The Fossil Mammal Hall

When we think of mammals we often think of furry, warm blooded creatures. Because we too are mammals, it is easy for us to recognize what a mammal is. When a paleontologist looks at fossils, there is also a set of skeletal features that set mammals apart from other vertebrates. Mammals, aside from having hair, and producing milk, are specialized over dinosaurs, reptiles and birds in having two major unique features which are preserved in the fossil record. Mammal teeth are uniquely shaped depending on their position in the mouth. They are also replaced only once during life. The teeth of mammals are divided into incisors, canines, premolars and molars. The second major feature of mammals is their arrangement of the jaw and ear bones. Reptiles have a number of bones that compose their lower jaw (dentary, angular, and articular). Mammals have only one bone of the lower jaw, the dentary. The other bones have evolved into the ear bones and form the malleus, tympanic, and stapes. The incus ear bone of mammals is part of the upper jaw in reptiles (quadrate bone). There are other features that are unique to mammals, including a secondary palate, and the increased size of the inferior temporal fenestra in which the temporalis muscle passes through, enabling mammals to chew more effectively then reptiles. There is a problem in recognizing a mammal from only fossil bones. Fossil synapsid reptiles from the Periman period 250 million years ago, show many skeletal features of mammals, but not all. Thrinaxodon at the base of the Triassic period 245 million years ago, has a secondary palate and teeth that can be divided into incisors, canines and molar-like teeth. Thrinaxodon was a meat eater and had long canines to slice into its prey. But what makes it a reptile is that had articular and quadrate bones as part of the jaw joint. Most books refer to these fossil synapsid reptiles as mammal-like reptiles, mostly because paleontologists debate where to draw the line between reptile and mammal. For more information on this and other transitions in the fossil record. Visit fossil transitions. The oldest completely known true fossil mammal is Morganucodon from the early Jurassic, 200 million years ago. Morganucodon is different from the mammal-like reptiles in having only one lower jaw bone, a secondary palate, and the unique anatomy of mammal teeth, being replaced only once in life. Morganucodon was very small, only about 13 centimeters long and is known from China and the United Kingdom.

Mesozoic Mammals

The Mesozoic, also known as "the Age of the Reptiles" was a time of great diversification for the small early mammals. For 135 million years the mammals would never get much larger than Morganucodon. But despite the small size, mammals during this time specialized by developing and expanding cusps on their teeth. This allowed them to chew much more effectively then ever before. Morganucodon belongs to the Order Triconodont, which is named because each of these mammals have three cusps in a row on their molar teeth. The mammals placed in the Order Symmetrodonta have three cusps arranged as a triangle. Other more complex patterns of tooth cusps are found in the Order Docodonta. Docodon, is often found in the same localities as dinosaurs from the Jurassic period. The best locality for these little guys, is Como Bluff, Wyoming, more famous for its dinosaur fossils. A well preserved jaw of Docodon was recently discovered, while preparing a Stegosaurs from Canon City, Colorado, for the Denver Museum of Natural History. Dryolestes, belonging to the Order Eupantotheria, has 8 molar teeth, (a mouthful set of 68 teeth!) A tooth of a new mammal related to Gonwanatherium from the Cretaceous of Madagascar was recently described by Dr. Dave Krause at the State University of New York, Stony Brook. These mammals were previously only known from South America, and they are unique in that had ever growing teeth, like modern rodents. We can infer that all of these early mammals laid eggs, and did not have nipples to give milk to their young.

Monotremes

It is a great joy, to a paleontologist that a primitive group is alive today. The biology and behavior of the group can be extended back into the past, with much greater confidence in reconstructing the fossil creature’s lifestyle and biology. Living monotremes include Ornithorhynchus (the platypus) and Tachyglossus (the echidna) are such an example. The fossil record of monotremes is greatly unknown, beyond the Miocene epoch 15 million years ago. A 60 million year old tooth from South America, as well as 110 million year old jaw fragments from Australia have been included in the group. Modern monotremes lack permanent teeth, but fossil montremes from the Miocene have teeth which can be compared to fossil teeth found much earlier, giving scientists clues to which Mesozoic mammal group monotremes are most closely related to. Monotremes also have an extra bone in their skull, the septomaxillae. This bone is found in mammal-like reptiles, but not in marsupials. Strangely, the septomaxillae bone is also found in Xenarthra mammals (sloths and armadillos). Monotremes are most famous for laying eggs, but they also lack nipples, milk is "leaked" out of the skin, and lapped up by the young.

