STL Science Center

STL Science Center

30 August 2020

Man or Nature

 Megatherium went extinct approximately 12,000 years ago. They were endemic to the southern continent of the Americas (South America). We know that the extinction events at the end of the Pleistocene era helped the giant ground sloths hasten their extinction. However, we also know that Megatherium, in particular the species M. americanum, was likely hunted in substantial numbers which helped them to become extinct. We mentioned yesterday that an adult Megatherium had few, if any, predators. One predator it did face had the social cooperation and tools necessary to kill these near-mammoth sized animals without extreme risks to themselves. Everyone thinking about humans is very correct. Human beings migrated across the isthmus of Panama somewhere around the time that Megatherium was beginning to severely decline, a little before 12,000 years ago. The migrating peoples of what we now know as South America were right on the heels of the extinction events that were driving down giant ground sloth numbers. Their hunting of Megatherium likely pushed an on the brink group of animals over the edge indefinitely. We know that the earliest human settlers of South America killed and ate giant ground sloths thanks to the discovery of a site known as Campo Laborde in the Argentine Pampas. In the Buenos Aires province near the east coast, a site was discovered and analyzed by Politis et al. (2019) that held evidence of the killing and butchering of the giant ground sloths by early human settlers of the region. The site is dated to around 12,600 years ago (BP: before present in the paper). Large numbers of stone tools have been found and Megatherium ribs with cutting marks match the overall striking characteristics of the stone tools. Coincidentally, North American ground sloths (Megalonychdidae) were on the menu as well for early humans. Fossil trackways from New Mexico somewhere between11,000 and 13,000 years ago (an estimated window of time is reported in this paper) appear to show a Megalonychdid sloth defending against human attackers. The interpretation by Bustos et al. (2018) is not about our Megatherium sloths, but if it is from slightly earlier (circa 13,000 BP), it may show that the migrating human groups encountered and knew how to hunt sloths long before they encountered Megatherium and hastened its extinction. Either way, the interpretive figure from the articles accompanying the Bustos et al. (2018) publication look a lot like dancing with giant ground sloths, and I will fully endorse that.

Credit: Alex McClelland, Bournemouth University

Literature Cited

Bustos, D., Jakeway, J., Urban, T. M., Holliday, V. T., Fenerty, B., Raichlen, D. A., Budka, M., Reynolds, S. C., Allen, B. D., Love, D. W., Santucci, V. L., Odess, D., Willey, P., McDonald, H. G., Bennett, M. R. (2018). Footprints preserve terminal Pleistocene hunt? Human-sloth interactions in North America. Science Advances 4:eaar7621. DOI: 10.1126/sciadv.aar7621

Politis, G. G.; Messineo, P. G.; Stafford, T. W.; Lindsey, E. L. (2019). "Campo Laborde: A Late Pleistocene giant ground sloth kill and butchering site in the Pampas". Science Advances. 5 (3): eaau4546. doi:10.1126/sciadv.aau4546.

29 August 2020

 During the early Pleistocene the South American landscape was filled with giant animals. Out birds from last week were (specifically Phorusrhacos) not a significant feature of the landscape anymore. Not that they were waiting specifically for the terror birds to become less important fauna of this landscape, but mammals were becoming more dominant and larger during this time. This coincided with stabilization of the animal movements across the isthmus of Panama, but the animal we will look at this week was not necessarily interested in that migration; though some of the descendants of the migration into South America likely impacted the evolution of this animal from its ancestors over the years since the Great American Biotic Interchange had started. The image below, by Dmitry Bogdanov, shows three of Argentina's large land animals of the early Pleistocene. From left to right they are Argentavis, a large flying bird, Glyptodon, a relative of the extant armadillos, and Megatherium, otherwise known as the Megathere, or Giant Ground Sloth. As a genus, Megatherium has been divided into two recognized subgenera (more on this later) and 6 recognized species. Ultimately, Megatherium and the Glyptodon are related, as both are in the Superorder Xenarthra, which also includes modern armadillos, tree sloths, and South American anteaters (other animals called anteaters, like the aardvark actually belong to different groups of animals). One of the largest ever known land mammals, Megatherium was approximately 6 m (20 ft) from nose to tail and 4 tons. These estimations make Megatherium as large as modern elephants and second, in history, to very few mammals; a few species of contemporaneous mammoths were larger. Possibly at least partially bipedal, Megatherium was capable of reaching vegetation that other herbivorous mammals, like Glyptodon, would not have been able to reach. Their shear size would have made them vulnerable to very few predators as full grown adults.

