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Greetings and welcome to the final day of Prehistoric Shark Week! All week we have covered sharks that swam in Arkansas during the Cretaceous Period. The dinosaurs get all the press, but we had a diverse marine ecology during that time. Last week, we met a few of the non-shark denizens, such as mosasaurs, elasmosaurs, and more. This week, we have seen nurse sharks, goblins, sand tigers, and an array of rays, skates, and angel sharks. We wrap up the festival of marine animals with the question that everyone wants to know. Where did the most famous sharks of all time, the Great White and Megalodon, come from and how does Arkansas play into this?
The Great White, or simply White Shark, is named Carcharodon carcharias, meaning sharp tooth pointer, although more popularly named for its white belly, is well known as the largest living predatory fish in the sea, reaching up to and, probably over, 20 feet. Megalodon, listed either as Carcharocles megalodon or Carcharodon megalodon, depending on whether or not one believes it is directly related to or convergent with White Sharks, is the largest known predatory fish ever, reaching sizes up to three times that of the White Shark. It appeared in the fossil record about 16 Mya, but went extinct 1.6 Mya (contrary to what a fictitious documentary on the Discovery Channel claimed).
During the Cretaceous, the southwestern part of the state was covered by the Western Interior Seaway, which for us, was essentially equated to having the Gulf coast not just on our doorstep, but flooding it. Those waters were warm, rich in nutrients, and a hotbed of marine life. In those waters, a few sharks of interest made their home.
All of the sharks we will be talking about are lamniform sharks. These sharks are known for being at least partially endothermic, meaning they used their core muscles to create their own heat and maintain an elevated body temperature, giving them the ability to be active hunters even in cooler waters. Of course, it also meant they were hungrier, needing more food, keeping them always on the prowl. This is what allows the White to be such a fearsome hunter today, giving it the power and energy to breach completely out of the water during attacks.
Squalicorax is an extinct shark of the time that is commonly thought to have resembled Whites. These sharks got up to five meters, although they were typically around two meters. Squalicorax is also called the Crow Shark, which some people have speculated it got that name from evidence of its scavenging. However, squalus means shark (and is the scientific genus name for dogfish) and corax means crow, so the name Squalicorax literally means crow shark. Now as to why it was named that way to begin with, no one knows because when Agassiz named it in 1843, he didn’t leave a record as to why. They hunted and scavenged a wide range of animals, everything from turtles to mosasaurs. Unfortunately, the relationships between Squalicorax and other lamnids is uncertain, so whether or not it could have been ancestral to anything, much less Whites or megalodons, is unknown at present.
Another candidate is a shark named Isurus hastalis, an Oligocene shark that lived 30 Mya. Isurus also includes the modern day mako shark. However, a researcher by the name of Mikael Siverson concluded that the Isurus teeth were not makos, but worn down teeth similar to modern Whites. So he changed the name to Cosmopolitodus. It has also been suggested that these sharks originated from a shark called Isurolamna, which lived in the paleocene 65-55 Mya.
A more recent view, and one which I back (with freely admitted bias because it allows me to say they evolved from Arkansas sharks:) ), is that both Whites and megolodons evolved from an extinct lamnid called Cretolamna, the Cretaceous lamna. This shark had large, strong teeth and was very successful. It had a worldwide distribution and lived from the Cretaceous to the Paleocene. Cretolamna fossils have not been reported in Arkansas thus far, but they were a member of the family Cretoxyrhinidae, of which the shark Serratolamna was a member. The teeth of Cretolamna and Serratolamna are extremely similar, as one might expect from genera in the same family. However, Serratolamna teeth have serrations and Cretolamna does not, making Serratolamna teeth closer in shape to the White Shark. Serratolamna did not have the same worldwide distribution and did not last as long as long as Cretolamna, though. It is impossible to tell which one was directly ancestral to the later sharks, but Cretolamna, due to its more cosmopolitan range, has gotten the nod. It was named first and is much better known than Serratolamna, giving it an edge when people find and identify fossil shark teeth. Thus, it is not a big stretch to say that Serratolamna, or a very close relative, eventually evolved into Carcharodon carcharias as well as Carcharocles (or Carcharodon) megalodon.
