Palaeogene of southern France: new size estimates and a possible taxonomic identification of the egg-layer". Geology Magazine: 1-10 <Full Article>

Mustoe GE, Tucker DS, Kemplin KL (2012). “Giant Eocene bird footprints from northwest Washington, USA”. Paleontology 55(6): 1293-1305 <Abstract>

Buffetaut E (2008). “First evidence of the giant bird Gastornis from southern Europe: a tibiotarsus from the lower Eocene of Saint-Papoul (Aude, southern France)”. Oryctos 7: 75-82 <Full Article>

Agnolin F (2007). "Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina". Revista del Museo Argentino de Ciencias Naturales 9: 15-25 <Full Article>

Witmer LM & Rose KD (1991). “Biomechanics of the jaw apparatus of the gigantic Eocene bird Diatryma: implications for diet and mode of life”. Paleobiology 17(2): 95-120 <Full Article>

Cockerell TDA (1923). “The supposed plumage of the Eocene bird Diatryma”. American Museum Novitates 62: 1-4 <Full Article>

Friday, September 2, 2016

What is a Tapir?

Tapirs (Tapiridae) are forest-dwelling perissodactyls that first appear in the fossil record during the early Eocene, about 55mya. The earliest tapirs were small animals no bigger than modern house cats, with Oligocene forms growing substantially larger reaching the size of domestic pigs. Tapirs appear to have a rather conservative body plan which has allowed them to survive relatively unchanged apart from size since their Eocene origins. The modern genus Tapirus, for example, would have been instantly recognizable to us if we could travel back in time 10 million years or more. Such conservatism is in stark contrast to other modern perissodactyl groups, horses and rhinos, which have achieved a diverse array of morphotypes and ecologic niches over their long evolutionary histories. The presence of tapir bones at a fossil site is seen as a reliable indicator that the locality once covered by a well-watered forested environment during prehistoric times. There are four accepted species of tapir alive today together with a possible fifth species (Tapirus kabomani) described in 2013 whose status is still being questioned.

The four accepted species of modern tapir. From left-to-right, top-to-bottom:
the Lowland Tapir (Tapirus terrestris), Baird's Tapir (Tapirus bairdii),
Mountain Tapir (Tapirus pinchaque), and the Asian Tapir (Tapirus indicus).
The most notable attribute of tapir anatomy is the fusion of the nose and upper lip into a short trunk or proboscis, similar to those of elephants. This appendage is a highly mobile and dexterous tool used to gather and manipulate food. To support this structure, tapir skulls have had to undergo considerable modifications compared to other perissodactyls. The nasals have migrated to the back of the skull and the eyes have been moved forward. Certain elements of the facial skeleton itself have been retracted and reduced and the airway has become inclined relative to the long axis of the skull. These cranial features in modern tapirs that are associated with the presence of the proboscis are present as far back as the late Eocene-early Oligocene genus Colodon.

The skull of the early Oligocene tapir Colodon occidentalis (top)
is remarkably similar to that of the modern Lowland Tapir (middle-left).
Both animals have skulls with greatly retracted nasal bones and ample
insertion areas for the nasal cartilage and musculature (middle-right)
which comprise the highly mobile proboscis (bottom).

image: Figure 1 from Colbert, 2005.
Middle-right image: Figure 2 from Witmer, 1999.
Bottom image from Wiki.
All tapirs, both living and extinct, are small to large forest-dwelling browsers with muscular, compact bodies and digitigrade feet. There are four toes on the front feet and three on the rear feet. For the first months of their lives, tapir calves have a coat of short, brownish fur patterned with pale stripes and spots which serve as efficient camouflage on the forest floor. Tapir cheek teeth are bilophodont, that is, the transverse crests have joined to form a continuous ridge which helps break softer plants. Ironically, the canine teeth are greatly reduced in size and often lost in adult tapirs, while the third upper premolars are enlarged and are caniniform in shape.

