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Plesiadapis
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.
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Ophiacodon
Mounted specimen on display at the American Museum of Natural History, NYC.
Reconstruction by Dmitry Bogdanov
When: Late Carboniferous to Early Permian (~305 - 280 million years ago)
Where: North America
What: Ophiacodon is a basal synapsid, meaning it is more closely related to mammals than modern reptiles. It was a fearsome predator of its day, with a long snout (over half of its skull length) full of pointy teeth. These teeth are so sharp that they are reminicent of the fangs of snakes - the name Ophiacodon means ‘Snake Tooth’. Ophiacodon reached up to 10 feet (~ 3 meters) in length and is thought to have preyed upon its fellow tetrapods rather than insects, as many previous predators focused on.
Ophiacodon occurs very early in synapsid evolutionary history, it is even more basal than the ‘sailbacks’; almost at the base of the spilt between the major living aminote clades. Ophiacodon is part of the group Ophiacodontidae, which ranged from Mid Carboniferous to the Early Permian. About a half dozen genera are known for this group, and they all were good sized predators. Ophiacodontids and other basal synapsid clades were once grouped together under the name pelycosaurs, however, this group is not monophyletic/a natural group. That means that some synapsids that were held as pelycosaurs are more closely related to mammals than they are to other ‘pelycosaurs’. ‘Pelycosaurs’ can be though of as an evolutionary grade in the synapsid lineage; these are the members of our group that more closely resemble reptiles - they had sprawling gaits and their their teeth were not differentiated - there is not even a distinct canine. Ophiacodon and its kin are assuredly synapsids, however, as shown by the single opening behind their eye sockets - their single temporal fenestra.
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Perspicaris
Reconstruction by Marianne Collins
When: Cambrian (~505 million years ago)
Where: Specimens first known from the Burgess Shale of Canada, now possibly found in other sites in North America of similar ages.
What: Perspicaris is another enigmatic stem arthropod from the Burgess Shale fossil lagerstatten deposit. It is a bivalve arthropod, like our last Burgess Shale fossil Tuzoia, but has more bits than just its eyes sticking out, so more conclusions as to its life style can be drawn. Sticking out from the front of its carapace are eyes and relatively thick antennae and emerging posteriorly is a powerful tail. Perspicaris swam though the water column under its own power, but swam to the bottom of the paleo-ocean to feed. Sediment preserved in the gut of a few specimens shows this animal was a deposit feeder; consuming loose mud on the ocean floor and processing all minute food particles from these sediment grains.
The evolutionary relationships of Perspicaris are not well known, like Tuzoia it is proposed to either be related to a subgroup of crustaceans or to be a stem taxon to all living arthropods. As you probably have concluded, the relationships between all of these taxa is an area of paleontology that really needs more people working on it!
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Tuzoia
Reconstruction by Marianne Collins
When: Cambrian (~520 - 500 million years ago)
Where: First described from Burgess Shale formation in BC, Canada, now found fairly worldwide in beds of simular ages.
What: Tuzoia is a bivalve arthropod first known from the Burgess Shale formation. It may seem odd that an animal that looks like an upside down taco with sunglasses on is an arthropod, but this is not as crazy as it first appears! In the crustacean subgroup of Arthropoda many taxa have a bivalve (2 parts that cover a great deal of the animal with a hinge in the middle) shell, if not as adults then in their larva forms. One example of a living group that has this type of shell as adults is the ostracods, and more well known taxa such as lobsters have shells such as this in their juvenile stages. To complicate matters even more, there are a number of fossil taxa, some also known from this locality, that have simular shells.
The placement of Tuzoia -within- arthropoda is more problematic. It is possible that it on the lineage leading to crustaceans, or at least somewhere in that general vicinity, but it is conservatively held as a ‘stem arthropod’ in most classifications for now. As most of the animal is hidden within its shell, most of the features that would confidently inform its position are unknown. I didn’t include any of the images, as they are really hard to interpret, but there are specimens known with stalked eyes and possible antennae peeking out from its spiky ornamented shell. As removing the shell would potentially destroy any preserved soft-parts beneath it, this has not been done on any specimens, to my knowledge. In my opinion they should toss this sucker into a CT scanner to find out what is underneath there.In life Tuzoia swam freely though the water column. Without knowledge of its mouth parts its difficult to know if it was a filter feeder or a small predator. Tuzoia reached a maximum size of about 7 inches (~180 mm).
For more information and images see: http://www.burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=125&m=5&&ref=i
And check out the Burgess Shale tag on Daily Fossil to see other odd odd animals from this formation!
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Rhynchonkos
Reconstruction by Smokeybjb.
