Mounted specimen on display at the Harvard Museum of Natural History
Reconstruction by Dmitry Bogdanov
When: Triassic (242 - 230 million years ago)
What: Dinodontosaurus is a synapsid, or ‘mammal like reptile’. It was one of the most common large herbivorous animals in the mid Triassic. These beasts reached lengths of 8 feet (2.4 meters) and are estimated to have weighed hundres of pounds. They fall within the clade Dicynodontia, so named for their two large front teeth. A fossil find in Brazil of over 10 Dinodontosaurus, including juveniles, shows these animals lived in herds and cared for their young.
Synapsids were extremtly common in the Permian, but were hit hard by the end Permian extinction. Some groups, such as the Dicynodonts exemplified by Dinodontosaurus, however, made it though the extinction just fine. The extinction at the end of the Triassic period, however, was brutal to this clade, wiping out the vast majority of species. Some dicynodonts made it though this extinction, but the clade continued to dwindle throughout the rest of the Mesozoic, with the last dicynodont vanishing in the mid Cretaceous. In the synapsid family tree dicynodonts are fairly far up there, falling far closer to gorgonopsids than to the basal “pelycosaurs”.
Fossil and model both on display at the American Museum of Natural History, NYC
Model created by Louis Ferraglio
When: Early Devonian (~410 to 392 million years ago)
What: Gemuendina is an odd little placoderm fish. It is known only from the early Devonian of Germany, from deposits that have been reconstructed to represent areas with anoxic bottom waters. Anoxic means ‘lack of oxygen’, so there was no oxygen in the sediments where the dead or dying fish fell, and thus scavengers and decomposing organisms could not disturb the remains. Specimens of Gemuendina have only been preserved in such conditions because their armor was not a solid shield, as seen in some other placoderms, but rather a series of unfused relatively thin bony plates. This placoderm bears a close resemblance to a ray, with its flat body and series of horizontal fins. This is another excellent example of convergent evolution. Unlike rays, however, the eyes of Gemuendina were on the top of its head, not the side, and its nostrils were on top as well, not on its ventral (under) surface.
Within Placodermi Gemuendina falls into the clade Rhenanida. This group shares the characters of a ray-like body and the lose series of unfused plates that covered their flat bodies. While the ray-like body is a shared derived feature that unites the group, the seires of individual plates is likely a retained primitive feature that was also found in the first placoderms, which gave rise to all of the rest, including massive forms such as Dunkleosteus. Gemuendina is one of the earliest well-known members of the clade, but isolated plates from tens of millions of years earlier in the Silurian period may represent the true first rhenanids. Though the fossils are rare and fragmentary, rhenanids swam throughout the Devonian waters all over the world.
Mounted specimen on display at the American Museum of Natural History, NYC
Reconstruction by Jay Matternes
When: Late Paleocene to Early Eocene (~ 61 - 55 millon years ago)
Where: North America and Europe
What: Plesiadapis is a small tree-dwelling mammal that was fairly comment in the late Paleocene of North America and Europe. This ancient mammalian taxon was about the size of a house cat, and though it may look very reminiscent of a squirrel it is a member of the primate family, as part of the larger group Plesiadapiformes. The latest research has shown that Plesiadapis was actually atypical for its namesake clade; this genus tended to be much larger than the average plesiadapiform and was not as well adapted for climbing as its smaller relatives, lacking a hand specially adapted for grasping. Plesiadapis could climb trees, but it would have been an arboreal quadruped, like the living squirrels, rather than a grasping locmotion as seen in most primates today. Another features reminiscent of rodents in Plesiadapis (and this is found in most of its kin) is its enlarged front teeth and the reduction or loss of teeth between these massive incisors and the grinding cheek teeth. Plesiadapis has been reconstructed as a frugivore - meaning its diet was primarily comprised of fruit. As much of North America and Europe was covered with lush sub-tropical forests during its range, Plesiadapis would have had quite a large selection of fruits to feed on.