A mounted tapir skeleton at the Montbeliard Museum of Natural History (Wiki).
The modern tapirs are generally thought of as short-haired animals associated with the warm forests of the tropical zone. However, tapirs have undergone most of their evolution and dispersal in the seasonally cool, temperate forests of Eurasia and North America. The vast expanses of grassland and desert habitat in the Middle East and northern Africa have served as a natural barrier preventing the group’s dispersal into Subsaharan Africa, and their dispersal into South America was a relatively recent event that took place during the Pleistocene. Among the four living tapirs, the Mountain Tapir is adapted to live in the cold, montane forests of the Andes Mountains and is clad with a thick coat of wool-like fur. Thus, this species is a good model when attempting to reconstruct the more northerly distributed tapirs that existed during the Pliocene and Pleistocene.

A Lowland Tapir calf at Hamburg Zoo in Germany. Tapir calves gradually lose their striped
coat pattern until they attain their adult coloration after their first year of life (Wiki).
All tapirs demonstrate a fondness for water and will spend large parts of the day swimming or resting in rivers and lakes. They will instinctively retreat to deep water when they sense a predator. Tapirs consume a wide variety of leaves and fruits in their respective habitats and are important seed dispersers in their ecosystems. Some plants have, in fact, formed mutual relationships with these animals in which they may only germinate after passing through the digestive tract of a tapir.

A Lowland Tapir swimming. All tapirs are partly aquatic in nature and spend
much of their time in bodies of freshwater (Wiki).
References & Further Reading
Voss RS, Helgen KM, Jansa SA (2014). “Extraordinary claims require extraordinary evidence: a comment on Cozzuol et al (2013)”. Journal of Mammalogy 95(4): 893-898 <Full Article>

Cozzuol MA, Clozato CL, Holanda EC, Rodrigues FHG, Nienow S, de Thoisey B, Redondo RAF, Santos FR (2013). “A new species of tapir from the Amazon”. Journal of Mammalogy 94(6): 1331-1345 <Full Article>

Colbert MW (2005). “The facial skeleton of the early Oligocene Colodon (Perissodactyla, Tapiroidea)”. Palaeontologia Electronica 8(1): 1-27 <Full Article>

Holbrook LT (2001). “Comparative osteology of early Tertiary Tapiromorphs (Mammalia, Perissodactyla)”. Zoological Journal of the Linnean Society 132: 1-54 <Full Article>

Witmer LM, Sampson SD, Solounias N (1999). “The proboscis of tapirs (Mammalia: Perissodactyla): a case study in novel narial anatomy”. Journal of Zoology 249: 249-267 <Full Article>

Jefferson GT (1989). “Late Cenozoic tapirs (Mammalia: Perissodactyla) of western North America”. Contributions in Science 406: 1-22 <Full Article>

Macdonald, David W. The Princeton Encyclopedia of MammalsPrincetonNew JerseyPrinceton University Press, 2009 <Book>

Saturday, August 20, 2016

More Than One Way to be a Carnivore: An Exposition of Carnivory

From an early age, many of us are introduced to the terms carnivore, herbivore, and omnivore; animals which consume other animals, plants, or both animals and plants respectively. These terms, while useful, are highly generalized and may overlook the often significant complexity in the diet of a given species. Many animals often considered to be ‘carnivorous’, for example, may ingest particular types of plants depending on the situation. Similarly, many animals traditionally thought of as ‘herbivores’ have been observed feeding on other animals, albeit in small quantities. This shows that animal feeding behaviors are not as fixed as we are taught in school and, in some instances, it may be necessary to use alternate means of classification. To simplify matters, most animals may be grouped into one of three categories based on the amount of animal matter that they consume: hypercarnivore, mesocarnivore, and hypocarnivore*.

*The following descriptions will focus exclusively on mammals, but the aforementioned terms still apply for other animal groupings.
Modern examples of a hypercarnivore (Jaguar), mesocarnivore (Red Fox),
and hypocarnivore (Red Panda). Scale represents the percentage of animal
matter which makes up the diet within each respective level of carnivory, note
that this varies between species.
A hypercarnivore is an animal for whom over 70% of the diet consists of animal matter, with non-animal matter (plants, fungi, or algae) being consumed rarely as a dietary supplement if at all. The prefix hyper- comes from the Greek language and in simplest terms translates to “over”. Thus, the word hypocarnivore describes an animal that is highly predacious and requires a huge amount of animal protein to sustain itself. Some hypercarnivores, such as cats, have a reduced ability to digest sugars or carbohydrates, and must therefore rely entirely on animal matter (the term “obligate carnivore” may also be used in this instance).