Allotheria (Multituberculates)

This is the most successful group of mammals to have ever lived! But you probably have never hear of them. The fossil record of these guys extends way back to the late Jurassic about 160 million years ago, to 36 million years ago. They lived over 100 million years and survived the K/T boundary, 65 million years ago when the dinosaurs went extinct! The multituberculates are named, because of the numerous cusps on their molar teeth. But the most distinct feature of these mammals is their greatly enlarged premolar, which is a long thin blade, serrated like a steak knife. It is not known what these mammals ate, some scientists have suggested that they were omnivorous, eating insects and plant material. Others have proposed that they de-shelled hard seeds, by slicing through them with their enlarged premolar. The multituberculates and monotermes both have a parafibula bone, which is part of the knee joint. Multituberculate mammals were the first group to specialize by living in trees, much like living squirrels. After the extinction of the dinosaurs they also become larger, reaching the size of a beaver (Taeniolabis). Recent expeditions to the Gobi desert in Mongolia, lead by Dr. Michael Novacek, of the American Museum of Natural History, has lead to hundreds of skulls being discovered of Cretaceous multituberculates. The cause of the final extinction of the multituberculates is related to the origin of another highly successful group, the rodents.

Marsupials

This group of mammals is very diverse and has survived from the Cretaceous to the recent by either being isolated from placental mammals or by taking advantage of new niches. Marsupials are divided into groups based on the number of incisor teeth, and the number of toes. Syndactylous marsupials have reduced their second and third toes, giving them a very characteristic foot print, unlike any other mammal. The Virginia Opossum (Didelphys) belongs to the primitive group, called Didelphoidea, which have more incisors and more toes. This group extends back to the Cretaceous, where they were diverse just after the dinosaurs went extinct. Didelphoidea includes some very large carnivores, such as the saber-tooth marsupial Thylacosmilus, from the Pliocene of Australia, which reviles any of the saber-tooth cats, from North America; and the bear-like marsupials such as Borhyaena. Others such as Dromiciops probably lived in burrows, and Argyrolagus looked like a gerbil and lived in South America. The more advance group includes the classic marsupials like kangaroos and koalas, which are syndactylous and extend back to the Miocene epoch, about 15 million years ago. Many other types of unique marsupials, only known from Australia, have fossils extending only as far back as the Miocene. The fossil record before the Miocene, in Australia, is largely unknown, and there is a good chance that important discoveries will be made someday from earlier deposits. Many strange fossil marsupials are known from Australia including Diprotodon which was as large as a rhino; and Thylacoleo which was carnivorous, having very sharp front teeth, and flat plate-like back teeth. Marsupials, multituberculates, and monotremes all have an epipubic bone. Recently an epipubic bone was described from a skeleton of Zalambdalestes, from the Cretaceous, which many paleontologists had viewed as a placental mammal based on dental evidence. It is difficult to say if Zalambdalestes is a marsupial with placental teeth or a placental with an epipubic bone. The true function of the epipubic bone, and its relationship to the marsupial pouch has yet to be uncovered.

Archonta (primates, bats, tree shrews and flying lemurs)

Archonta is a term used to group primates, bats (Order Chiroptera), tree shrews (Order Scandentia) and flying lemurs (Order Dermoptera) together, as well as many extinct mammal groups such as primate-like microsypoids, and rodent-like plesiadapids and carpolestids. Bats are accomplished flyers, and have a fossil record extending back to the Eocene, about 45 million years ago. The origin of bats, has not been resolved, because earlier fossils from the Paleocene 60 million years ago are only known from teeth, which do not tell us if they were flying mammals. Modern bats are divided into two groups microchiopterans and megachiropterans. Megachiropterans are larger and do not have much of a fossil record. The relationship between the two groups is subject to some debate.

Primates are the most well studied group of all mammals. Fossil evidence for primates is also very extensive, going back to the early Eocene 55 million years ago. Primates are identified by the structure of the ear bones, in particular the shape of the petrosal bone. The rodent-like plesiadapids and carpolestids are often grouped together with true primates, because no fossil ear bones have yet to be discovered, showing if they are truly primates or not. These groups, do not have a post-orbital bar (cheek bone), which all modern primates have. Plesiadapids were very diverse after the extinction of the dinosaurs, but went extinct just as the rodents were diversifying in the late Eocene 40 million years ago. The first true primates are the adapids and omomyids. The adapids looked remarkably like modern lemurs, which some people suggest that they are ancestral to. Omomyids, were small and had large eyes. They looked a lot like modern Tarsiers. Most scientists believe that humans and most modern primates evolved from the omomyids, rather than the adapids. Antropoid primates include South American monkeys (Platyrrhini) and old world monkeys (Catarrhini) as well as Apes and humans (Hominodidea). Fossil apes have a well studied fossil record, going back 22 million years ago in Africa.

