Tuesday, December 6, 2016

Anthracobunids: Amphibious Mammals of Eocene Asia

Anthracobunids (Anthracobunidae) are an extinct group of herbivorous mammals from the Eocene of southern Asia. These animals were formerly considered to be members of Tethytheria, the group of mammals which contains elephants, hyaxes, sirenians, and their relatives. However, a recent study in 2014 has shown that anthracobunids were actually basal members of Perissodactyla, related to tapirs, rhinos, and horses.

Defining Characteristics
Anthracobunids have a complete dentition of 44 teeth. The incisors were relatively small and simple, with relatively large canines. The cheek teeth were low-crowned with bunodont* or bilophodont* cusps, suggesting that these animals fed on relatively both soft to moderately hard, non-siliceous plant matter. Based on tooth measurements, anthracobunids have been estimated to be about the size of modern tapirs, with body masses ranging from 100 to 275kg (220 to 610lbs). Unfortunately, complete skeletal remains are virtually unknown for this family. All described anthracobunid fossils are limited to skull and mandibular fragments, isolated teeth, and incomplete postcranial elements. 

Figure 1 from Cooper et al., 2014, showing much of the known fossil material for
the entire family Anthracobunidae.
A-B: the crushed skull of Anthracobune pinfoldi.
C-D: second premolar of A. wardi.
E-F: skull fragment of A. wardi.
G-I: complete mandible of A. wardi.
J-L: proximal phalanges of A. wardi.
M: head of a metapodial from A. pinfoldi.
N: phalangeal fragment of A. pinfoldi.
O: terminal phalanx of A. pinfoldi.

Stable isotope values and long bone geometry studies have suggested that anthracobunids fed on land and spent considerable amounts of time in water. The postcranial bones were hyperostotic, meaning that they were much denser and more compacted than those of most terrestrial mammals. This condition, which increases buoyancy, is an adaptation associated with aquatic or partly aquatic mammals which spend most of their time in shallow water such as hippopotami, early whales, tapirs, and rhinos. All evidence suggests that anthracobunids shared behavioral and ecological similarities with tapirs and certain species of rhino. These animals likely had relatively restricted home ranges which overlapped with a permanent source of freshwater such as a river or lake in which they would frequently wallow and wade. They would forage in the forested environments which bordered these water sources and would readily retreat to water to escape predation, for thermoregulation, relief from terrestrial parasites, or simply to rest.

Tentative reconstruction of Anthracobune wardi based on specimen H-GSP 96258,
an exceptionally complete mandible missing only its incisors. The cranium was
approximated by referencing the skulls of early rhinos and tapirs. Although no
postcranial material is known for this animal, its body proportions and much of
its ecology and behavior would have been similar to that of modern tapirs.

Genera & Species
As of 2014, the Anthracobunidae contains 3 genera with 4 species between them. Two other monotypic genera, Ishatherium (I. sabathuensis) and Nakusia (N. shahrigensis), have been excluded from the family based on differences in dental morphology.

Anthracobune (1940)
The name Anthracobune is derived from the Greek anthrakos, meaning “coal”, and bune, meaning “mound”. Thus, the genus name translates into “coal mound”. Members of this genus lived during the middle Eocene of Pakistan. 2 species are known; A. wardi and A. pinfoldi. A. wardi, the type species of the family, was originally placed in Anthracobune by Pilgrim in 1940, only to be placed in the now defunct genus Lammidhania wardi by Gingerich in 1977. It was later moved back into Anthracobune 37 years later by Cooper et al. in 2014. 

Jozaria (1983)
Jozaria is represented by a single specimen belonging to the only known species of this genus (J. palustris). Geological evidence indicates that the animal inhabited a brackish marsh environment and it may have fed on the soft aquatic vegetation found there.

Obergellia (2014)
Containing the singular species, Obergellia occidentalis, this animal from the middle Eocene of India and Pakistan is the most recently described member of the Anthracobunidae. Although its fossil remains were first discovered in 1980, the genus Obergellia was erected by Cooper et al. in 2014. The name honors the late married vertebrate paleontologists Friedlinde Obergfell and A. Rango Ral. It differs from other anthracobunids in a suite of dental and mandibular characters.

Bilophodont: cheek teeth with two transverse crests or ridges.
Bunodont: referring to cheek teeth with rounded, bumpy cusps.