Hypercarnivory in terrestrial mammals is often, but not always, accompanied by a suite of changes to the skull and dental morphology. The face may be shortening and/or deepening of the face may occur in association with the reduction or loss of pre- and post-carnassial dentition. The carnassials themselves (pictured to the right), laterally compressed cheek teeth adapted for cutting through flesh, are often lengthened to expand the slicing surface while reducing the capacity for crushing. Hypercarnivorous taxa are relatively easy to identify in the fossil record because of these adaptations. Notable mammalian hypercarnivores include all members of Felidae (cats), Phocidae (seals), Otariidae (sea lions), and Cetacea (whales), as well as most members of Mustelidae (weasels), Herpestidae (mongooses), Chiroptera (bats), and Eulipotyphla (shrews).

Additionally, some hypercarnivores may be specialized in taking certain types of prey and are often categorized accordingly: insectivores (invertebrate-eaters), piscivores (fish-eaters), or molluscivores (mollusk-eaters) to name a few examples. It should be noted, however, that animals tend to be opportunists, very rarely limiting themselves to a single food source. River otters, for example, are adapted to a mostly fish-based (piscivorous) diet but will readily feed on many other types of animals including crustaceans, frogs, birds, and small terrestrial mammals whenever they are encountered.
Most otters, like this Giant Otter (Pteronura basiliensis), are piscivores;
hypercarnivores that specialize in eating fish (wiki).
A mesocarnivore is any predatory animal for whom animal matter comprises 31-70% of the diet. These animals actively feed on animal matter but will readily feed on non-animal foods such as fruits and fungi, hence the prefix meso- which means “middle”. Compared to hypercarnivores, mesocarnivores may be more numerous in a given terrestrial ecosystem due to their diet being more adaptable. Furthermore, the proportion of animal to non-animal matter may vary depending on location and seasonality. Red Foxes, for example, may feed almost exclusively on small animals for much of the year but may shift to a more plant-based diet during the autumn months in some parts of their range. Mesocarnivorous mammals often have relatively elongate skulls with a more compete dentition adapted for dedicated to piercing, slicing, and crushing.
The African Civet (Civettictis civetta) is a mesocarnivore which typically
feeds on small animals and carrion, but also readily feeds on fruits. Note
the expanded grinding area of the carnassials and molars (wiki).
The prefix hypo- means “below” or “beneath”. Thus, the term hypocarnivore describes an animal for whom animal matter makes up 30% or less of the total diet. To extract as much nutrients as possible from the plants they eat, hypocarnivores have expanded the grinding aspect of their dentition, having either reduced or lost those areas dedicated to slicing. Also, the stomach and intestines may be longer and more complex. Notable hypocarnivores among the Carnivora include most bears, raccoons, palm civets, and the Red Panda. These animals independently evolved from mesocarnivorous ancestors that became specialized for a more plant-based diet. Other examples include all primates, peccaries, pigs, fruit bats, and most rodents.
Upper dentition (P4, M1, & M2) of the Red Panda (Ailurus fulgens), a hypocarnivore
which descended from a predatory ancestor. Note loss of the shearing blade on the
carnassial, broadening of the molars, and the addition of extra grinding cusps
for processing tough plants such as bamboo. 
Like hypercarnivores, some hypocarnivores may specialize in feeding on particular types of foods, in this case non-animal matter, and may be categorized accordingly: frugivores (fruit-eaters), folivores (leaf-eaters), granivores (seed-eaters), and nectarivores (nectar-eaters), and fungivores (fungus-eaters). As with mesocarnivores, hypocarnivores may show considerable versatility in their diets depending on location and season. The diets of Black Bears and Brown Bears, for example, generally consist of 90 to 95% non-animal foods for much of the year. Immediately following winter hibernation, however, the bulk of their food intake may consist of winter-killed ungulates and other animals that failed to survive the winter. Other spikes in carnivory are associated with the spawning or birthing seasons of certain types of fish and ungulates respectively. The Polar Bear is a rare instance of a hypercarnivorous species evolving directly from a hypocarnivorous ancestor.
Most modern bears such as this Black Bear (Ursus americanus), shown here
feeding on dandelions, are hypocarivores whose diets are mostly plant-based.
However, these animals display extreme versatility and may become more
carnivorous depending on the season or other environmental factors. The
Polar Bear (Ursus maritimus) is a rare example of a hypercarnivore which
evolved directly from a hypocarnivorous ancestor.

