Wednesday, August 7, 2013

Chalicotheres: The Clawed Hoofed Mammals

Moropus elatus skeleton, one of the best known of the
Chalicotheres are an unusual family of perissodactyls* that appeared during the early Eocene of Mongolia. They would have appeared rather different from any animal alive today. Like tapirs they were pure browsers that inhabited lightly wooded and forested environments, like rhinos they had three toes on each foot with the middle toe bearing all the weight, and their faces would have looked somewhat horse-like. These animals are unique among ungulates in that they possess hooves that have become modified into claws. This family survived in Africa until the late Pleistocene when they all became extinct.

Evolution & Extinction
Chalicotheres appear to have originated in Asia and appear to share a close relationship with brontotheres. The earliest known member of this group, Protomoropus gabuniai, is known from early Eocene (55mya) of Mongolia. The family extended its range to North America by the late Eocene and they reached Africa by the early Miocene. In North America during the late Miocene, chalicotheres had been replaced by the megalonychid ground sloths which had just immigrated from South America. At the same time, chalicotheres in Eurasia and Africa were facing ever-increasing competition from large apes. Africa was the final stronghold for this family. The last chalicothere, Ancylotherium hennigi, survived in here long after its kind had gone extinct elsewhere in the world and it until the very end of the Pleistocene about 13,000ya.

Like tapirs, chalicotheres are never abundant in the fossil record and do not show significant variations in their dentition and overall body plan. This suggests that chalicotheres, early on, became specialized for a particular lifestyle (in this case as mid-level browsers) and never deviated from this general morphology throughout their history. Unlike horses and rhinos, they never evolved high-crowned cheek teeth for feeding on more abrasive types of plants found at ground level. Thus, their overall diversity was low and only a handful of species are known to science. This degree of specialization may partly explain the decline and eventual extinction of this family. Because of their extensive adaptations for browsing, the presence of chalicothere bones at a fossil site is a good indicator of an ancient woodland or forest environment.

Claws, Locomotion, & Posture
Chalicotheres possessed three toes on each foot, all of which ended in a large claw. These claws grew from and were anchored to deep fissures in the middle of the third phalanges*. This suggests that the claws of these animals were very strong and able to resist heavy impacts and tension. A similar condition can be seen in the claws of creodonts (archaic mammalian predators that preceded the order Carnivora). Like other perissodactyls, most of the animals’ body weight would have been carried on the middle digits of each foot.  To deal with this, the bones of the second manual digit are fused to form a structure that strengthens the inside edge of the hand. 

The Chalicotheriidae is divided into two subfamilies, each characterized by a different type of locomotion.
  • The Schizotheriinae had a more conventional digitigrade* posture in which the animals walked on padded toes like a dog or a cat. This is an efficient form of locomotion that increases the length of the limb to enable a longer stride. To prevent wear and to allow unhindered movement, the tips of the claws were held above the ground when walking or running thanks to tendons in the toes.
  • The chalicotheriinae preserved their claws by walking on their knuckles. For this, the bones of the front knuckles are thickened and well-developed for bearing weight. In the living animal, thickened skin pads would have covered the upper surface of the knuckles to cushion the digits and prevent abrasion. Gorillas, anteaters, and ground sloths have the same adaptation which is designed to protect either their touch-sensitive fingers or pointed claw tips. 

Chalicothere forelimbs were very muscular and were longer than the hindlimbs, causing the back to slope downward towards the tail. This feature was taken to extreme in the gorilla-like Chalicotherium. These animals would have ran in a moderately quick loping gate. To withstand the weight of the body and facilitate an upright posture, the lumbar region is shortened to increase lower-back strength. The lumbar vertebrae also had elevated dorsal processes that would have supported the tendons and muscles needed to maintain an upright posture for extended periods. 

Browsing Specialists
Chalicotheres have a number of adaptations that suggest that they were browsing specialists that fed on above-ground vegetation at shoulder level or higher.