Argentavis, Glyptodon, and Megtherium on a hillside ©Dmitry Bogdanov

28 August 2020

This image shows the bones that were studied by Degrange et al. (2019) in white and the remainder of the reconstructed skeleton (Paul 2002) in grey. As we mentioned very briefly yesterday, the skull is massive. We can appreciate that by taking into account the scale bar from the paper, shown at the bottom, which equals 1 m. However, one part of Phorusrhacos we have not discussed really are the enormous legs that this animal possessed. The femur (and therefore thigh) is shorter than the leg (tibiotarsus) and the bone (tarsometatarsus) that is made up of bones that we possess in our ankle and the superior (upper) portion of the foot. This leg and foot are immense, like the skull, and have led many to consider the hypothesis that in addition to being able to run down prey and strike with their beaks, Phorusrhacos (and related birds) were able to stomp and kick their prey to death in a similar manner to the way that cassowary or secretary birds can kick attackers or prey (in the case of the secretary bird). The secretary bird (on a casual stroll in this video) looks a lot like what we might picture a theropod dinosaur looking like, as it skulks around the savannah. Add in the last portion of that video, where the bird kicks and stomps some lizards into submission, and then extrapolate that to a bird that is nearly 100 kg and 2.4 m (7.9 ft) tall. Then picture the animal it is trampling being a small mammal that looks something like a weasel. Those are actually pretty scary, and awe inspiring, legs on a flightless bird.
Literature Cited:
Degrange, F. J., Eddy, D., Puerta, P. Clarke, J. (2019). New skull remains of Phorusrhacos longissimus (Aves, Cariamiformes) from the Miocene of Argentina: implications for the morphology of Phorusrhacidae. Journal of Paleontology. 93 (6): 1221-1233. DOI: 
Scale bar = 1 m, reconstruction of Phorusrhacos is modified from Paul (2002). Adapted from Degrange et al. 2019.


26 August 2020

What Does it Eat?

 The simple answer to the question "What does Phorusrhacos eat?" is pretty much anything smaller than itself that it could chase down or surprise. However, we could instead scour the literature and time scales and create a much more definitive and accurate list as well, if one were so inclined. Knowing that Phorusrhacos is well represented in the Santacrucian South American Land Mammal Age, I think that looking at the group of animals that are known and of a size that they could have been prey is a little more informative than making a giant pile of South American fauna. In popular venues (like BBC's Walking with Beasts) Phorusrhacos is shown attacking Smilodon cubs, Macrauchenia (as a scavenger), and smaller animals like Diadiaphorous. The majority of the animals that Phorusrhacos attacks in popular culture shows are pure imagination because they were not contemporaneous with the large bird. Of the three mentioned, Diadiaphorous is the only likely prey item because it did live at the same time. Necrolestes (possibly a mole-like creature), Cladosictis (a small marsupial carnivore), and Peltephilus (a genus of canine-sized armadillos) could have also been on the menu for Phorusrhacos. "Anything smaller" could have included other terror birds, like Patagornis and Psilopterus, other birds in general (Liptornis and Thegornis, for example), and the young of large ungulates (like Nesodon), ground sloths (like Pelecyodon), and Astrapotheres (large animals that looked like elephants or tapirs). While images of Phorusrhacos like the one show here are really intriguing, Glyptodon would not have been on the menu either, as it was a Plesitocene mammal, and Phorusrhacos was long gone by the time it came around.

The image shows a Phorusrhacos standing on the skeletal remains of a Glyptodon, and animal which existed after the extinction of Phorusrhacos. ©H. Santiago Druetta

24 August 2020

Flightless Tradition

The ancestors of Phorusrhacos were mainly flightless animals. Studies like Alvarenga et al. (2003) placed these giant birds in the family of rails (Ralliformes), indicating that their closest living relatives are flighted birds. Garcia et al. (2020) published a tree of the rail family that did not include Phorusrhacos; their data was genetic though, and (to my knowledge) no genetic studies have been possible within the Phorusrhacidae. 

Ignoring rails for the moment, we turn to the immense height of Phorusrhacos. At 2.4 m (7.9 ft) tall, Phorusrhacos would have towered over modern human beings. These birds were barely above the middle height of their family though. Other members of the family Phorusrhacidae were over 3.2 m (10 ft) tall. Phorusrhacos was not the smallest of these birds either though; that honor currently belongs to two genera currently: Procariama simplex (Rovereto, 1914) and Psilopterus bachmanni (Moreno and Mercerat 1891) measuring in at 70 cm (2.3 ft) tall. Psilopterus genera are often estimated to be slightly taller than this, but the lower end estimate is the same as for Proceriama. A nearly 1 m or 2.5 foot tall terror bird is no laughing matter of course, but we can all agree that a bird the size of Phorusrhacos is very awe inspiring bird. I think we should appreciate that these birds existed and that they were all very large and dominant birds. I also think we might want to heave a sigh of relief that they have not persisted in history to the present day.

Though, I admit I would love to keep one as a pet, maybe get a saddle...

From Alvarengo and Hofling, 2003, showing the height comparison of a number of Phorusrhacid birds compares to a 1.75 m tall human

Sources to Consider

Alvarenga, H. M.F.; Höfling, E. (2003). "Systematic revision of the Phorusrhacidae (Aves: Ralliformes)". Papéis Avulsos de Zoologia. 43 (4): 55–91. doi:10.1590/S0031-10492003000400001

Degrange, F. J.; Tambussi, C. P.; Taglioretti, M. L.; Dondas, A.; Scaglia, F. (2015). "A new Mesembriornithinae (Aves, Phorusrhacidae) provides new insights into the phylogeny and sensory capabilities of terror birds". Journal of Vertebrate Paleontology. 35 (2): e912656. doi:10.1080/02724634.2014.912656

Garcia-R, J.C.; Lemmon, E.M.; Lemmon, A.R.; French, N. (2020). "Phylogenomic reconstruction sheds light on new relationships and timescale of rails (Aves: Rallidae) evolution". Diversity. 12 (2): 70. doi:10.3390/d12020070.