Furthermore, virtually all animals which we traditionally think of as ‘herbivores’ will feed on small amounts of animal matter on occasion and would thus be more accurately considered to be hypocarnivores. Duikers, for example, are small antelopes endemic to the forests of Africa that are well-known to feed on numerous types of small animals such as insects, frogs, and birds in addition to their more regular diet of soft leaves and fallen fruits. There are numerous other examples carnivory among herbivores, some of which have been caught on film as provided below; 
  • River Hippopotami, which feed almost exclusively on grasses, have been filmed scavenging from the carcasses of animals which have either drowned or were killed by large predators (video). 
  • Giraffes, which normally feed on leaves and fruits high above the ground, are known to chew on the bones of other animals when they stumble upon them (video). 
  • Deer, cattle, and horses have also been filmed eating dead or unattended bird chicks (video1, video2, video3, video4). 

These behaviors may seem unusual to the casual observer, but they are likely the result of animals attempting to compensate for the lack of certain nutrients which may be lacking in an otherwise plant-based diets. Although these particular animals lack the physical attributes necessary to actively hunt and kill live prey animals, they will clearly take advantage of animal protein when they can obtain it.

References & Further Reading
Roemer GW, Gompper ME, Valkenburgh BV (2009). “The ecological role of the mammalian mesopredator”. BioScience 59: 165-173 <Full Article>

Valkenburgh BV (2007). “Déjà vu: the evolution of feeding morphologies in the Carnivora”. Integrative and Comparative Biology 47(1): 147-163 <Full Article>

Holliday JA, Steppan SJ (2004). “Evolution of hypocarnivory: the effect of specialization on morphological and taxonomic diversity”. Paleobiology 30(1): 108-128 <Full Article>

Valkenburgh BV (1988). “Trophic diversity in past and present guilds of large predatory mammals”. Paleobiology 14(2): 155-173 <Full Article>

Monday, April 11, 2016

Perissodactyla: The Odd-toed Ungulates

Perissodactyla is the order of mammalian herbivores which includes modern tapirs, rhinos, and horses. This group is characterized by having limbs in which the third digit of each foot is enlarged and reinforced to carry the main body weight. As a result, these animals have a tendency to reduce the number of functional toes from five to just three (as in modern rhinos) or one (as in modern horses). For this reason, members of this group are commonly referred to as the “odd-toed ungulates”. Artiodactyls, the “even-toed ungulates”, differ in that the main weight of the body is distributed equally through the third and fourth digits with the number of functional toes being four or two. The name Perissodactyla was coined by renowned English scientist Sir Richard Owen in 1848.
Representatives of six families of perissodactyls, living (top) and extinct (bottom).
Left to right/top to bottom: the tapir Tapirus terrestris, the horse Equus ferus
the rhino Ceratotherium simum, the brontothere Brontotherium hatcheri
the palaeothere Palaeotherium magnum, and the chalicothere Morpus elatus.
Paleogene- The earliest definitive perissodactyls are known from the early Eocene of Europe and probably descended from a phenacodontid codylarth, a group archaic herbivores which may have been perissodactyls themselves according to some experts. This relationship, if correct, would push the origin of Perissodactyla back nearly 10 million years to the early Paleocene. Perissodactyls underwent an explosion in diversity shortly after this initial appearance: the horse, rhino, tapir, chalicothere, paleothere, and brontothere were already well-established by the early-middle Eocene. The first representatives of each of these families were all small animals comparable in size to house cats, with rhino-sized animals appearing by the middle Eocene. The largest perissodactyls, the indricotheres of Oligocene Eurasia, were true giants with one species growing up to 5m tall at the shoulder and weighing nearly twice as much as the largest modern elephants.