References & Further Reading
Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, et al (2014). Anthracobunids from the middle Eocene of India and Pakistan are stem perissodactyls”. PLoS ONE 9(10): e109232 <Full Article>

Kumar K (1991). “Anthracobune aijiensis nov. sp. (Mammalia: Proboscidea) from the Subathu Formation, Eocene from NW Himalaya, India”. Geobios 24(2): 221-239 <Full Article>

Wells NA, Gingerich PD (1983). “Review of Eocene Anthracobunidae (Mammalia, Proboscidea) with a new genus and species Jozaria palustris, from the Kuldana Formation of Kohat (Pakistan)”. Contributions from the Museum of Paleontology 26(7): 117-139 <Full Article>

Sahni A, Kumar K (1980). “Lower Eocene Sirenia, Ishatherium subathuensis, gen. et nov. sp. From the type area, Subathu, Simla Himalayas, H. P.” Journal of the Paleontological Society of India 23 & 24: 132-135 <Full Article>

Friday, November 4, 2016

Marsh's Chalicothere (Moropus elatus)

Marsh’s Chalicothere (Moropus elatus) is the best known North American chalicothere, and perhaps the most well-represented of the Chalicotheriidae as a whole. This large, browsing herbivore lived during the early Miocene of what are now the states of Nebraska and Colorado.

A mounted skeleton of Marsh's Chalicothere (Moropus elatus)
on display at the Smithsonian Museum of Natural History (wiki).

American paleontologist Othniel Marsh coined the genus name Moropus in 1877, which translates into “Slow Foot” or “Sloth Foot”. This name was inspired by the discovery of the animals’ large claws which were originally thought to have come from a type of ground sloth. The thought behind the specific name elatus is unclear, but it seems to be derived from the Latin word meaning “to carry”, “produce”, or “lift up”. Though not officially given a common name, this species will be referred to as Marsh’s Chalicothere for the purposes of this blog, which honors Othniel Marsh who described it.

Habitat & Distribution
Marsh’s Chalicothere lived in North America during the early Miocene where it inhabited open woodland and savanna environments. This species is particularly well-represented at the Agate Fossil Beds National Monument, an early Miocene (Arikareean) site which has produced one of the most diverse Neogene faunas in North America.
Three mounted skeletons of Marsh's Chalicothere depicted reacting to an
off-screen threat (see below). Photo taken at the Agate Fossil Beds
National Monument Visitor Center and Museum, 2015. 

Physical Attributes
Because it is known from several complete or nearly complete skeletons, Marsh’s Chalicothere is the best known chalicothere from the New World. It was a large herbivore which towered over many of the early Miocene animals with which it coexisted, weighing up to a ton and standing up to 2.4m (8ft) at the shoulder. The animal had a narrow skull which was set on top of a long neck. The forelimbs were slightly longer than the hindlimbs, giving the animal a slightly lopsided appearance. The feet had three toes each, all ending in prominent claws instead of hooves.

Marsh’s Chalicothere was specialized for a high-browsing lifestyle. The structure of its nasal bones suggest that they supported mobile lips adapted to grasp and break leaves and small branches into their mouths, similar to those of modern browsing rhinos. Additionally, reduced incisors and a procumbent lower jaw suggest that these animals had protrusible tongues which they would have used to bring food within range of its aforementioned lips in the manner of a giraffid or sloth. Elongated necks and forelimbs enabled a vertical reach of up to 4m when on all fours. Furthermore, the structure of the lumbar vertebrae and pelvis indicate that these animals were capable of actively rearing up on their hindlimbs, effectively increasing their vertical reach to 5m. The closest modern analogue to the feeding behavior of Marsh’s Chalicothere is the Gerenuk (Litocranius walleri), a long-limbed and long-necked African antelope adapted to browse on tree foliage unreachable to other animals its size. In addition to quadrupedal browsing, these animals frequently engage in bipedal feeding, during which they stand completely vertically. Although they are capable of balancing on their hindlimbs for extended periods, Gerenuk will more often use their forelimbs to help support themselves against overhanging branches and will even use their hooves to pull down twigs. With its clawed forearms and ability to stand erect, it is easy to imagine Marsh’s Chalicothere adopting the same behavior, essentially taking the role of a giant Gerenuk in its environment.
Three Gerenuk (Litocranius walleri) browsing from a tree in Samburu National
Park, Kenya (wiki). The erect posture which these antelopes adopt when browsing
on high foliage is a likely feeding behavior for Marsh's Chalicothere.