Glires (rabbits, pikas, and rodents)

Glires is a name used to group rodents, rabbits, pikas, and the extinct groups Anagalidea and Mimotonidae. Some mammalogists include elephant-shrews as well, although fossils from Africa, seem to not support this. Anagalids are very primitive, and include Zalambdalestes (see above). Anagalids and mimotonids are known from the Paleocene of Asia, 65 million years ago. Anagalids went extinct about 23 million years ago, and mimotonids gave rise to lagomorphs (rabbits and pikas) during the late Eocene, about 35 million years ago. Lagomorphs differ from rodents in having two sets of incisors per jaw, as well as having unique skull and tooth morphology. Modern pikas belong to the family Ochotonidae, which were much more diverse during the Miocene, even out numbering rabbit genera.

Rodents first jumped into the scene in the late Paleocene, about 57 million years ago. They quickly diversified during the Eocene into many groups, which lead to the extinction of many more primitive groups, including plesiadapids, multituberculates, and arctocyonids (discussed later). Rodents are unique from all other mammals in having one set of ever growing incisors, and having no canines. Rodents are grouped together based on their jaw muscles and skull morphology into eight major groups. Although the current classification of rodents is in a state of flux. It is recognized that the unique characteristics of the jaw muscles of rodents, that are used to classify rodents, have evolved from separate lineages independently. The first rodents, (Ischyromyoidae) are known from complete material as early as the late Paleocene and looked a lot like modern squirrels. Ischyromyoids are grouped with the family Mylagaulidae from the Miocene of North American, which had horns over their nose and formed burrow. Beavers and squirrels as well as prairie dogs are grouped together in the Sciuromorpha which have a fossil record extending back to the early Eocene, about 40 million years ago. Strangely most fossil beavers lived in the ground. Daemonelis formed corkscrew-like burrows that are preserved in western North America. Geomyoids are a large group of rodents that include pocket gophers are known as early as the late Eocene. The major group of rodents are the Muroids, which include most mice-like forms. They are extremely diverse and have a long fossil record back to the Eocene. Hystricomrpha, is a very large group, that includes the Caviomorpha (guinea pigs) and the very weird ctenodactylids (naked-mole rats). This group appears to have arisen from Asia or Africa during the Eocene. Although most modern groups are known from South and North America, except the naked-mole rat, which today live in Africa. The study of fossil rodents is very insightful to paleontologists who are interested in climatic and evolutionary patterns from the last 5 million years, because they are so common in recent cave deposits.

Lipotyphla (shrews and moles)

Sometimes also referred to as Insectivora, this group includes shrews, moles, and hedgehogs. Insectivores are distinguished by having high cusps on their teeth, which are arranged in a W shape called dilambdont, or V shape called zalambdont. The oldest-known insectivore is Paranyctoides, from the late Cretaceous (about 70 million years ago) of North America, and is related to modern hedgehogs. In slightly younger beds from the Cretaceous, is Batodon the first shrew-like form. Clearly hedgehogs, moles and shrews have had a long history apart from each other. Sadly fossil insectivores, because of their small size often get over looked. New methods of using fine screens to sift through chemically broken down rock, have lead to many new discoveries of fossil insectivores. Using these new methods, Jonathan Bloch, from the University of Michigan, recently described the smallest known mammal from the Willwood formation in Wyoming. Shrews are so small they have reduced their skulls, many don’t even have bony arches around their eyes. Insectivores are often considered primitive, but in reality they are highly specialized predators on insects. Two groups of moles are known, one including the modern genus Solenodon, from the West Indies and the other including talpids. Solenodon appears to be related to modern tenrecids, a group of insectivore mammals from the island of Madagascar, near Africa, because they both have zalambdodont teeth. Talpids, include moles from North America and Eurasia, appear to have originated from the late Eocene, about 40 million years ago, from the same group as modern shrews. Talpid moles and shrews both have dilambdont teeth.