The placement of Plesiadapiformes has been somewhat controversial in the past decade or so. There is uniform agreement that these animals fall somewhere near the group Euarchonta within placental mammals, but exactly where has been much debated. Euarchonta contains not only primates, but also the Scandentia (tree shrews) and Dermoptera (flying lemurs). Some early studies placed plesiadapiforms closer to the dermopterans than primates, but more recent studies tend to find this clade as either the first branches to spring off the primate lineage or just outside of Euarchonta itself, as stem taxa to all three orders. One last point to make things even more confusing! The group Plesiadapiformes? It is probably not a monophyletic (natural) group in reality. It is looking more and more like that some taxa previously grouped within Plesiadapiformes fall closer to living primates than to other taxa within the group.
To sum up that confusing mess, Plesiadapiformes are very important in understanding primate evolution, as at least some members of this assemblage of taxa are the first animals on the primate lineage. As this lineage includes me and you there is a lot of study focused on this group right now! Nice to see animals that are primarily paleocene taxa finally getting some attention.
Pachycrocuta - The Giant Hyena
Mounted specimen from the Zhoukoudian Museum, Beijing.
Reconstruction by Mauricio Antón
When: Pliocene to Pleistocene (~ 5 million to .5 million years ago)
Where: Europe, Asia, and Africa
What:Pachycrocutais a prehistoric member of the Hyaenidae. Today hyenas are restricted to Africa and western Asia, but their fossil record has revealed they were once much more wide spread. Pachycrocuta has been found in Africa and Asia, but most specimens have been found in Europe, with many localities in the Iberian Peninsula. The largest species was Pachycrocuta brevirostris, which stood over 3 and a half feet (~100 cm) at the shoulder and is estimated to have weighed over 400 lbs (190 kg). This makes it about the size of a modern lioness! Cave deposits in both Spain and China have revealed multiple almost complete skeletons, suggesting that these animals lived in packs and utilized these caves as their dens.
As Pachycrocuta is even more heavyset and adapted for bone crunching than the living bone-crunching hyenas, it has been suggested that this fossil form was even more dependent on scavenging kills than living species. But there really is not much evidence to pack this up other than thought-experiments. As it appears that some large cat species were displaced when Pachycrocuta moved into their ranges, it is more likely it was a direct hunter that would take advantage of pre-killed remains when it could drive away other predators. Like 99% of carnivores today. The predator/scavenger divide is really not a fast or hard line at all. Even more evidence of hunting comes from the remains of interactions between Pachycrocuta and Homo errectus. These two species overlapped and bones of our poor relative have been found in Pachycrocuta dens in China!
Reconstruction by Gerhard Boeggemann
When: Jurassic (~ 156 - 151 million years ago)
What: Europasaurus is the smallest sauropod known. Now remember I said smallest SAUROPOD known, so we are still looking at an animal that was about 20 feet (~6 meters) long. Keep in mind though, this includes the long neck and tail - taking those out of the picture and the animal’s body length was about 6.5 feet (~ 2 meters). This length, coupled with the shoulder height of Europasaurus ( about 5 feet [~1.5 meters]), gives us a sauropod with a body that was about the size of a modern elephant. There are a number of proposed tiny sauropods out there, but I have selected Europasaurus as it is known from several almost complete skeletons, from juveniles to adults, so we can be sure this is not just a baby sauropod that would have grown much larger. Above I have included a diagram from the original description of Europasaurus rather than a reconstructed mounted skeleton to show you just how much of the skeleton is known. Why is Europasaurus so small? Its our good friend Island Dwarfism back again! The specimens have been found in Germany, on what would have been one of many islands in the area during the late Jurassic, owing to the higher sea levels.