Adult chalicotheres lacked canines and upper incisors with which to crop vegetation. Instead, they would have possessed a long, prehensile tongue and muscular lips that would pull food into their mouths. They had low-crowned, square-shaped, cheek teeth with which to grind relatively soft leaves and fruits (the name "chalicothere" was coined because the cheek teeth, in their worn state, are said to resemble a chalice or goblet). The skull itself is small but rather long and narrow, an adaptation that enables modern browsers to select individual plants and twigs from among many. Modern browsing mammals often have thick eyelashes which help to protect their eyes during feeding, and chalicotheres likely had this characteristic as well. 

Moropus elatus shown
using its long tongue and
lips to browse.
The necks of schizotheriines were long, narrow, and flexible. Elevated spinous processes on the lumbar vertebrae would have supported strong muscles and tendons that would strengthen the lower back, enabling the animals to stand on its hind legs and hold that position for extended periods of time. These animals were able to browse at many different heights either by feeding quadrupedally as most herbivores do today or by rearing up on their hindlimbs, effectively doubling their vertical reach. The modern Gerenuk (Litocranius walleri) has a similar feeding behavior in which it balances itself on its hindlimbs to feed on high tree foliage. 

Chalicotheriines had a more specialized feeding method coompared to schizotheres. They had relatively shorter necks and much longer arms, suggesting that they relied more on their forelimbs to gather food and pull it toward their mouths. Inward-facing hands were well-adapted to be used as hooks to either grip tree trunks or to pull branches close to their mouths, similar to the way modern gorillas feed. The short, yet powerful, hindlimbs were well-suited for maintaining a sitting or squatting posture for extended periods of time. Pad-supporting bone on the ischium* acted as a cushion, stabilizing them on their haunches while its torso stood fully vertical.

These morphological differences suggest that chalicotheriines would have lived in more thickly forested environments where they could use their unique feeding strategy to full advantage. Schizotheriines were able to live in more open woodland and savanna environments, where their more efficient gait enabled them to travel farther distances to exploit widely dispersed food sources. Furthermore, analysis of chalicothere teeth suggests that schizotheriines were preferentially feeding on more leaves, twigs, and possibly bark, while chalicotheriines were ingesting more seeds and fruits in addition to leaves. Because of the rather long forelimbs, chalicotheres would have needed to crouch down in order to drink so that their faces could reach the water.

Other Behaviors
Skeleton of Moropus elatus. A typical schizotheriine that
demonstrates the more conventional body build with
digitigrade feet and forelimbs only slightly longer than
the hindlimbs.
Because of their body proportions, chalicotheres were relatively slow moving and unable to keep up sustained speeds for extended periods. Despite lacking the necessary speed and agility to outrun predators, or horns with which to fight them off, chalicotheres would have been a formidable target for any would be predator. Their long and sharp claws and powerful forelimbs certainly could have doubled as defensive weapons. Because the short hindlimbs carried most of the animals’ body weight, they would have been able to pivot on their hindlimbs rather quickly. Thus, when attacked from behind a chalicothere would have been able to turn rapidly so that it could face its attacker, brandishing its sharp claws in defense. Like tapirs, it is likely that chalicotheres also had thickened skin on their backs that could deflect the claws and teeth of an attacking predator long enough for the victim to free itself and fight back.

It can also be inferred that chalicotheres lived in herds as most herbivores do today. Based on their closest modern relatives, these groupings likely consisted of 3 to 30 individuals and led by an adult male. Younger males would have left their maternal herds at sexual maturity and became part of a bachelor herd until they were old enough to challenge dominant males for breeding rights. Male chalicotheres seem to have had skulls that were somewhat deeper and heavier than those of females, suggesting a fighting style similar to that of the modern Giraffe (Giraffa camelopardalis), whereby two males would stand side-by-side and swing their long necks and heads like wrecking balls until one rival submitted. The genera Tylocephalonyx and Ancylotherium possess a high, thick dome at the back of their skulls that likely evolved for such confrontations, and could have delivered punishing blows to an opponent. Like all of today's perissodactyls, chalicotheres would have given birth to a single large, fully-developed offspring after a long gestation of 10 months to a year depending on the species.