22 August 2020

The Name of Many Terrors

The holotype is a mandible in the Museo de La Plata in Argentina and it is labeled MLP-118. The name that the scientific community knows this mandible by is Phorusrhacos longissimus (Ameghino 1887). In more colloquial terms we refer to it as a "Terror Bird". Today remains of this bird are known from the Santa Cruz and Monte Leon Formations from the Miocene epoch (23 MYA to 5.3 MYA). Originally described as a toothless mammal, Florentino Ameghino appears to have named the animal for a wrinkled appearance of bone on the mandible; the name means "Wrinkle bearer" (for more on other possible translations see Lydekker, below). In 1889 Ameghino offered a change to the name, for grammatical reasons, to Phororhacos, but the original maintains seniority and is in use because of this, though the 1889 spelling is recognized by some as an alternative spelling. In 1891 Ameghino again published on remains of the bird, then recognizing it as a bird rather than a mammal and adjusting the initial description. 

Standing at approximately 2.4 m (7.9 ft) and with a skull 65 cm (26 in) long, Phorusrhacos was a giant in the Miocene of South America and was very capable of hunting and preying upon most other forms of life. Weighing around what a male ostrich weighs (130 kg, 290 lb), sporting a raptorial hooked beak, and strong legs that appear to have been capable of providing a long stride and a high speed, Pharusrhacos was an apex predator capable of running down, pinning, and dispatching prey with a swift stroke of its head and beak. Flightless and fierce, this was indeed a terror bird.

A drawing of the terror bird Phorusrhacos by Charles R. Knight published in Animals of the Past, 1901.

Sources to Consider

Ameghino, F. (1889). Contribución al conocimiento de los mamíferos fósiles de la República Argentina. Actas de la Academia Nacional de Ciencias de la República Argentina en Córdoba 6:xxxii-1027

Ameghino, F. (1891). "Mamíferos y aves fósiles argentinas. Especies nuevas, adiciones y correcciones". Revista Argentina de Historia Natural. 1: 240–259.

Fernicola, J. C., Cuitiño, J. I., Vizcaíno, S. F., Bargo, M. S., & Kay, R. F. (2014). Fossil localities of the Santa Cruz Formation (Early Miocene, Patagonia, Argentina) prospected by Carlos Ameghino in 1887 revisited and the location of the Notohippidian. Journal of South American Earth Sciences, 52, 94-107.

Lydekker, R. (1893). "On the extinct giant birds of Argentina". Ibis series 6 (5): 40–47.

19 August 2020

Awesome... Antennae? or Forks?

Walliserops trifurcatus, Houston Museum of Natural Science, Houston, Texas, USA Photo by Daderot

 Walliserops, a genus consisting of four recognized species, is a really interesting group of trilobites. Known from Devonian rocks of Morocco, there are a number of interesting anatomical features in these trilobites. All species of Walliserops exhibit some asymmetry, that is, the halves of their bodies are not identical on both sides, due to the curvature of the spine on the occipital lobe of the cephalon. Three spines in total originate from the cephalon. The occipital spine in the middle and farthest back (most caudal) and two more lateral spines located near or on the palpebral lobes, caudal to the compound eyes. Many trilobites have some kinds of spines on the cephalon or on the thorax (or both) and these are not what make this genus so interesting. What makes them truly interesting is the trident shaped fork protruding from the forward-most ridge of the cephalon. Each species has a distinctly shaped and different length trident. The exact purpose these tridents is not understood and, at one point, it was thought that they signalled differences between the sexes. This has been considered an interesting hypothesis, but is not considered a fact, as there is not enough evidence to support this claim. A plausible hypothesis states that the tridents were likely similar to the horns of the rhinoceros beetle, which are used for sparing against rivals during mating season and occasionally for digging. Of course, whatever the reason for the trident, it is a very interesting anatomical structure and makes the members of this genus appear very well protected (considering all of their spines as well).

Sources to Consider:

Brett, K. and Chatterton, N. (2001). Parabolops, a new asteropygine trilobite from southern Morocco with an unusual trident-like anterior cephalic frontal process. 3rd International Conference on Trilobites and their Relatives. University of Oxford.

Knell, R. J. and Fortey, R. A. (2005). "Trilobite spines and beetle horns: sexual selection in the Palaeozoic?". Biology Letters. 1: 196–199. doi:10.1098/rsbl.2005.0304

Whittington, H. B. (1997). "Mode of Life, Habits and Occurrence". In R. L. Kaesler (ed.). Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. Boulder, Colorado & Lawrence, Kansas: The Geological Society of America, Inc. & The University of Kansas.