This smallest of all sauropods is in the clade Macranaria within the sauropod family tree. This group actually contains some of the biggest sauropods (and thus dinosuars) ever known! Talk about some size diversity within a group! An adult Europasaurus could have easily walked between the legs of some of its larger cousins. The great range of ages in the recovered specimens of Europasaurus have allowed for detailed study of the microstructure of the bones, revealing how these animals grew. From this study it has been proposed that Europasaurus managed to stay small on its restricted island habitat by not stopping its growth exceptionally earlier than its relatives, but instead by dramatically slowing down its growth rate.
I hope this cute little sauropod makes up for ruining all of those childhoods with the reality (or should I say unreality) of Brontosaurus a few posts ago. ;)
Indricotherium - The largest terrestrial mammal
Skull on display at the American Museum of History of Natural History, New York City
Reconstruction was part of the traveling Extreme Mammals exhibit, photo from when it was at the AMNH.
When: Eocene and Oligocene (~ 34 to 23 million years ago)
Where: Asia and Eastern Europe
What: Indricotherium is the largest terrestrial mammal known. It is a member of the rhinoceros family. Some material was found earlier in the 20th century, but the first fairly complete skull and skeletal elements were found by Roy Chapman Andrews in 1922 while on an expedition for the American Museum of Natural History in Mongolia. Indricotherium would have stood about 16.5 feet (~5 meters) tall at the shoulder and is estimated to have weighed in excess of 20 tons. There were sauropod dinosaurs that were smaller than Indricotherium! This giant was a herbivore and filled a simular niche to the sauropods and modern giraffes, so much so that it is sometimes referred to as the ‘giraffe rhinoceros’. It stripped tall trees bare of leaves using its large front teeth and mobile lips.
In the family tree of mammals Indricotherium is in the order Perrisodactyla (horses, rhinos, and tapirs). Within this group it in the rhinoceros clade. While rhinos today all look pretty much the same their fossil record shows this group used to be extremely diverse, with two completely extinct major sub groupings. Indricotherium is in the group Hyracodontidae (the running rhinos). This lineage divered from that leading to modern rhinos over 55 million years ago. Not all rhinos in this group were giant sized, the very first ones were no larger than wolves! All members of this group lacked horns like Indricotherium.
There is a bit of controversy and confusion surrounding the topic of what genus name to apply to this animal. Indricotherium has been proposed by some workers to be synonymous with Paraceratherium and Baluchitherium, but this is not universally accepted. I have used the name Indricotherium for this entry as both examples shown above are based upon material that has held the name Indricotherium.
Reconstruction by Pavel Riha.
When: Late Triassic (~236 - 203 million years ago)
Where: found only in Scotland.
What: Scleromochlus is a very important Triassic archosaur. It was about 7 inches (~18 cm) long, with forelimbs much shorter than its hind-limbs. Its lower leg is also elongated relative to its thigh, and it has elongated feet bone as well. Scleromochlus is a very rare taxon, known from only one fossil locality in Northern Scotland. It is an extremely important fossil for our understanding of the evolution of the archosaurs, as it is most often found to be the first species outside of the clade that contains dinosaurs and pterosaurs. This group is called the Ornithodira. The anatomy of Scleromochlus has been variably interpreted as indicating a running animal, a climber, or a hopping creature.
As it is very possible Scleromochlus strongly resembles the last common ancestor of the dinosaurs and pterosaurs, a better understanding of how it lived is critical for our knowledge of the evolution of the ornithodirans. This is especially true for the pterosaurs, of which very little ‘transitional’ material is known. It has even been suggested in the literature that Scleromochlus is in fact a basal pterosaur, but most studies do not support this placement.