Ecology & Behavior
Marsh’s Chalicothere was not built to run fast for extended periods, and thus would have been unlikely to occur on extensive open areas. Instead, it would have favored woody or bushy cover to conceal themselves where they would feed on the leaves, twigs, and bark of various trees and shrubs. For this reason, I have given my reconstruction a pelage similar to that of the modern Bushbuck (Tragelaphus scriptus) or Greater Kudu (T. strepsiceros) with vertical white stripes for concealment among the trees.

Because of their size and formidable claws, healthy adult individuals would have been virtually immune to predation to anything short of the equally massive Giant Entelodont (Daeodon shoshonensis) or Lion-sized bear-dogs of the genus Amphicyon. Direct evidence of predation on the bones of Marsh’s Chalicothere by the Giant Entelodont is known in the form of large puncture wounds which match the dimensions of the entelodont’s premolars. It should be noted, however, that these marks could have easily been left during the act of scavenging on animals which had died of other causes. Juvenile chalicotheres were far more vulnerable to attack from a broader range of predators that included several medium-sized nimravids and amphicyonids. To better protect their young it is likely that female chalicotheres aggregated in small nursery herds perhaps led by a single mature male as many modern browsing ungulates do. To defend themselves and their young, chalicotheres were capable of turning rapidly on their hindlimbs in order to face a potential threat and could lunge forward while kicking out with their forelimbs.
Skeleton of the Giant Entelodont (Daeodon shoshoensis) depicted standing over
the carcass of Marsh's Chalicothere. The Giant Entelodont was one of the few
contemporary predators capable of singly threatening a fully grown chalicothere.
Photo taken at the Agate Fossil Beds National Monument
Visitor Center and Museum, 2015.

References & Further Reading
Agate Fossil Beds National Monument: Official National Park Handbook

Coombs MC, Hunt RM, Stepelton E, Albright LB, FremdTJ (2001). “Stratigraphy, chronology, biogeography, and taxonomy of early Miocene small chalicotheres in North America”. Journal of Vertebrate Paleontology 21(3): 607-620 <Full Article>

Colbert EH (1935). “Distributional and phylogenetic studies on Indian fossil mammals: a classification of the Chalicotheriodea”. American Museum Novitates 798: 1-16 <Full Article>

Saturday, October 15, 2016

Xenungulates: The Strange Ungulates

The Xenungulata (zeen-un-gu-la-ta), whose name literally means “strange ungulates” or “strange hoofed mammals”, is an archaic and poorly known group of South American mammals that are currently only known from the Paleocene and early Eocene. 

Mounted skeleton of Carodnia vierai. Photo by Lily P. Bergqvist from
In 1958, Paula Couto coined the order Xenungulata together with the family Carodniidae, which today contains a single genus with three known species; Carodnia feruglioi, C. vieirai, and C. inexpectans. Later, in 1987, a second family called Etayoidae was established with the two species known today; Etayoa bacatensis and Notoetayoa gargantuai. As with most early Paleogene animals, xenungulate fossils are very scarce. In spite of their scarcity, however, xenungulates are known to have lived throughout the South American continent and were the largest terrestrial mammals in their environment with body mass estimates for C. vieirai ranging from 450 to 720kg.

By far the best known member of the group is the species C. 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 procumbant, laterally flattened incisors and particularly large canines which may have been used as combat weapons against rivals or predators. The cheek teeth were low-crowned and well-suited to a browsing diet which likely comprised mostly of 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 an elephant-like gait.

Xenungulates have been considered to be close relatives to both the South American pyrotheres (Pyrotheria) and North American uintatheres (Dinocerata) 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.

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
Antoine PO, Billet G, Salas-Gismondi R, Lara JT, Baby P, Brusset S, Espurt N (2014). "A new Carodnia Simpson, 1935 (Mammalia, Xenungulata) from the early Eocene of northwestern Peru and a phylogeny of xenungulates at species level". Journal of Mammalian Evolution DOI: 10.1007/s10914-014-9278-1 <Full Article>

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

Animal Behavior: More Than One Way to be a Carnivore

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 lineages 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-shaped 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.

Side-by-side comparison of the "single-pulley" astragalus of perissodactyls (left)
and the "double-pulley" astragalus of artiodactyls (right). These elements are
highly diagnostic with their general shapes remaining consistent across their
respective orders. Functionally, these bones enable a more efficient running gait
by restricting movement to the fore-and-aft plane. 

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>