Your Daily Fossil

Hybodus

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. 

Saurodon - a sword eel

Mounted reconstruction on display at the Rocky Mountain Dinosaur Resource Center,  Woodland Park, Colorado 

Reconstruction by Charles Bonner

When: Cretaceous (~ 89 - 83 million years ago)

Where: North America

What: Saurodon is one of the large fish which swam though the Cretaceous Seaway, the marine waters that covered much of North America during the late Mesozoic. This particular species was ‘only’ about 8.5 feet (~2.6 meters) long, with a relatively skinny body and large pointed lower jaw. These features are what gives the family Saurodontidae the nick-name ‘sword eels’. The Saurodontidae fall into the later group Ichthyodectidae, a completely extinct clade that contains some of the largest fish on record. Today the living relatives of these gigantic fishes are in the clade Osteoglossomorpha and are some of the largest bony fish that swim though today’s waters. 

This was not a very specious group - there are only three described species - but they have been known to science for almost two-hundred years. The first Saurodontidaewas named in 1824 by Richard Harlan (the discover of Harlan’s ground sloth) - but was misidentified as the jaw of an extinct marine reptile. This was corrected only six years later when the first Saurodon specimen was found, and it was clear that the fragmentary specimen which was previously named belonged to a large fish, not a marine reptile. The use of the long lower jaw in Saurodon and its kin is not well understood, but it has been hypothesized that perhaps these predatory fish dug prey out from the deep muds at the bottom of the seaway. 

Kronosaurus

Mounted specimen on display at Harvard Museum of Natural History

Reconstruction by Jaime Chirinos

When: Cretaceous (~ 125 - 99 million years ago)

Where: Australia

What: Kronosaurus is an australian plesiosaur. Yes, it is a plesiosaur even though it lacks the long neck that many people associated with the group. Plesiosauria is roughly divided into two groups;  Plesiosauroidea - the long necked forms and Pliosauroidea - the short necked forms. Kronosaurus is an example of the latter clade, and shows many of the defining features of this group - such as an enormous head with massive jaws, a short neck, and a relatively short tail- the opposite in many ways of their cousins the plesiosauroids. This australian sea monster was one of the largest of its clade, with estimates of up to 33 feet long (~10 meters). Its teeth reach almost 5 inches (~12 cm ) long in crown length - the part above the gumline. The total tooth would have been over double in size. The large size of its teeth, combined with distinct shape and the lack of clear cutting surfaces also  for their easy identification if they are found as isolated material. 

The Kronosaurus specimen seen above was found in on private property in central Queensland, Australia in the 1920s. A crew from Harvard was shown where the specimen was weathering out, and set about excavating the fossil. After years of work, the specimen was boxed up into over 80 crates, weighing in at over 6 tons and shipped to the states, where it was mounted at the Harvard museum. Decades later the original discoverer of the material finally got the see the results of the preparation and mounting of what he termed ‘his dinosaur’ at the age of 93. In life Kronosaurus was a top predator; there are fossils of Elasmosauridae plesiosaurs that show bite wounds that could have come from Kronosaurus! No fish for this animal, it was after much bigger prey, leading to amazing plesiosaur vs plesiosaur encounters. Or so I like to imagine!

Cheirolepis 

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! 

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. 

 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! 

Livyatan melvillei 

When: Miocene (~12 - 13 million years ago)

Where: Peru

What: Livyatan is a gigantic toothed whale. It is fairly closely related to the living sperm whale, and is thought to have been about the same size, at 45 feet (~14 meters) long. This is an estimate as the whole body was not found, but its head was fairly well preserved, and its skull alone is 10 feet (~3 meters long) Unlike the modern sperm whales, it had a full set of teeth in both its upper and lower jaws, and its lower jaw was not reduced compared to its skull. Inside these giant jaws were giant teeth, the largest of which are 1.2 feet (~36 cm) long. What did they eat with these massive jaws and gigantic teeth? Well, living sperm whales eat very large prey, such as giant squids and megamouth sharks with their comparatively small jaws and teeth. It has been suggested that Livyatan was feeding upon other whales at the time! Such as the reconstruction above where a Livyatan  dramatically ruins the day of a Cetotherium (an extinct baleen whale). 