When: Late Permian (~275 - 270 million years ago)
Where: Oklahoma, USA
What: Rhynchonkos is a very rare amphibian that lived in the swamp land covering what is now Oklahoma in the Permian. It was about 4.5 inches (~11 cm) long, not counting the tail, with an extremely elongated body and tiny tiny limbs. The elongation of its body compared to other amphibians was accomplished via replication of vertebrae, not elongation of each individual bone. Rhynchonkos had at least 36 pre-sacral (before the hips) vertebrae. Its mouth was full of rows of tiny teeth, and it is likely that it ate insects and small fish in its swampy home. Older literature about this animal refers to it as Goniorhynchus rather than its current name. This change is due to the fact that the fossil taxon was named in 1970, however, a moth was given the name Goniorhynchus in 1896. Stupid insects. At least it wasn’t a beetle this time! The name Rhynchonkos was applied in 1981.
The phylogentic relationships of Rhynchonkos are fairly uncertain. For some time it was held as a close relative of modern caecilians (a group of limbless amphibians), but later fossil finds have cast doubt upon this affiliation. Within other fossil ‘amphbians’ Rhynchonkos has been placed in Lepospondyli (along with our friend Diplocaulus). This group as a whole has a much debated relationship with living amphibians. Some studies have them having nothing to do with living amphibians (lissamphibians), where as others link specific taxa with certain groups of living amphibians. Such as the now disputed Rhynchonkos - caecilian link. It may seem obvious to link this almost limbless fossil with the limbless amphibians, but amphibians (and lizards too!) seem to like to lose their limbs at the drop of a hat. It is very common in swimming and burrowing forms.
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Neoproterozoic, Paleozoic, Mesozoic, Miocene and Pleistocene, by Alain Bénéteau for the Toulouse Museum.
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Fasciculus
When: Cambrian (~505 million years ago)
Where: Canada
What: Fasciculus is yet another fantastic fossil from the Burgess Shale fossil site in BC, Canada. It is extremely rare, with only one known specimen. This single specimen was found in 1917, but not fully described in detail until 1996. It is about 4.5 inches (11.4 cm) wide, and is perserved in intricate detail. Unlike most fossils from the Burgess Shale, Fasciculus is confidently linked to a living group. It is placed as a stem Ctenophora,which are the comb-jellies. Comb-jellies are not closely related to ‘normal’ jellyfish, which fall into the cnidaria along with the hydras. Ctenophoras are cup-like animals that swim though the water, with multiple sets of cilia arranged in longitudinal series, so that they resemble combs, giving them their common name of comb-jelly. In Fasciculus you can see these rows wonderfully preserved. The discovery of a stem member of this lineage once again shows that most, if not all, modern phyla were well separated even as long ago as the Cambrian.
For more read up on this very rare fossil and its contemporaries at: http://burgess-shale.rom.on.ca/en/index.php
Reconstruction by Marianne Collins
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Diadectes
When: Early Permian (299-271 Million years ago)
Where: North America
What: Diadectes is a large reptiliomorph. That means that it is on the line towards amniotes (reptiles and mammals), but is not quite there yet. Diadectes is closer than most reptilomorphs, and is found in many studies to be the sister taxonto the amniotes. It is thought to have been amphibious, but spending most of its time on land. The dentition of Diadectes clearly marks it as an herbivore; its front teeth were procumbent(they tilted forward) and its cheek teeth were blunted and fairly wide compared to other stem reptilomorphs. Diadectes was one of the first large terrestrial herbivores, though it was not the only one of its time. Edaphosaurus was a synapsid (‘mammal-like reptile’) contemporary of Diadectes. The herbivorous diet of these two taxa was attained convergently, as there was by this time a large amount of plant material covering the land.
Diadectes looks very reptilian, and in fact some early studies placed it as a true reptile, but odds are the clade is belongs to is not directly ancestral to any of the amniotes. Though Diadectes with its large bulky form and size of up to 9 feet (~3 meters) is convergent on later reptiles such as Scutosaurus, the first true reptiles were small lizard like insect eating forms, such as Hylonomus.
Reconstruction by Matt Celeskey.
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Haikouichthys
When: Early Cambrian (~530 million years ago)
Where: China
What: Haikouichthys is one of the oldest fish known. Though first described from a single specimen in 1999, it is now one of the most well understood Cambrian chordates, as a spectacular find in China unearthed over five hundred individual specimens. Haikouichthys was primarily soft bodied, and so most of these fossils are impressions. There is a limited amount of information one can glean from a squished fossil, but when you have literally hundreds of individuals of a single species, all squished in a slightly different way, most of the morphology of an animal can be determined. This ancient fish was tiny, at only about 1.2 inches (~3 cm) long on average. It had well developed anteriorly placed eyes, a notocord, and rudimentary fins along the midline of its body. It was not an armored fish, but several specimens suggest it had a cartilaginous head shield in life, a precursor to the ossified dermal armor of the later placoderms.
Haikouichthys was closely related to the ancestor of all later fish, not just the placoderms, and as fish gave rise to tetrapods, it is also one of the first representatives of our own lineage. Phylogenetic analysis place Haikouichthys near the base of all vertebrates, with some minor ambiguity as to its placement relative to the living jawless fishes, the hagfish and lamprey. These two forms are not commonly mentioned when ‘living fossils’ are highlighted, but they are the only living examples of a once diverse assemblage of jawless and mostly finless fish.
Reconstruction from Talifero via wikimedia.