I hoped you have enjoyed Prehistoric Shark Week and the previous week of Cretaceous Arkansas marine predators. Let me know if there is another group that you think deserves special consideration for a celebratory week.
For Day 4 of Prehistoric Shark Week, I would like to mention another modern day shark that has been around since the Cretaceous: the sand tiger sharks. Tomorrow, I will discuss a couple of Cretaceous sharks that may be the ancestors of the two most famous sharks in the world – the Great White and the giant Megalodon.
The Sand tiger is a common shark in the Cretaceous sediments, or at least, their teeth are, which means they were probably pretty common back then. The teeth tend to be long and thin, with two small cusps on either side of the large, center blade. Elasmo-branch.org reports that the center blade is smooth-edged with a strongly bilobed root, large bulge in the center of the root (aka lingual protruberance), and nutrient foramen in the center.
There are actually two sharks that are often called sand tigers in the Cretaceous rocks. One is Carcharias holmdelensis, the Cretaceous version of Carcharias taurus, the modern day sand tiger shark. Also going by the name grey nurse shark, amid several others, sand tigers are large-bodied sharks that will eat pretty much anything, but since it is a fairly slow and placid shark most of the time, it doesn’t seem to go after anything that requires a lot of effort. They are known for gulping air to allow themselves to float in the water column without expending much effort. So although they look scary, they appear to be too lazy to live up to appearances.
The other shark that gets called a sand tiger, is Odontaspis aculeatus, one of the ragged toothed sharks, which also go by the name sand tiger. These sharks were until recently in the same family as Carcharias, but have since been pulled out into their own family. They are very similar, as one might has guessed from the numerous times these sharks have been grouped and split over the years. As Elasmo-research.org put it, “Chaos reigned until Leonard Compagno examined museum specimens from all over the world, corrected misidentifications and sorted out synonyms.”
For Day 3 (a little late, yes) of Prehistoric Shark Week, I want to bring to your attention the diversity of chondrichthyans that have opted for a flatter bauplan.
Sharks are generally split into two groups, the galeomorphs, which are mostly the more typical torpedo-shaped sharks, including the sharks that most people think of when they envision a shark. The other group is the squalimorphs. These sharks lack an anal fin and many of them have developed a penchant for flatter bodies and broad pectoral fins, and in some cases pelvic fins as well (although not all, such as the dogfish and frilled sharks). Up until recently, the batoids, otherwise known as skates and rays, were considered part of this group, the consensus being that they were a more specialized type of squalimorph shark that had taken flat to an extreme. But the most recent molecular studies have indicated that they are a group unto themselves. The batoids have a long fossil history, with a number of ray teeth found in the Cretaceous deposits of Arkansas, particularly the eagle ray family Myliobatidae. Their teeth are typically flat rectangles on top with a comb-like surface below. Another type of ray that can be found are the guitarfish, or Rhinobatos casieri. These pectoral fins of these fish extend to their head, giving them a triangular shaped front end of a more traditional shark-like back end.
Skates and rays are generally very docile and would not be very threatening, spending their time scrounging about on the sea floor for benthic (living in or on the sea floor) invertebrates and the occasional fish. The same can’t be said for the last member of this group, the sawfish. Armed with a rostrum (its elongated snout) with teeth out to the side, the fish looks like it has a chain saw for a nose. The sawfish will swim into a school of fish and thrash its rostrum rapidly back and forth, spearing and stunning several fish, which it can then gobble up. They can also use it to dig up clams and crabs from the sediment. While they won’t attack humans, any human who provoked one may easily wind up perforated by the rostrum, probably not deadly but certainly painful. Most modern sawfish reach a respectable two meters, but the largest species, the green sawfish (Pristis zijsron) can top seven meters (24 feet). This is as large as the Cretaceous versions. Modern sawfish are typically put into the family Pristiformes. The Cretaceous ones are in their own family, called Sclerorhynchiformes and are not directly related, in that the Cretaceous ones are not thought to be ancestral to the modern ones. They are both put into the group Pristirajea, so they are thought to at least be related. But with the uncertainties in the relationships of the modern fish, the relationships with fossil forms are necessarily less certain. In any case, Arkansas sports several different species from this group, including Schizorhiza stromeri, Sclerorhynchus sp., Ischyrhiza mira, Ischyrhiza avonicola, and Ptychotrygon vermiculata. We were postively awash in sawfish.