18 August 2020

Triarthrus - Preserved Legs and Antennae

Pyritized Triarthrus Trilobite

Often when we imagine a "typical" trilobite we picture an animal something like this genus, Triarthrus. Known from the Upper Ordivician soils of North America (midwestern states as well as New York and regions of Canada), China, and Scandinavia, Triarthrus is one of the final lineages of Olenid trilobites which was highly successful and very diverse during the Cambrian. Triarthrus is so well represented in the fossil record and considered so highly "typical" of trilobites that it is often used as the textbook example of what a trilobite looks like. One of the best preserved species within this genus is T. eatoni, is known from the state of New York and Canada (multiple sites across the provinces of Ontario and Quebec). Shale and iron pyrite deposits in these areas, especially the Franklin Shale in the state of New York, have produced specimens so exceptionally preserved that legs and associated appendages, gills, and antennae are perfectly retained in the fossils. A single location in New York, the Beecher's Trilobite  Bed, is world known; Triarthrus makes up approximately 85% of all of the material that has been recovered in that location. An entire community, including the shed and discarded remains of younger life stages, of T. eatoni are known from the Franklin Shale. 

This community graveyard of Triarthrus has allowed for a great deal of interpretation concerning the life cycle of these animals. The discarded exoskeletons of members that could float about with plankton were interspersed with the exoskeletons of 2mm sea floor dwelling individuals and, of course, a large number of larger exoskeletons of adults. Though 2mm may appear to be minuscule, adults of T. eatoni were not very large overall either, at approximately 5cm (50mm). Triarthrus individuals are preserved in a variety of configurations; lateral dorsal, and ventral views are all well represented. 

Other discoveries of T. eatoni have led to the discovery of trilobite eggs as well. Because of all of these exquisitely preserved Triarthrus specimens, we know much about its life from birth to death. The image below shows a ventral view of a well preserved T. eatoni with eggs preserved near the cephalon shield. The eggs were photographed in the study of this fossil by Thomas A. Hegna using a scanning electron microscope. 

Pyritised trilobite (Triarthrus eatoni) w/ eggs from a quarry in ...
Image credit: Thomas A. Hegna et al, doi: 10.1130/G38773.1.

17 August 2020

Short Time

Rather than rush a post today (the first day of the semester was rather busy), I want to encourage you all to listen to Dr. Melanie J. Hopkins discuss trilobite fossils. She is an authority on trilobite evolution and paleobiology and knows a lot of valuable information about these intriguing arthropods. Take a 20 minute break from whatever you might be doing and listen to someone that really knows their science share their knowledge with you!

16 August 2020

General Anatomical Survey of Trilobites

Calling trilobite anatomy simple is not fair to these highly diverse creatures. Some of their fossils may make their anatomy appear to be highly simplified, but many of those fossils are lacking imprints and other evidence of the complex soft structures, fine details, and even fragile hard structures that also make up the bodies and appendages of trilobites. There are many great finds that do preserve these types of structures, so we do have information about them. These structures are variable across species, of course, and will be better described in individual circumstances. There are abundant excellently preserved core exoskeletons of these animals, and this generalized regional anatomy is our focus today. Discussing them in the simplest terms, however, we are able to break down trilobites into six general regions. 
We can break them down from one side to the other (4 - 6 below) and describe the right pleural, axial, and left pleural lobes. These three lobes are where the name trilobite originates from. Pleura is a word of medieval Latin and Greek origin that means "to the side of the body" and is often used in zoology to describe the sidewalls of arthropods. The pleural lobes are literally the "side lobes". The axial lobe, then, is the lobe that makes up the center of the body. 

We can also break down trilobite anatomy by discussing it from head to tail (1 - 3 below). The head region is known as the cephalon (1), the middle region is the thorax (2), and the tail region is the pygidium (3). The cephalon can be broken down into many different smaller regions based on facial sutures (natural fractures separating components of the head) that include preocular and postocular (before and after the eyes, respectively), the rostrum (the "nose" or front end part), and the hypostome ("mouth" part). The cephalon includes the mouth area of the trilobite, houses the compound eyes, and would have had attachments for antennae. The large lateral (side) flanges or fringes are also a part of the cephalon, regardless of how long they may have been; some trilobite species had lateral fringes that extended to being nearly the length of their body. 
The thorax is the middle region of the body and is made up of multiple articulated segments. The number was variable between species and could be as few as two and as many as 103. The thorax helped to protect vital organs of the trilobite, including the gills, and served as the attachment site for the limbs; the thorax  therefore protected the limbs as well. The articulation of these segments is such that they allow for "rolling up" as can be seen in modern pill bugs (I always called them "roly-pollies" when I was a kid). Fossils of this activity have been discovered and it is thought to have been a protective measure that trilobites employed against predators. The pygidium is the most posterior portion of the trilobite. It is formed by the fusing of a number of smaller segments, like those making up the thorax, and the most posterior portion of the animal, the telson.

1 – cephalon, 2 – thorax, 3 – pygidium, 4 – right pleural lobe, 5 – axial lobe, 6 – left pleural lobe

 Sources to consider:

Bruton, D. L.; Nakrem, H. A. (2005), "Enrollment in a Middle Ordovician agnostoid trilobite", Acta Palaeontologica Polonica (3 ed.), 50: 441–448, retrieved June 22, 2009

Paterson, J.R.; Edgecombe, G.D. (2006). "The Early Cambrian trilobite Family Emuellidae Popock, 1970: Systematic position and revision of Australian Species". Journal of Paleontology. 85 (3): 496–513.

Whittington, H. B. (1997), "Morphology of the Exoskeleton", in Kaesler, R. L. (ed.), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida, Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 1–85.