When: Eocene (~48-40 million years ago)
Where: Found at the Messel fossil site in Germany
What: Leptictidium is one of the more common mammals found in the Messel fossil pit in Germany. The adults ranged from about two (~60 cm) to three (~90 cm) feet in length, with most of this length being in the long tail. Leptictidium had extremely short forelimbs relative to the length of its legs, and has been reconstructed as the first bipedal mammal. There is debate as to its precise mode of locomotion, with some researchers proposing that the animal was a fast runner and others suggesting it was saltatorial (hopping). More recent studies have supported a hopping and leaping mode of locomotion. Thanks to the extraordinary preservation of fossils from Messel, we know the tail was bald for much of its length, that Leptictidium had a short ‘trunk’, like the elephant shrews of the modern day, and that this animal ate insects and small vertebrates. Contemporaries of Leptictidium include the tiny horse Propalaeotheirum and the predatory giant flightless bird Gastornis.
Leptictida is the larger clade that includes Leptictidium and its kin. The first members of this group appear in the latest Cretaceous of western North America and the order quickly spreads throughout the northern continents, lasting until the early Oligocene about 30 million years ago, when the forests worldwide started to give way to grasslands. Previously leptictids were thought to be related to either the living lipotyphla (hedgehogs, shrews, and moles) or elephant shrews, but recent studies of the relationships of mammals have placed them outside of placental mammals entirely, making them stem eutherians and not members of Placentalia.
Mounted specimen from the National Museum of Natural History in Leiden, the Netherlands.
Reconstruction by Mauricio Anton with a common hedgehog for scale.
When: Miocene (~11 - 5 million years ago)
Where: One island that now is part of Italy
What:Deinogalerix is a comparatively giant relative of the hedgehog. It lived on what is now the Gargano peninsula in Italy, but during the Miocene this region was a separate island. Much of Italy during this time period was a series of isolate islands, owing to the higher water level. Deinogalerix was about five times the size of a common hedgehog, more the size of a small fox. However, with a skull about 1/3rd the total length of its whole body, it was proportioned very differently. An eight inch (~20 cm) skull on a 24 inch (~60cm) body isn’t too out of proportion for many of the Lipotyphla (the order that includes hedgehogs, shrews, moles, and solenodons), and it appears Deinogalerix saw no reason to shrink down its head just because of its growth spurt. I have called this animal a hedgehog, and it is in that grouping, but it did not look much at all like the little spiny animal shown above. Within the hedgehog family, its closet relatives are not true hedgehogs, but rather the gymnures or ‘moon-rats’. These animals have not developed spines as protection and are covered with a coat of long course hairs.
Mounted specimen on display at the America Museum of Natural History, NYC
Reconstruction by Charles Knight.
When: Miocene to Pliocene (~12 - 3.5 million years ago)
Where: North America, Europe, Asia, and Africa
What: Gomphotherium is a four tusked extinct proboscidean. Unlike modern elephants which only have enlarged upper incisors as their tusks, Gomphotherium and its kin had enlarged upper and lower incisors. Neither set of tusks grew as large as living elephants, but the lower jaw was heavily modified and elongated to support the lower tusks. If you look at the photograph of the mounted specimen above, you can see that the actual bone of the mandible extends to almost the tip of the upper tusk. Based on the structure of the skull of Gomphotheriumit is thought the animal had a trunk, though again not one as log as the living species of elephants. Gomphotheriumis on the small side compared to the mammoth and mastodon in the photo with it, and also is a bit smaller than the living african elephant, but about the same size as the asian elephant - standing about 10 ft (3.2 meters) tall at the shoulder. These fourtuskers were proportioned very differently from the asian elephant, however. Their legs were much shorter in proportion to their body. The genus Gomphotheriumoriginated in North America, but spread throughout most of the world before going extinct in the Pliocene.
Gomphotheriumin the group Gomphotheriidae (shocking I know). Gomphotheres ranged almost world-wide for over ten million years, and it is possible the last one died less than 10,000 years ago. I say only possible as relationships of gomphotheres, and really proboscideans as a whole, are really not well understood. Gomphotheriidae may be a paraphyletic series of taxa (not a ‘real’ group), with some taxa more closely related to the living species than others. Basically if you are interested in paleontology the study of proboscideans is an area that desperately needs more people in it. You also get to look at other cool extinct forms like Deinotherium!