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.
Tiktaalik - the fishapod
Model by Tyler Keillor and this particular set up on display at the Harvard Museum of Natural History
When: Late Devonian (~375 million years ago)
Where: Found on Ellesmere Island in Nunavut, Canada
What: Tiktaalik is a very critical specimen on the line of tetrapod evolution. In the tetrapod family tree it falls between sarcopterygians (‘lobed fin’ fish) that looked much like the living Coelacanth and more advanced tetrapods, such as Acanthostega.The discovery and announcement of Tiktaalik was very exciting, as fossils on both side of transitional period were known for a long time, but nothing really in the middle. Of course as with the discovery of any ‘missing link’ now we have two more ‘links missing’: one on either side of Tiktaalik ;). The most important part of the specimen is the anatomy of its forelimb - there was a well developed wrist inside the fin of Tiktaalik! Not only that, but it possibly has the first ‘fingers’ seen in the tetrapod lineage. Unfortunately the back end of Tiktaalik is unknown… for now!
In life Tiktaalik would have been an aquatic animal, as its limbs could not support its weight on land - but they would have been very helpful for maneuvering the creature around the shallow waters of prehistoric Canada. Based on the spiracles - openings behind the eyes- of the skull it has been preposed Tiktaalik could have had a form of primitive lung.
If you want to know more about Tiktaalik - check out its website at: http://tiktaalik.uchicago.edu/. And for more in depth reading, I cannot recommend the book ‘Your Inner Fish’, written by the discover of Tiktaalik - Neil Shubin, enough! It is a really great explanation of how Tiktaalik fits into the evolution of tetrapods and explaining homology in general! Shubin has done a fantastic job of promoting public science education using this great Tiktaalik specimen as a starting point.
Just look at all of these models getting ready to go out to museums. Maybe one is near you!
Fossil specimen from the Museum für Naturkunde in Berlin, Germany
Model by Dan Erickson and on display at the American Museum of Natural History
When: Permian to Cretaceous (260 to 80 million years ago)
Where: World Wide
What: Hybodus is a very wide spread, both temporally and geographically, fossil shark. I will be upfront here and say that I may be grossly over representing its temporal range, the literature is rather confusing and there have been a number of species going in and out of Hybodus over the years. So you may want to consider this an article on hybodontiform sharks in general, rather than just the one genus. Shark fossils are fairly rare in the fossil record when compared to other fish because sharks do not ossified their skeleton. However, Hybodus and its kin can be identified from fragmentary remains by their distintive teeth (two kinds in their jaws, both flat and pointy) and their ossified dorsal spines. These spines can be easily seen on both the fossil and the model above, they were most likely involved with stabilization of Hybodus as it swam. The relatively few full body specimens preserved complete the picture, showing us that Hybodus was a streamlined shark with a very heavy ribcage compared to most sharks, and that the males had not only ventral claspers, as seen in modern sharks, but also a series of spines on the side of the head - which are depicted above.
Hybodontiform sharks were the dominate group of sharks in the Jurassic period, and were even very common in the late Cretaceous after modern sharks had originated and diversified. Studies of this archaic shark clade have shown they were most likely over all slow swimmers, but they could enjoy brief bursts of speed if needed. The diverse teeth forms of hybodont sharks imply they did not just eat fish, but also were able to prey on hard shelled invertebrates. In the shark family tree Hybodontiformes is the first group outside of Neoselachii - the clade that contains all living sharks and rays.
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.
Fossil specimen on display at the American Museum of Natural History
Reconstruction by Hirokazu Tokugawa
When: Permian (~299 to 265 million years ago)
Where: North America
What: Cotylorhynchus is a member of one of the most basal groups of synapsids, the Caseidae. Cotylorhynchus was a herbivore, and reached lengths of up to 20 feet (~6 meters) long, with a massive barrel chest, putting weight estimates at around 2 tons. This animal is very large for its time… well at least its body is. Cotylorhynchus has one of the most extreme cases of ‘tiny head’ I have ever seen. Even more so than the sail-backed Edaphaosaurus! Which is closer to modern mammals than Cotylorhynchus is. It is one of the most primitive animals known that unambiguously falls on the synapsid lineage. It is so basal that it does not even have any differentiation seen in its dentition, though there are less teeth than found in the non synapsid contemporaries of this wee-headed creature.
Reconstruction by Marianne Collins
When: Cambrian (~505 million years ago)
Where: British Columbia, Canada
What: Louisella is a worm-type organism from the Burgess Shale formation in the Canadian Rockies of BC. This organism was about 12 inches (~30 centimeters) long, with a proboscis at its anterior end that could be inverted into the body or extruded. In the images above it is inverted into the body in the fossil specimen, but shown at its full extended length in the reconstruction. This structure was ringed by a series of spines with shorter and more robust spikes on the end of the proboscis. These structures would rub past one another as the animal extended and retracted its proboscis, allowing it to ‘chew’ its food. The rows of short fringes on one surface of Louisella are thought to possible have been the animal’s gills. This worm has been reconstructed as a burrowing and carnivorous creature, and due to the grinding capability afforded to it by its proboscis, it likely ate animals of a relatively large size.
Louisella is currently held as a stem fossil on the lineage leading to the Priapulida worms, also known as the ‘Penis worms’. I swear I am not making that up. These worms are very rare even today, with less than 20 living species known. They, like their ancient relative, are burrowing creatures which hunt other invertebrates. Fossils of priapulid worms are also rare, only a handful of Louisella specimens are known. What is far more common though are their distinctive shaped burrows, which are sort of an interlocking L-shape. The appearance of this type of burrows is one of the biostratigraphic markers of the start of the Cambrian period.
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.
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!
Painted reconstruction by Brian Choo.
When: Devonian (~390 to 360 million years ago)
Where: Europe and North America
What: Cheirolepis is one of the oldest of the ray-finned fishes. It was about ~10 inches (25 cm) long on average. It was covered with thick scales that were in tight articulation with each other with an even heavier covering on the head. This ‘head shield’ was made of jointed dermal bones. Cheirolepis had a heteroceral tail, meaning the bones forming the tail were not in the middle of the structure. The tail appears symmetrical however due to the well developed fin. Its pectoral fins were also covered with jointed dermal bones. Cheirolepis had well developed teeth, both on the margins of its mouth on its jaw bones, but also deeper inside its mouth on interior bones. It would have been able to open its mouth extremely wide; preying on animals up to half its size. It was a fast swimming predator of the devonian seas, not even the heavily armored placoderms were safe - the teeth on the inner skull bones would have been well suited for crunching their armor plating.
The ray finnied fishes, the Actinopterygii, are today by far the most common fish in the world. Cheirolepis is a Devonian actinopterygian, but the first members of the group date to the Silurian. Only about ten genera of ray-finned fishes are known from the Devonian, and all look very simular to Cheirolepis. The fish that dominated the devonian waters were the lobefins: the Sarcopterygii. As you have probably gathered, a major difference between these forms even in the ancient Paleozoic is how the fin is formed. Actinopterygians have a simple serial arrangement of bones all in a row from the back of the fin to the front. Whereas, Sacopterygians have much sturdier bones in their fins, connecting to each other down the length of the fin.
In this picture the Sarcopterygiifin is on the left and the Actinopterygii on the right.
After the devonian the ray finned fishes had a dramatic explosive radiation. Even today though there are some living ray-fins that look a lot like Cheirolepis, such as Amia - the bowfin. Today lobe fined fish are extremely rare, using a non cladistic definition. However there are more sacopterygians around than you might first think! Tetropods are descendant from these lobe finned fishes. As humans are tetrapods, you too are a Sacopterygian!
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.
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!