Individual Species
Marsh's Chalicothere (Moropus elatus)

Digitigrade: a type of locomotion in which only the toes touch the ground with each step.
Ischium: one of the three pelvic bones; situated below the ilium and behind the pubis.
Perissodactyl: the mammalian order which contains horses, rhinos, tapirs, and their extinct relatives.
Phalanges: the bones which comprise the fingers and toes.

References & Further Reading
Geraads D, Tsoukala E, Spassov N (2007). “A skull of Ancylotherium (Chalicotheriidae, Mammalia) from the late Miocene of Thermopigi (Serres, N. Greece) and the relationships of the genus”. Journal of Vertebrate Paleontology 27(2): 461-466 <Full Article>

Agusti, J. & Anton, M. "Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe". Columbia University Press. New York. ISBN 0-231-11640-3 <Book>

Turner & Anton. "Evolving Eden: An Illustrated Guide to the Evolution of the African Large-Mammal Fauna". Columbia University Press, New York. 2004. ISBN 0-231-11944-5 <Book>

Savage, RJG & Long, MR. "Mammal Evolution: an illustrated guide". Facts on File Publications, 1986. pp 216-220 <Book>

Coombs MC (1979). “Tylocephalonyx, a new genus of North American dome-skulled chalicotheres (Mammalia, Perissodactyla)”. Bulletin of the American Museum of Natural History 164: 1-64 <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>

Thursday, August 1, 2013

Cenozoic Timeline: Evolving Oceans

Most of the Earth has always been covered by salt water. Starting from the late Cretaceous we begin to see the continents drifting into their modern positions. These geologic events had profound effects on global climate, which gradually became cooler and drier on average from the Eocene to the present day. Also, we see very early on how marine animals recovered following the devastation of the late Cretaceous and achieved its modern composition. Unlike terrestrial or freshwater animals, marine animals are able to achieve a truly worldwide distribution because there are no barriers to obstruct their expansion. As a result, many oceanic species are quite long-lived. The modern Great White Shark, for example, has existed for at least 16 million years, and its relative Megalodon was around for 28 million years.

Paleocene (66-56mya)
Tropical reefs like this one could be found in shallow
waters all over the world during the late Paleocene.
The oceans all around the globe were warm and free of ice during the Paleocene. As a result, sea levels were much higher than they are today: much of the eastern United States lay at the ocean floor and Europe was a series of volcanic islands similar to Indonesia. The early Paleocene featured a low diversity and abundance of marine life, the flora and fauna still recovering from the ravages of the K/T extinction event. Sharks became the top predators after the extinction of the mosasaurs. Penguins evolved 63mya, replacing the Hesperorithid birds of the late Cretaceous. Coral reefs could be found in all oceans. Apart from the absence of marine mammals and Carcharhinid sharks, the marine fauna of the late Paleocene would have been very similar to that found in tropical seas today.
Eocene (56-34mya)
Ambulocetus natans, one of the first whales of the
early Eocene. This species lived in both saltwater
and freshwater habitats.
Tropical conditions continued throughout the Eocene, with oceans teeming with fish, turtles, and crocodiles. The marine fauna was enriched significantly during the early Eocene by the debut of three important animal groups which still survive to the present day. The first requiem sharks (Carcharhinidae; the family which contains Tiger Sharks, Bull Sharks, Oceanic Whitetip Sharks, and others) appear during this time.  The first sirenians (manatees and dugongs; herbivorous marine mammals of shallow waters) evolved off the coast of Africa around 55mya, having branched off from the ancestors of today’s elephants. By far the most significant new arrival of the early Eocene were the first whales. The first whales would have had a lifestyle very similar to that of today’s seals and sea lions, feeding along the coastal waters and returning to the land periodically to rest and give birth. The late Eocene Basilosaurids became the first whales to become fully incapable of returning to the land. For the first time, mammals had become the top predators of the oceans. The toothed whales and baleen whales appeared during the latest Eocene. Meanwhile, the earlier walking whales appear to have disappeared by 36mya, corresponding with the spread of the marine Gavialids (a family of narrow-snouted crocodilians of which the modern Indian Gharial and False Gharial are the only survivors).
Dourodon atrox, a basilosaurid whale of the late Eocene.
These were the first whales to have become fully incapable
of leaving the water, having rigid flippers and vestigal
hind legs.