15 August 2020

Things We Do Not Talk About

There is an entire Class of animals that we have not really discussed. They are always right there when we look up fossils. We should take a closer look at these alien, but very familiar, animals. Appearing between 540 and 520 million years ago, the Class we know as the Trilobites, are ovoid, sometimes clearly showing appendages, arthropods that existed until shortly before the end Permian extinction. No existing direct descendants of trilobites exist today. However, in the age of the greatest diversity of trilobites, from the Early Cambrian to the end-Permian, there are 11 known orders of trilobites that includes more than 30 genera and many more species. Talking about "the trilobite" fossils is actually quite misleading. Usually when someone mentions trilobites they conjure up images of trilobites such as Ogygopsis, or Meroperix. Perhaps you might think of a less ovoid member of the group, one that has large head elements, like Paradoxides, or with long tail structures, like Cheirurus. We will look at general anatomy of trilobites tomorrow, then, during the rest of the week, we will discuss some specific examples of genera or species of iconic trilobites.

Plate from Barrande's work Système silurien du centre de la Bohême

13 August 2020

Freshwater Sharks

 Growing up we often kept Rainbow sharks in aquaria. These streamlined pets were fast when they wanted to be and were fairly aggressive. They often jumped from out of the tanks as well. We stopped keeping them after a while. However, these are not true sharks and freshwater sharks are globally rare in modern times.The most common are those of the genus Glyphis, commonly called "river sharks", and Carcharhinus leucas, commonly known as the bull shark. The bull shark is known worldwide (and by many names such as Zambezi shark, Lake Nicaragua shark, and freshwater whaler, to name a few), whereas the Glyphis sharks are less well known, by the general public, but they also enjoy a global distribution. Four extant Glyphis species inhabit mainly southeast Asian, Indian, and Australian rivers including the Ganges, Brahmaputra, and Adelaide Rivers (this is a small fraction of the rivers the extant species inhabit). Two extinct species, G. hastalis and G. pagoda are known from British and southern Asian fluvial fossil records respectively. The most important difference between Glyphis sharks and bull sharks is that the Glyphis sharks are true freshwater-only sharks, whereas the bull shark can adapt and survive in freshwater, but is more suited to marine environments. We could discuss the shallow, brackish water suitability and aggressive nature of bull sharks for days, but we are more interested in the history of freshwater sharks today. 

Glyphis sharks are more closely related to ocean-going tiger sharks and reef sharks than they are the original, well represented freshwater sharks. Many fossils from the Order Xenacanthida are known from the Carboniferous Period. The most well known are Xenacanthus and Orthacanthus. Xenacanthus may be the better known genus, on the internet at least, and consisted of many more species than Orthacanthus, even having some species survive the end Permian extinction event and last into the Triassic. Orthacanthus, however, appears to have been an older lineage and died out well before the end Permian extinction event. Both genera of freshwater shark were apex predators in freshwater environments and were likely in direct competition in freshwater environments throughout their existence. Orthacanthus, has a well documented predatory lifestyle. Coprolites (fossilized feces) from Orthacanthus have been discovered containing the remains (often teeth but also bones) of smaller Orthacanthus (cannibalism), other Xenacanthid, tetrapods, and various other fish. The coprolites are known to come from Orthacanthus because skin and other soft tissue impressions in fossils have allowed paleontologists to reconstruct rectal anatomy of Orthacanthus and match this anatomy to the shapes of coprolites.

Xenacanthus restoration, ©Nobu Tamura
Orthacanthus restoration, ©Nobu Tamura

12 August 2020

The Cretaceous Sharp Nose

 During the Cretaceous, when the crushing Ptychodus sharks were breaking open shells, sharks were often not the greatest predators in the neighborhood. They competed with plesiosaurs, tylosaurs, and even Xiphactinus, a very large fish with impressive dental hardware. Despite being outclassed in many of the world's oceans, sharks were still capable of becoming apex predators and competing directly with their fish and marine reptile neighbors. One of the sharks most capable of direct competition in the Cretaceous seas and oceans was the rather large (up to 8 m, 26 ft, and 3400 kg, 3.3 tons) mackerel shark, Cretoxyrhina. Mackerel sharks are sharks in the Order Lamniformes and are fast, powerful, ocean swimming sharks. The most notable living member of this group is the Great White Shark, which grows to approximately the same size as Cretoxyrhina is thought to have grown to. The genus Cretoxyrhina consists of four recognized species with C. mantelli being the first described; initially teeth of the shark were described in 1822 by Gideon Mantell as a modern shark's teeth and a second formally published description, honoring Mantell, was written in 1835 by Louis Agassiz. These teeth were all collected from England, but a number of exceptionally well-preserved vertebral columns (and some associated other elements including teeth) are known from the work of George F. Sternberg, and his father Charles H. Sternberg, that was conducted in the state of Kansas in the mid-western United States of America. 