Ungulates, whales and strange mammals

When we think of modern ungulate mammals (mammals that have hooves) we picture a cow or horse. But the strange thing about hooves is that they have evolved not once, but about seven separate times! Paleontologists believe that all ungulates arose from a very diverse group, often called Condylarthra. The Condylarthra are so diverse that the group is currently being broken down into separate groups. Condylarths originated during the late Cretaceous. One of the oldest members is Protungulatum which belongs to the arctocyonids and looked more like a primate than any cow or horse. The teeth of arctocyonids from the early Eocene look remarkably like the first artiodactyl, Diacodexis, and probably gave rise to them. Diacodexis is known from the earliest Eocene and looked alike like a small deer. Artiodactyls are unique from all other mammals in having one of the ankle bones (astraglus) shaped liked a pully which allowing them a extra joint of movement in the leg. In addition, artiodactyls differ from perissodactyls in having an even number of toes. Artiodactyls are divided into pigs and Hippos (Suina), Camel-like mammals (Tylopoda) and ruminats, which include cows and deer, as well as the giraffe. Suina have been around since the early Eocene (55 million years ago), but were most diverse in the Miocene 15 to 6 million years ago. The group Tylopoda include the commonly found oredonts, from the Western United States, as well as the diverse group of camels and lamas, that were very diverse in North America during the Miocene and Pliocene. Surprizingly camels became extinct in North America only about 1.8 million years, and oredonts became extinct everywhere about 6 million years ago. Ruminants are known from the Eocene to the present. They include all mammals that have a specialized extra stomach, where food is stored and broken down by bacteria. Often food is regurgitated to be chewed a second time. Next time you see a cow or deer, watch it chew on this regurgitated grass.

The Condylarths Phenacodus and Meniscotherium appear to be related to the first perissodactyls, which include horses and tapirs as well as rhinos. The first horse is Hyracotherium from the Early Eocene (55 million years ago). Check out the Florida Museum of Natural History’s online exhibit Fossil Horses in Cyberspace! Tapirs are known back into the Eocene, with such forms as Hyrachyus, and Helaletes, which were common in North America. One of the most interesting groups of perissodactyls are the brontotheres. Brontotheres were very large, with latter forms developing twin nasal horns, like Megacerops. They are only known from the Eocene and early Oligocene of North America and Asia. The amazingly bizarre chalicotheres are related to horses, and had claws. Calicotherium from Europe had very long front limbs, which it used to pull branches out of the trees to eat. Brontotheres where replaced by modern rhinos, which dominated North America, Asia and Europe during the Oligocene and Miocene.

You might be surprised to known that during the Eocene the climax predator was an ungulate! Mesonychids, were fearful creatures that had long-narrow jaws, filled with pointed teeth. They were also very quick on there feet, having developed a second joint, in there ankle, like artiodactyls and perrisodacyls. Mesonychids are closely related to whales. Toothed whales and mesonychids share numerous similarities in their skull and jaw. Modern whales originated from the same stock, as the mesonychids during the early Eocene. The first whales are known from Pakistan. Pakicetus is recognized as one of the earliest whales, it had the shark-like teeth of the mesonychids, which it used to eat fish. Modern whales are grouped into those that have teeth (odontocetes) and those that have baleen (a specialized filter in the mouth used to sift through the water to collect food) called mysticetes.

Tethytheria is a name giving to groups of African ungulates that don’t fit in other groups. These include Proboscidea (elephants), two aquatic groups the extinct Desmostylia and the Sirenians (sea cows). These group also includes Arsinoitherium a giant two horned rhino-like ungulate from the Oligocene of Egypt. Living hyraxes from the Middle East, are sometimes included in this group, or sometimes included with perissodactyls. Hyraxes were once very diverse during the Late Eocene and Oligocene of Egypt (43 to 25 million years ago). Tubulidentata, is a group, that includes one living genus, Orycterpus, the aardvark. The aardvark shares features with the Condylarths, and appear to have splintered off from that group, early during the Paleocene and Eocene (60 to 35 million years ago). Although the fossil record of the Tubulidentata, only extends back to the Miocene (about 20 million years ago).

The Condylarth families Mioclaenidae, from North American and Didolodontidae, from South America, are related to the origin of the six orders of South American ungulates. These fossil ungulates from South American, where so diverse and unique that George Gaylord Simpson (the father of modern mammalian paleontology) concluded that such diversity was evidence of a long history of isolation for South American. They include the horse-like Diadiaphorus, the camel-like Theosodon, the rabbit-like Henricosbornia and the rhino-like Toxodonta. These groups exhibit a carbon copy of what was going on with other mammals else where during the Cenozoic (65 to 3 million years ago). This has lead to many examples of convergent evolution, indicating that similar patterns of natural selection were operating in different parts of the world. It was not until the "Great American Interchange" 3 million years ago that many North American groups entered South America, leading to the extinction of all South American ungulates.