Two significant tectonic events occurred during the Eocene; India collided with southern Asia around 45mya, and Arabia connected with west Asia 34mya around the Eocene/Oligocene boundary. As a result, the Tethys Sea had disappeared, possibly setting in motion the cooling trend that continued from the Oligocene to the Pleistocene.

Oligocene (34-23mya)
Skeleton of Paleoparadoxia tabatai, a desmostylian. These
poorly-known animals were marine grazers that fed in the
shallows and clambered back on land to rest. Its closest
living relatives are thought to be the sirenians, the group
that includes manatees, dugongs, and sea cows.
The Oligocene sees the beginnings of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans, and of the global climate in general, had already begun by the latest Eocene. This cooling trend intensified when South America finally detached from Antarctica during the early Oligocene and started to drift north toward North America. This allowed the formation of the Antarctic Circumpolar Current which rapidly cooled Antarctica and caused a permanent ice cap to form at the South Pole, although the fringes of the continent remained ice-free until the middle Miocene.

Reconstructed skeleton of Megalodon (Carcharocles 
megalodon) on display at the Calvert Marine Museum.
After evolving during the middle Oligocene, this giant
shark ruled the seas for almost 30 million years.
The Basilosaurids had disappeared during the early Oligocene while the toothed whales and baleen whales continued to diversify. Among the toothed whales, the diverse extinct family of shark-toothed dolphins (Squalodontidae) persisted from the late Eocene, and by the late Oligocene they were joined by river dolphins (Platanistoidea), dolphins (Delphinidae), and sperm whales (Physeteridae). Among the baleen whales, the toothed Aetocetids also carried on from the latest Eocene, and by the late Oligocene they had given rise to the toothless Cetotheriids, of which the Pygmy Right Whale is the only modern survivor. Also, a new group of marine herbivores appeared in the fossil record during this time, the desmostylians. These animals appear to have been relatives of sirenians and early elephants, and may have appeared rather strange to a modern observer. When alive, these creatures would have resembled a cross between a sea lion and a hippo. In response to this great increase in prey diversity, the giant shark Megalodon evolved in the late Oligocene about 28mya, and would survive unchallenged until the Pleistocene.

Miocene (23-5mya)
Acrophoca longirostris, a long-bodied seal closely related
to the extant Leopard Seal (Hydrurga leptonyx), and
possibly ancestral to it. It is known from late Miocene
rocks off the coast of South America.
Further decreases in temperature occurred during the middle Miocene around 15mya, probably reflecting ice build-up in the Southern Hemisphere. Formally confined to the center of the continent, the permanent Antarctic ice cap expanded to its modern form at this time, covering the whole continent.

The Great White Shark (Carcharodon carharias) was the
same 15mya as it is today. It evolved in response to
the adaptive radiation of pinnipeds at that time.
The Miocene cooling may explain the gradual decline of the marine gharials, which would continue to disappear into the Pliocene leaving behind only a few freshwater species by Pleistocene times. Their decline, however, matches an increase in the diversity of pinnipeds (sea lions, walruses, and seals). The first pinnipeds of the late Oligocene, Puijila and Potamotherium, were small and otter-like, not far removed from their basal Ursid ancestors. By the early Miocene these had given rise to the archaic sea lion Enaliarctos, which would itself give rise to a number of new species 16 to 14mya. The pinniped radiation also corresponds with the extinction of the giant penguins which had existed since the early Paleocene. The Great White Shark as we know it today appeared 16mya during the middle Miocene, seemingly in response to the adaptive radiation of the pinnipeds. Cetaceans continued to grow in diversity as well. Porpoises (Phocoenidae), beaked whales (Ziphidae), rorqual whales (Balaenopteridae), and right whales (Balaenidae) all evolved in the middle Miocene, with Monodontids evolving later in the epoch. In spite of this increase in marine mammal diversity, the desmostylians become extinct during the late Miocene about 7mya.