Neogene- Perissodactyls dominated the terrestrial herbivore communities of the Northern Hemisphere for much of the Paleogene and the first half of the Neogene. However, the group has undergone a noticeable decline in diversity from the middle Miocene onward, an event which seems to coincide with the continued radiation of artiodactyls. The group remained largely successful as a whole however. After brontotheres became extinct at the beginning of the Oligocene, rhinos became the dominant group of giant herbivores in all terrestrial ecosystems, second only to elephants. Chalicotheres, which never seem to have been particularly abundant, had a broad distribution throughout the Northern Hemisphere and eventually reached Africa by the middle Miocene. The extinction of chalicotheres in North America by the late Miocene (Hemphillian) seems to coincide with the introduction of megalonychid ground sloths to the continent. Tapirs have retained a specialized lifestyle as forest-dwelling browsers and have changed very little throughout the Neogene: tapirs from the early Miocene would have differed very little from their modern counterparts. Horses remained diverse and prominent elements of terrestrial faunas throughout the Neogene and, together with tapirs, became the only perissodactyls to colonize South America when the Isthmus of Panama was formed during the Pliocene.

Quaternary- Perissodactyls were significantly more specious during the Pleistocene with at least twice the number of species than there are today. As a whole however, the group had been reduced to a smaller remnant of their former glory due mostly to the replacement of many types of horses with various deer, antelopes, and pronghorns. The mass extinction which took place at the end of this epoch saw the complete extinction of horses from the New World, of tapirs and rhinos from the temperate regions of North America and Eurasia respectively, and of the last surviving chalicotheres in Africa. At present, there are 3 modern families with 17 species between them ranging in size from 150 to 3,500kg, all of which are either threatened or endangered in the wild do to human activities such as poaching and habitat alteration.

Defining Characteristics
From the beginning of their evolution, perissodactyls developed a specialized tarsal configuration which was specialized for running. A saddle-saped astragalus* forms a pulley-like articulation with the tibia which prevents lateral rotation of the foot, enabling stronger and more efficient forward propulsion. The clavicle (collar bone) has been greatly reduced or lost to enable unhindered motion of the forelimbs during running and to maximize the stride. The first digit (the equivalent of our thumbs and big toes) are absent in all species. All perissodactyls are hindgut fermenters, that is, digested food which has left the stomach is stored in an enlarged cecum* where it is further broken down by bacteria to release the nutrients. Compared to the more complex digestive system found in ruminants*, this method is relatively inefficient and may partly explain the aforementioned decline of perissodactyls throughout the latter half of the Cenozoic. An enhanced ability to obtain nutrients combined with faster reproductive rates could have potentially made ruminants able to exploit new niches and food sources more readily, particularly in the face of environmental changes which would have placed greater stresses on competing herbivores.
The hindfeet of the three modern perissodactyl families: tapir (left), rhino (center), 
and horse (right). Note that the middle digit of each foot is the largest and bares most 
of the body weight, or all the weight in the case of modern horses.
There are two main suborders within the Perissodactyla; (1) Ceratomorpha, which includes tapirs and rhinos, and (2) Hippomorpha, which includes horses, paleotheres, brontotheres, and chalicotheres. Occasionally, chalicotheres are placed in a third suborder Ancylopoda.
  • Phenocodontidae: early Paleocene to middle Eocene
  • Anthracobunidae: early to middle Eocene
  • Tapiridae (tapirs): early Eocene to present
  • Rhinoceratidae (rhinos): middle Eocene to present
  • Palaeotheriidae (paleotheres): early Eocene to late Oligocene
  • Equidae (horses): early Eocene to present
  • Brontotheriidae (brontotheres): early Eocene to early Oligocene
  • Chalicotheriidae(chalicotheres): early Eocene to late Pleistocene

Astragalus: the ankle bone which connects the foot to the rest of the leg.
Cecum: a pouch connected at the junction between the small and large intestines.
Ruminant: herbivorous artiodactyls which possess a specialized, multi-chambered stomach.

References & Further Reading
Rose KD (ed). 2006. The Beginning of the Age of Mammals. The John Hopkins University Press, Baltimore, Maryland:243-257 <Book>

Macdonald DW (ed). 2006. The Princeton Encyclopedia of Mammals. Princeton University Press, Princeton, New Jersey <Book>

Augusti J, Anton M (2002). Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. Columbia University Press, New York: 36-37 <Book>

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>