 Charles Sternberg found a skeleton and 250 teeth in 1890 that were sold to a German museum (it was destroyed during WWII), but George Sternberg found many skeletons of Cretoxyrhina. The first (1891) contained portions of the jaw as well as a last meal of Xiphactinus and 150 teeth. In 1950 and 1965 he found even more of the shark, including specimens with preserved gills, portions of the skull, complete jaws, and pectoral fins. These are exceptional discoveries because, as many of us may know, sharks are cartilaginous fish, and much of their skeleton is difficult to preserve. Teeth are typical remains that we possess for sharks because of their durability, continuous replacement (allowing for many more teeth to be potentially preserved compared to whole individuals), and the number each animal possesses; remember that Charles Sternberg found 250 teeth in his 1890 fossil.

These large sharks, and their teeth, are associated with numerous other fossils, indicating what kinds of animals they fed on regularly. These include fish like, and including, Xiphactinus, plesiosaurs, mosasaurs, turtles, other sharks, and even pterosaurs and dinosaurs. Pterosaurs are thought to have sometimes fished over ocean waters, and it is thought, as shown in the Mark P. Witton illustration below, taht the sharks may have actively hunted low flying pterosaurs such as Pteranodon. Dinosaurs are often considered a scavanged meal in relation to Cretoxyrhina, showing that the shark was an opportunistic hunter as well. One hypothesis, involving a Claosaurus (shown below also), is that dinosaurs sometimes died on the shore, during a flooding event, or in some other way that enabled their bodies to float out into the ocean where they were eaten by sea creatures of all sorts. Evidence for Cretoxyrhina feeding on dinosaurs includes not only the hadrosaur Claosaurus, but also the nodosaurian Niobrarasaurus. The evidence for larger prey (fish have been found in the stomach areas of Cretoxyrhina) such as plesiosaurs, pterosaurs, and the dinosaur mentioned is mostly based on Cretoxyrhina teeth embedded in bones, where they were broken off during biting and feeding. Finding shark teeth in bone is very interesting. A plesiosaur vertebral column I worked with had shark teeth embedded. I did not diagnose the teeth, but the animal was from a time and area populated by Cretoxyrhina, so it is a likely culprit to include in the list of suspects.

Cretoxyrhina fossils from Newbrey et al. (2013) of G.F. Sternberg's 1950 discovery, FHSM VP-323, and another fossil, FHSM VP-2187
Cretoxyrhina breaches (jumps out of the water) to attack Pteranodon
Cretoxyrhina and two Squalicorax sharks circle a floating Claosaurus

11 August 2020

Crushing it in the Cretaceous

Yesterday we went over a bit of the development of sharks and discussed an early group of sharks that has persisted since the dawn of recognizable sharks and their jaw arrangements. Today we are going to look at a Cretaceous era group of sharks in the genus Ptychodus. These sharks possessed interesting teeth that would not be recognizable as shark teeth but were efficient and crushing and smashing food items, specifically large bivalves and crustaceans. Large numbers of the fossils of Ptychodus are known from the middle of North America in what was, during the Cretaceous, the Western Interior Seaway, but the distribution of Ptychodus species (22 species overall) is truly global. Part of the reason that large numbers of these fossils are known from the Western Interior Seaway is that this area, encompassing what is now the dry land stretching from the Gulf of Mexico to the Arctic Circle, is thought to be the last "stronghold" of Ptychodus sharks before their extinction around 85 million years ago.

The teeth of the Ptychodus sharks are peculiar, for a shark, as I noted above. Rather than individual conical, spiny, or serrated wedge shaped teeth, Ptychodus sharks possessed large crushing plates. These plates were known as early as 1822 from the Mantell illustrations by Mary Ann Mantell and likely could have been known earlier than this, but without published scientific scrutiny. The plates consist of multiple lobed projections and would have been used to crunch down on the shells of the sharks food items. In any kind of territorial battles it could have been used to bite down and crush other Ptychodus fins and bodies as well. My hypothesis is that being bitten by this shark would feel something like slamming one's hand in a car door. Or possibly more like being hit by a meat tenderizer. Regardless, it would likely hurt.
Ptychodus tooth plate illustration from Everhart, adapted from Shimada.

10 August 2020

The Gilded Age of Hexanchus

 The Devonian Era saw sharks developing and diversifying under the "rule" of larger armored fish like Dunkleosteus. Sharks diverged from their cousins, the Chimaeriformes, somewhere around the time of yesterday's animal, Cladoselache; this branching is not completely understood, even leading to some discussion, but nothing definitive, about placing Cladoselache in the Chimaeriformes group. As the Devonian Era came to a close (also the start of the Carboniferous Era, approximately 359 million years ago) approximately three quarters of all species on the planet became extinct, including many of the larger fish. This left an apex predator role open and sharks and chimaera expertly filled that role, continuing to diversify and increase their overall size. Many of the best known of the Carboniferous sharks are actually members of the Chimaera branch of the family tree. These include the anvil finned Stethacanthus and the wheel-mouthed Helicoprion. Despite being outshined by their Chimaera cousins during the Carboniferous, the sharks began to develop the characteristic jaws full of teeth that we now associate with sharks. The end Permian extinction (252 million years ago), which caused the extinction of approximately 96% of all marine, was significant for the shark species that survived. With their mouths full of teeth and few rivals (including their Chimaera cousins), sharks truly became masters of the ocean for some time; as marine reptiles evolved the balance of power would shift again. During the Jurassic Period (195 million years ago) the first modern sharks began to appear. These include the genus Hexanchus, which includes one of the oldest surviving lineages of shark: Hexanchus griseus, commonly called the Bluntnose sixgill shark.