Pliocene (5–2.5mya)
The formation of the Panamanian Land Bridge 3.5mya led
to changes in ocean circulation, which led to the formation
of the Greenland ice cap and the beginning of the glacial
activity that would characterize the Pleistocene.
Oceans were still relatively warm during the early Pliocene and subtropical conditions were present as far north as England. Another cooling event would soon transpire however. 3.5mya, the Isthmus of Panama formed when South America finally connected with North America. This event may have influenced the further cooling of the global climate; the once equatorial current which had been flowing since the Cretaceous had now been redirected to flow in a north-south course, causing cold waters from the north and warm waters from the south to mix and subsequent cold north Atlantic and north Pacific currents to circulate. Perhaps in direct correspondence, the Greenland ice cap developed during the middle Pliocene about 3mya and glacial activity would continue through the rest of the Cenozoic.
The Killer Whale (Orcinus orca) evolved during the Pliocene
and survives virtually unchanged. Later by the end of the
Pleistocene it would become the top predator of the world's
surface waters following the extinction of Megalodon.

Starting in the middle Pliocene, northern and southern parts of the world began to experience more pronounced, seasonal drops in temperature. For the first time we see the evolution of truly cold-adapted land mammals such as bison, mammoths, and lynxes. Despite these events, the marine fauna remained largely unaffected. The majority of the animals around at the time would have been very familiar to us today with modern types of cetaceans, pinnipeds, sirenians, sharks, turtles, penguins, and sea birds.

Pleistocene (2.5mya-13,000ya)
Polar Bears (Ursus maritimus) evolved from Brown Bears
(Ursus arctos) during the Pleistocene and specialize
in hunting on the sea ice. During peak glacial conditions
this bear's range would extend as far south as
British Colombia.
The Pleistocene was a time if great fluctuations in climate brought on by the continual advance and retreat of glaciers. This caused sea levels to drop by several hundred meters, polar conditions to extend further south, and colder and drier conditions worldwide.

Megalodon persisted throughout the Pleistocene but disappeared toward the end along with several types of giant whales, sirens, and turtles. These animals may have died out as a result of the same event that decimated many of the land mammal populations 13,000ya. Being a large, pelagic, slow-breeding carnivore, Megalodon was particularly vulnerable to drastic changes in prey stocks, and so may have become extinct as a result. Those predators that hunted in deep waters or along the coasts were relatively unaffected because prey is more abundant in these environments. Likewise, the Sperm Whale, Killer Whale, and Great White Shark are now the largest ocean predators.

Harrison & Bryden. "Whales, Dolphins, and Porpoises". Facts on File, November 1988

Haines, Tim. "Walking With Prehistoric Beasts: A Prehistoric Safari". DK Publishing, November 2001

Hooker, J. J., "Tertiary to Present", pp. 459-465, Vol. 5. of Selley, Richard C., L Robin McCocks, and Ian R. Plimer, Encyclopedia of Geology, Oxford: Elsevier Limited, 2005. ISBN 0-12-636380-3

Barron, J, Bralower, T., Huber, M., Lyle, A., Ravelo, C., Rea, D., Wilson, P. (April 2008). "Pacific Ocean and Cenozoic Evolution of Climate". Reviews of Geophysics 46 (2): 1-47

Photo Credits
  1. Reef system: Sean Conolly, 15 August 2006, Wikimedia Commons
  2. Coral Reef: Jim Maragous/U.S. Fish and Wildlife Service, 27 March 2011, Wikimedia Commons
  3. Ambulocetus skeleton: Ghedo,13 August 2011, Wikimedia Commons
  4. Dourodon skeleton: EveK, 2 September 2007, Wikimedia Commons
  5. Megalodon skeleton: Dr. Alton C. Dooley, Wikimedia Commons
  6. Great White Shark: Hermanus Backpackers, 10 March 2009, Wikimedia Commons
  7. Ice ridges of northern Alaska: Rear Admiral Harley D. Nygren, NOAA Corps, Spring 1949, Wikimedia Commons
  8. Killer Whales: Robert Pittman, 2005, Wikimedia Commons
  9. Polar Bear: Alan Wilson, 2007, Wikimedia Commons