The true squalimorph sharks that have shown up in the Arkansas Cretaceous rocks are best represented by the Angel shark (Squatina hassei), which looks like an early rendition of a skate, so it is little wonder that most researchers viewed skates and rays as simply more specialized versions of these sharks. Nevertheless, it appears this is case of convergence, not homology (similarity due to relationship). If it is homologous, it isn’t directly so. It is possible both groups had a common slightly flattened ancestor and each took their own route from there.
All of these fish are pretty docile hunters, scrounging around the sea floor for benthic organisms, all those animals that make their home in or on the sea floor sediments. They spend their time digging around the sand for crabs, clams, and other invertebrates, the occasional fish. When threatened by the presence of a predator, they hide on the bottom, using their shape to help them blend in with the seafloor. Neither the ones today or the ones in the Cretaceous would have bothered a human swimming around them.
Becker, Martin A., Chamberlain, John A., Wolp, George E. 2006. Chondricthyans from the Arkadelphia Formation (Upper Cretaceous: Upper Maastrichtian) of Hot Spring County, Arkansas
This week we will celebrate fossil sharks of the Mesozoic that have been found in Arkansas. Because all of our surface rocks of the period are from the Cretaceous, the sharks are limited to that time. There are other cool sharks from the Paleozoic, but they will have to wait for another time. Hunting for shark teeth in Arkansas can best be done in the chalk formations called the Annona and Saratoga in southwest Arkansas. But you can also find them in several other formations as well.
Many of the sharks found in the Cretaceous in Arkansas have contemporary species. While the species may vary, the genus name is very long-lived. For those who are unaware, scientific names follow a binomial system, with a genus and a species name, the genus being the first name and indicating a group of very closely related species. It is next to impossible to tell the difference between species of sharks just by their teeth unless, and many times even if, one is an expert, so I will be sticking with the genus names.
To begin the week, I present to you Ginglymostoma, the nurse shark.
The scientific name comes from the shape of its mouth. The origin of the name nurse shark is not clear, but it is considered likely to have originated with the Old English word Hurse, for sea floor shark.
Known for its puckered mouth and barbels on the sides of its mouth, nurse sharks spend most of their time near the sea floor scrounging for whatever small animal they can catch. They are very docile and will only bite if provoked. Humans are far too big for nurse sharks to be interested in, so unless one really goes out of their way to annoy a nurse shark, you’re pretty safe, even from the largest ones, which can get over 4 meters.
The creature in the picture is a representative of an early shark called Falcatus. It was a very strange shark, in which the males had a forward-pointing spike on its head. This is one of the earliest examples of sexual dimorphism in the fossil record. Sexual dimorphism is when the male and females look different. Humans are sexually dimorphic in a variety of ways, but the classic example is the peacock with its extravagant tail. They are so dimorphic that many people do not even realize the term peacock only applies to the male of the peafowl species. The females are called peahens.
Falcatus was chosen this week because of an interesting shark fossil found in 325 million year old limestone near Leslie, AR. The fossil was recently published and got a lot of press. Pictures of the fossil itself show what appears to be little more than a couple of lumpy concretions stuck together. There is little there that resembles anything like a shark. At least, until you look inside. A concretion is an inorganic structure consisting of layers of minerals that have been precipitated around a central core, much like a rocky onion. Oftentimes, a fossil lies at that core and served as the basis upon which the mineral precipitation got started. So one never knows what one will find cracking open a concretion. It may be nothing more than concentric mineral bands, or it may be an exquisite fossil preserved for the ages. In this case, the scientists got lucky. Not only was there an exquisite fossil, but one in which few had ever seen before: the skeletal structure of an early shark’s gill basket.
The researchers named the fossil Ozarcus mapesae, for the Ozarks in which it was found and for Royal and Gene Mapes, geology professors at Ohio University. The Mapes are also paleontologists and have collected a large number of fossils, one of which happened to be this curious-looking concretion. Small teeth on the exterior pointed to possibly more interesting material inside, so they CT-scanned it, which revealed the remains of the head of the shark. But to get better detail, they had to use a synchotron. This technology is new and expensive enough that it was not possible to look at the fossil this way until recently (a good example of why we preserve fossils in museums, you never know when future technology will allow examination in ways never thought of before).