Bluntnose sharks would fall into the common misnomer of "living fossil" as evidence of their existence is extensive in the form of fossilized teeth from the Mesozoic Era. The fact that more of its relatives are extinct than living and that this shark has a primitive body plan with a single dorsal fin, broad rounded pectoral fins, and a number of gills (six) that is between the more primitive sharks (possessing seven) and the more derived sharks (possessing five). The Bluntnose sixgill shark, one may complain, is not a fossil animal and therefore has "no place" on a fossil animal discussion platform. However, as a shark with more extinct relatives than living relatives, the living Bluntnose sixgill sharks is a living breathing representation of Hexanchus sharks that came before it. Not much has changed in this genus either; hence the "living fossil" misnomer. Hexanchus sharks (collectively known as "Cow Sharks" within the family Hexanchidae) belong to the order Hexanchiformes and are described as the most primitive group of sharks. To compare the living and the fossil is not always easy with these sharks, as many of the fossils are teeth and very little else (skin impressions do exist but are a bit more rare than teeth).

Bluntnose sixgill shark, image by NOAA Ocean Explorer from USA
Teeth of Hexanchus andersoni from Jurassic,on display at the Museo Civico di Storia Naturale di Milano

09 August 2020

Dawn of Sharks

 Regarded as the first genus of shark, Cladoselache is a group of 8 recognized species, all from North America. These sharks are commonly called "scaleless sharks". They retain many ancestral characteristics including very fish-like heads and more than five gills (most modern sharks have five gills). The fins of these sharks possessed spines consisting of dentine and enamel, making them on of the strongest portions of the external surface of the body. The skin itself was almost entirely scaleless, possessing scales only near the eyes, mouth, fins, and portions of the tail. We know this because there are numerous exceptionally well-preserved members of the genus; so well preserved that we have internal organs such as the kidneys in the Cladoselache fossil record. Smaller muscles, a semi-scaled tail, and a semi-lunate shape that was nearly symmetrical all came together to form a lightweight, lower (than modern sharks) powered shark that was considered capable bursts of speed rather than maintained high speeds. 


Those short bursts of speed would have been immensely important to these small ~ 1.8 m (5.9 ft) long sharks. They are known from the oceans that covered modern North America approximately 380 million years ago during the Devonian time period. At this time, the apex predator of these oceans was the giant placoderm (armored) fish Dunkleosteus. These first scaleless sharks were were on the menu for most of the time that Dunkleosteus ruled the oceans. However, the scaleless sharks diversified during this time and their descendants outlived Dunkleosteus and eventually became the apex predators we know today from these fairly humble fishy origins.Cladoselache fyleri (Newberry, 1889) - fossil shark from the Devonian of Ohio, USA. (CMC VP7204, Cincinnati Museum of Natural History & Science, Cincinnati, Ohio, USA)

07 August 2020

Hammerhead Body

The body of Atopodentatus is fairly unremarkable in reconstructions. Other than the hammerhead face of the animal, its body is something we would consider rather typical for a swimming marine reptile. There are basic flippers on the forelimb and hindlimb; show here as early flippers with the digits extending just beyond the edges of the webbing. In appearance these flippers are not unlike the feet of ducks. They are less like the flippers we typically see in marine reptile reconstructions (plesiosaurs and ichtyhosaurs in particular) or in living marine reptiles (think of sea turtles). These flippers likely influence the evolution of the tail as well.

The tail of the animal is not paddle-like, and therefore probably not adapted for powered swimming. Considering the evolving flippers, that tail might not have been used in swimming at all. We could hypothesize that over time and evolution that the tail would become smaller (thinner, shorter, or any combination) like those of turtles or some of the plesiosaurs that we know. The tail would probably aid in turning a very small amount, if at all. However, it is unlikely that this algae grazing reptile needed to turn quickly in its daily life, so the tail not serving as a rudder or a propeller is not unexpected. I cannot speak to its overall speed, and I don't know if there is much study in that area either yet and saying that it likely didn't need to turn quickly is not an indictment of its overall traveling speed. I imagine, just as an off the cuff idea, that Atopodentatus was much closer to a manatee  or a sea turtle than other animals in terms of its swimming speeds. That would allow for short bursts, but overall indicate a nice slow meandering through much of its daily life.

The body itself, in the image below, is fairly average sized. There are reconstructions that show Atopodentatus with a more manatee-like pot belly as well. This imagery meshes well with the increased size we see in many herbivorous animals. One of the reasons for that, many of us know, is the need for increased digestive organ space. In some animals that can include elongated intestinal tracts, multiple stomachs, or organs like gizzards or crops that serve pre-digestive roles and often contain rocks to help grind down plant matter. Could Atopodentatus have had any of these kinds of extra long intestines or accessory organs? It is certainly possible! We would likely need a mummified fossil to be entirely certain, but some of the better preservations of this animal may also have clues. Could those accessory organs then lead to increase abdominal sizes in Atopodentatus? It is likely that they could, and, as noted before, some interpretations of reconstructions have included enlarged abdomens. That idea has permeated to the toy industry as well, so be prepared for young scientists whose first interactions with Atopodentatus are with an elongate, hammerheaded, and pudgy marine reptile.