The name ‘Livyatan melvillei’ is meant to bring to mind Melville and his famous white sperm whale Moby Dick. Originally the name published was Leviathan melvillei, but it had to be changed, as the genus name of Leviathan was already taken! It belongs to a poorly known species of mastodon named by a researcher in the mid 1800s. Thus, the spelling of this giant whale’s name had to be altered, as once a name is applied to something it is there forever! Let this be a lesson to carefully check your species names before you publish them, as there are a few cases of something like this happening. Mostly it seems species of theropod dinosaurs are accidentally given names that have already been applied to beetles. Whoops! 

The area of Peru where Livyatan  was found is today a harsh desert, but geologists think that during the Miocene this area was an ocean paradise; a warm shallow lagoon. Dozens of marine species have been found in this desert, not only a variety of toothed and baleen whales, but also sharks and pinnipeds. 

Reconstruction by C. Letenneur, Muséum National d’Histoire Naturelle, Paris, France

Ichthyosaurus

When: Early Jurassic (~200 to 189 million years ago)

Where: North American and Europe

What: Ichtyosaurus is an Ichthyosaur (shock!).  These fossils are common throughout western Europe, and thus the history of study extends back to the late 17th century. The first fragments known were found in Whales and published in 1699, but it was not until 1811 that a complete specimen was discovered on the southern coast of England by Mary Anning (who was an absolutely amazing paleontologist who you should all read up on).  This was only a skeleton, however, and the first reconstructions of the marine reptile omitted a dorsal fin. The full morphology of the 6 and a half foot (~2 meter) long  Ichtyosaurus (which is small for an Ichthyosaur!) was soon known due to the dozens upon dozens of complete skeletons found in the Holzmaden limestone quarry in Germany. These amazing skeletons commonly preserve soft tissue outlines, and so it could be seen that these reptiles were even more convergent upon dolphins than first assumed.  They even gave birth like dolphins do, as seen in the amazing specimen above. Before specimens demonstrating that Ichthyosaurs had developed vivipary (live birth) were discovered, it was thought this beast had to drag itself on land to lay eggs, and you can find some really early reconstructions out there showing this behavior. It is thought that many, if not all, of the specimens demonstrating live birth did not actually die during childbirth; rather the mother-to-be died and sank to the bottom of the ocean and the unborn fetus was ejected during decomposition. 

Ichthyosaurs compared to modern dolphins is a remarkable example of convergent evolution, though the reptilian roots of Ichtyosaurus and its kin are evident at a glance. it preserves hind flippers and has a vertical tail instead of a horizontal tail. Both of these trails relate to the different movement mechanisms in reptiles vs mammals; reptiles move side to side, whereas mammals have a more up-down movement. Thus, while the hind flippers of a marine reptile are useful for steering, they only increase water resistance for a dolphin. These two styles of movement are not just in aquatic animals. Think of how a lizard moves vs a dog. The lizard’s hindquarters shift side to side in relation to its front, whereas the dog pulls its rear legs forwards with no side to side motion.  

Skara 

When: Cambrian (~500 million years ago)

Where: Sweden

What: Skara is  one of the many tiny species of arthropods collected from an amazing swedish fossil locality. These fossils are notable for the intricate morphology still present, with many examples preserving individual cila, pours, and every tiny plate that makes up these animals’ shells. These fossils are made of phosphate, and what is preserved is actually a secondary coating of this compound which was deposited shortly after the animal died and was buried, thus these specimens are hollow and exceedingly fragile. Fossils with this type of preservation are called ‘orsten fossils’.  This  mechanism of fossilization has been found in several places around the world, the swedish locality is in Västergötland and was discovered in 1975 by Klaus J. Müller, a professor of micropaleontology at the University of Bonn. During the time of deposition, this part of Sweden, like most of the region, was covered by a shallow and warm sea. 