The condylarth family Hyopsodontidae, which are common in North American, but are also known from Asia, Europe and Africa, appear to have given rise to the Macroscelids, (Elephant Shrews) which live today in east Africa. Although, other scientists have placed Macroscelids as being related to the Glires (rodents and rabbits). The fossil record of Macroscelids extends back to the late Eocene of Africa. A number of weird orders of mammals, that are ungulate-like are known. They include the beaver-like Taeniodonta and Tillodonta, known from North America. And the early ungulate pantodonts which are common during the Paleocene, and represent the first large mammals known after the extinction of the dinosaurs. The Dinocerata, are a unique group, known from the late Paleocene through the middle Eocene (57 to 40 million years ago) of North America and Asia. Uintatherium was as large as a rhino and had dagger-like canine teeth, as well as a set of horns. It appears that they might be related to the Asia anagalids, the same group that gave rise to rabbits!

Xenarthra and other edentates (armadillos, sloths, and anteaters)

The Pholidonta, or scaly anteaters, extend back to the middle Eocene of Europe. Pholidonts have scales overlapping most of their body and lack any teeth. They are known today by one genus (Manis), the Pangolin, which inhabits Africa and Asia. Pholidonts are possible related to the palaeanodonts. Palaenodonts are a group of extinct anteater like mammals. The most well known is Metacheiromys, from the Eocene of North America, and looked like modern anteaters. Palaenodonts also include a fossorial group that lived in burrows, they had very flat heads, which they used to push up the soil, as they constructed their holes.

Xenarthrans include armadillos and sloths. Armadillos have a fossil record that goes back to the middle to upper Paleocene of South America. They are known form the bone scutes that modern armadillos still retain. Fossil armadillos got very large, and their fossils are common in Florida. One close relative of the armadillo, is the unique glyptodonts, which looked like giant turtles, they probably even had a small trunk like an elephant. Because they had a heavy shell on their back, glyptodonts are thought to have stayed close to the water living in swamps and went extinct in the Pleistocene less than a million years ago because of heavy glaciation, and climatic changes.

Carnivora / Creodonta (Sabertooth cats, bears, and dogs)

All carnivorous mammals have carnassial teeth, which are specialized for shearing and cutting through flesh. Creodonts have carnassial upper first molars, carnivores have carnassial upper fourth premolars. Creodonts are known from the late Paleocene to late Oligocene 60 million years ago to 28 million years ago. Creodonts are separated into three families. The hyaenodonts include the slender fox like Sinopa from the middle Eocene of North America, whereas, the oxyaenids include the robust tiger like Patriofelis. Members of the family Limnocyonidae, have long skulls and narrow snouts.

The early true carnivores are the miacoids (Caniformia). They were similar in size and appearance to ferrets. Miacoids are closely related to the viverravids (Feliformia), which had slightly longer skulls, and more slender bodies, although they are distinct from miacoids in dental anatomy. These two groups are known from the early Paleocene to late Eocene (about 63 million years ago to 35 million years). The family Canidae includes modern dogs, and are known from the late Eocene (about 38 million years ago). The family also includes the bear-like borophagines, which were a diverse group that thrived until the late Pliocene (about 1 million years ago). The family Procyonidae includes raccoons, ring-tail cats, and the red panda. They have a fossil record that extends back to the early Miocene (about 18 million years ago). Mustelidae is the family that includes ferrets, weasels, skunks and otters. More primitive mustelids were more robust and cat-like. Modern mustelids have shorten faces and are much smaller then their ancestors. Bears belong to the family Ursidae, and include the largest terrestrial carnivorous mammal, Arctodus. They are known from the fossil record as early as the late Eocene, but became diverse in the Miocene, with the appearance of the hemicyonids or "bear-wolves," which exhibited many canine features. The extinct family Amphicyonidae share many features with the hemicyonids, but occupied an early divergent evolutionary path away from the ursids. They became extinct during the Miocene with the appearance of the large bears and "bear-wolves."

Cats are represent in the fossil record by two families, the Nimravidae and the Felidae. Cats are also related to the family Hyaenidae, which includes the living hyenas of Africa. Nimravids are interesting because they exhibit many features which were copied by the felids in later sediments. For example nimravids are the first to be saber-toothed. They have such a diversity of canine sizes and shapes that the terms conical-tooth (for short round canines), dirk-tooth (for long finely serrated canines) and scimitar-tooth (for long, flat and coarsely serrated canines) have been erected to describe different forms. The felids originated during the Miocene, but did not become divergent until the late Miocene. The well known saber-tooth cat, Smilodon belongs to this family and lived only a few thousand years ago in North America. This family also includes Lynx, Puma (Moutain lion), and Panthera (Africa lion), as well as, the common house cat.