A reconstruction showing what Atopodentatus unicus would have looked like in life

06 August 2020

One More Head Post

In Luoping County in the Yunnan province of China in 2016 the newest fossil of Atopodentatus was discovered. That fossil, as we have seen, offered a very different perspective of the way that the jaws looked in this animal. That change, as we mentioned, required a new interpretation of how Atopodentatus was able to feed and what it fed on. Previously, the initial description described a filter-feeding marine reptile that pushed its head into the muddy bottom of its environment and push out the sediment, filtering that sediment and water in hopes that it would trap small arthropods hiding in the sediments. In the newer description, the change in the dental landscape called for a new interpretation. Instead, the new hypothesis proposed by Chun et al. 2016 ( detailed a mechanism in which the the small chisel-shaped teeth were used to scrape algal growths from rocks, plants, and other details of the underwater scenery. The authors also hypothesized that the more posterior teeth were capable of of helping to break up plants so as to be able to retrieve the algae from these materials as well.

As we can see from the paper, two new skulls were described, providing extra information about the organization of the mouth and face of Atopodentatus. These two skulls, in addition to the original (type) specimen allow for a very well defined reconstruction of the skull. Putting those three views of this animal together we end up with a reconstruction that fairly accurately portrays the actual organization of this animal. That allows us to better understand how it fed, how it swam (heads do have an aerodynamic property in all fluids after all), and possibly even how these animals may have interacted with one another. Could they bite each other in territorial disputes? Did they groom themselves, or one another on the beaches? Could they even get to the beaches? Remember, there is evidence of flipper-like digits on their limbs and we have not really determined their ability to "haul out" onto the beach in the first place. The head tells us a lot about behavior, but there is still more to this animal. We will look beyond the head tomorrow, so look at the fossils and reconstructions and consider what we might be able to glean from the bones and the interpretations of those bodies.

04 August 2020

Two New Faces

Two Atopodentatus restorations, both by Nobu Tamura. The first was produced shortly after the initial description was published and is, admittedly, quite terrifying in many ways. It turns out that this initial find (published in 2014) was a little crushed, side to side, and the upper and lower jaws (maxillae and mandibulae respectively) were misplaced. The subsequent find (published in 2016) of a differently arranged, and apparently better preserved, skull changed the way that paleontologists, and artists, interpreted the mouth, in particular, of Atopodentatus. This arrangement of teeth (and mouth) is definitely still odd and unique, but helped to better understand the likely feeding habits of this interesting marine reptile.

This fossil, in a short amount of time, shows how new discoveries can change not only how we view an animal, but also how we can, and often need to, change our perspectives and hypotheses due to new discoveries. It also helped to clear up some mysteries about Atopodentatus (e.g. How did it eat with the originally reconstructed mouth? What could it have possibly be eating?)

Restorations ©Nobu Tamura

02 August 2020

A Short Video for your Sunday

1) Minor correction, I had a typo in the name of Atopodentatus yesterday, it has been fixed though!

2) Now, please enjoy some background info on the world of Atopodentatus and then some more on the animal itself in this video.

01 August 2020

Vegetarian Hammerheads

We typically write about dinosaurs here. We then branched into mammals at different points. We sometimes even talked about birds, small reptiles, varied reptiles (turtles, snakes, etc.), and amphibians from time to time. We have discussed traditional looking animals (i.e. animals that we expect to look a certain way as in T. rex always looks kind of like we expect T. rex to look). We have also looked at very odd looking animals (Megacerops and Ceratosaurus, I'm thinking immediately of you). I think we ought to restart with an animal that is different looking and from one of the interesting fringe groups that we have discussed. To top things off, this somewhat recently described animal looks like pretty much none of its closest living relatives, making it far from what we might expect to see.

Enough intrigue and vagueness, we will start off this month by discussing Atopodentatus unicus. An herbivorous pantestudine (a group that includes the living turtles, testudines, and their ancestors the extinct "stem-turtles") marine reptile that has been assigned as a possible primitive (basal) member of the sauropterygia (the "lizard flipper" group that includes long and short-necked plesiosaurs). This classification follows the Schoch and Sues 2015 phylogeny that places sauropterygia within pantestudines (one can find other arrangements of the family tree that make this description hard to accept). Regardless, this animal appears to be related to turtles, but also has a long neck and long limbs with elongate digits that appear to be situated into primitive flippers. It also has a long tail and many ribs and gastralia ("belly ribs"). Despite all of these "normal" sounding bodily descriptions of a large marine reptile that make it sound like a very typical swimming air-breather of the Triassic, its head is anything but normal.

The name Atopodentatus translates to "Unusual toothed" and, with a "zipper-like" smile of teeth, it is unicus ("unique"). The anterior part of the skull can best be described as "hammerheaded", with the teeth forming their "zipper-like" pattern at the front of the mouth. The entire head is not hammer head shaped, only the mouth. This formation is very interesting. It looks almost like a hadrosaur ("duck-billed" dinosaur) mouth, but filled with small peg-like teeth. Approximately 8 million years older than any other herbivorous marine reptile, Atopodentatus is thought to have fed on algae embedded in the sea floor.

Restoration of Atopodentosaurus ©Nobu Tamura