All of these fossils are exceedingly tiny, the largest that has been found is only about 2 millimeters along its longest axis. Large animals evidently cannot be preserved by a phosphate coating. Skara is actually one of the larger specimens, coming in at a whopping 1.2 millimeters long! This form has a long flexible tail and trunk, with 5 pairs of anterior appendages. It has no eyes, and has been reconstructed as a benthic filter feeder. Skara has been placed in its own group, the Skaracarida.  It is a crustacean arthropod, there are over 60,000 species of  living crustaceans - including lobsters, crabs, shrimp, and barnacles, to name just a few. Within Crustacea Skara is closely related to the copepods.  These tiny tiny  fossils are extremely important, as they tell us a lot about the Cambrian world that was unknown to science before their discovery, such as how diverse non-trilobite arthropods already were at this early time. Additionally, as such small forms are preserved, juveniles of many fossil species are known and in some cases complete growth series. 

For more information about these amazing fossils, visit the CORE website: http://www.core-orsten-research.de/index.html  (Skara is in their logo!)

Metriorhynchus

Reconstruction by Dmitry Bogdanov. 

When: Late Jurassic (167-155 million years ago)

Where: Europe

What: Metriorhynchus is an extinct marine crocodile-relative. It is a member of the most aquatically adapted group of crocodile-line archosaurs ever known, the Metriorhynchidae. Like the rest of this group, it was more streamlined than most crocs, having lost the heavy dermal armor (osteoderms) that characterize the vast majority of crocodylomorphs. Its tail was finned and its  feet were transformed into paddles, making it an excellent swimmer. Its snout was very narrow, which is correlated with eating fish in living crocodiles. The group Metriorhynchus  belongs to is the only clade in the crocodile lineage to have ever developed paddles, even though for much of the history of crocodiles they have been heavily linked with aquatic habitats. 

It is unknown how much Metriorhynchus and its relatives came onto land. As its feet are paddles, it is tempting to think that it never left the water. It would have assuredly been very slow and awkward on land, dragging its bulk with its relatively small paddles. However, there is no evidence of live birth in Metriorhynchus  or its relatives. In fact, no archosaur (dinosaurs + crocodiles) has ever been shown to assuredly have developed live birth. Thus, it is likely that Metriorhynchus did drag its 10 feet (~3 meter) long body out of the water to lay its eggs, much like the modern sea turtles. 

Wiwaxia

When: Mid-Cambrian (515-500 million years ago)

Where: British Columbia, Canada, Northern Australia, and China

What: Wiwaxia is another odd form from the Burgess Shale fossil deposit in BC, Canada. This animal was small, only about 2 inches (~5 cm) long at the most, with many specimens much smaller than this. The smallest individuals represent juveniles, at only .13 inches (~.34 cm) long.  It was covered in small armored plates, with larger projections extending dorsally. It is thought the small plates protected it from smaller predators, and the large spikes made it hard for larger predators to crack this armor. The smallest individuals lack these spines, implying they grew in as the animal matured. Its ventral surface was soft and unarmored, with a set of hard ‘teeth’ at the anterior end, showing Wiwaxia fed by moving over the sediment and filtering out microscopic food particles.

As with many of the Burgess Shale fauna the phylogenetic placement of Wiwaxia is greatly debated. The story is the same as for taxa such as Hallucigenia and Opabinia; it is unknown if this form is a representative of a phylum that has no living members today, or if it is an early branch off a phylum that has persisted to the modern day. The proposed living relatives of Wiwaxia are polychaete worms and mollucs, but there is not espically strong support for either placement. 

For more images of Wiwaxia check out its page on the amazing Burgress Shale Website, presented by the Royal Ontario Museum. 

Also Wiwaxia is really fun to say out loud, you should try it!