The gill basket goes by many names, the branchial basket, branchial arches, gill arches, etc. but they all refer to the skeletal supports for the gills. To the shark, they are important for holding up the gills and forming the path for water to flow over the gills so the shark can breathe. For us, they are important because the arches evolved into a variety of structures, primarily the jaws and hyoid, a small bone in the throat to which the back of the tongue is attached. Because of the importance of jaws, the evolution of those structures has been a big topic of interest. The traditional view has been that the first jawed fish, the placoderms, evolved into the early precursors of the sharks, which then evolved into the early bony fish, the osteichthyans. The sharks then were expected to have a more primitive structure than bony fish. This story makes sense, given that the order of appearance in the fossil record pretty much matches what we would expect and the skeletal structures look more primitive in sharks than bony fish. Of course, despite the common view that sharks are relics of a bygone age, they have had over 400 million years to evolve after they split off from bony fish. How likely is it that they would have retained such ancestral characteristics for all this time? To answer that question, we can look at the fossil record to tell us what they were like at the earliest stages.
The big problem with examining the fossil record of sharks is that sharks are chondricthyans. They do not make bone. Other than the teeth, the skeleton is supported by a simpler set of calcium phosphate crystals. Unlike bone, which has a very structured arrangement of crystals and connective tissue, the bones in sharks are made of cartilage and a haphazard set of disorganized crystals, which fall apart shortly after the animal dies. Thus, finding any fossils of sharks that contain more than the teeth is extremely rare. Finding ones in which everything is still in place is almost impossible. Fortunately, over a long enough period of time, even the almost impossible will happen eventually. That is the thing with large numbers and vast amounts of time, our perception of what is unlikely doesn’t really work. For instance, given a 1 in a million chance that a tweet on Twitter will have something requiring the security team to deal with, that still gives them 500 tweets every single day. That “almost impossible” fossil was found with Ozarcus. This fossil provided our first look at what the throat of a primitive shark actually looked like.
Let’s have a bit of background to cover what we have known of early jaw evolution up to this point. Placoderms, armored fish from the early Paleozoic Era, were the first animals with jaws. The jaws themselves appear to develop from the first gill arch, according to a lot of embryological studies on modern animals.
The studies haven’t really answered where the bone came from though. In placoderms, it is pretty clear the armor came from modifications of the dermis, the basal layer of the skin. But they also have internal bone forming their skeleton. Modern sharks have no bone other than teeth and bony fish have jaws made from that dermal bone. In addition to the origins of the bone, there is the matter of how the bones are attached to the skull. The upper jaw is formed by a embryological structure called the palatoquadrate (the top part of the first gill arch), so named because bones called the palatine and quadrate form from it. The bottom jaw forms from what is called Meckel’s cartilage (the bottom part of the first gill arch). In modern fish, the palatoquadrate is braced against the skull only at the front, with the back unattached. The jaw joint itself is attached to a bone called the hyomandibular, which forms from the second gill arch. The hyomandibular acts like a swinging pivot, allowing the jaw to open very wide. When the jaw joint gets pulled forward, the back of the upper jaw can move down, using the point where the front part is braced as the pivot point. Sharks take this to an extreme, not bracing the upper jaw on anything at all, with the jaws attached solely by the hyomandibular bone (the “hyostylic” joint) allowing both the upper and lower jaw to move forward when they open their mouths. So when you see that shark opening its jaws and it looks like they are coming right out at you, they really are. For a great example of a hyostylic jaw joint, check out the goblin shark.
Unfortunately, when we look at the earliest fossil sharks and bony fish, both of them show a jaw in which the upper jaw is braced against the skull in both front and back (the “amphistylic” joint). So it doesn’t tell us much about how the sharks fit into the sequence. One might say that it seems logical to think the bony fish came first, loosening the jaw in the back and then the chondrichthyans took this one step farther. But surely, others might say, the fact that bony osteichthyans have a more advanced bony skeletal structure means they would have to have come later, right? Here again, the fossil record doesn’t help here because the earliest representatives of both groups appear close enough in time that it cannot be strongly stated which came first.
That is where things were, until 2013, when the fossil record started answering these questions. A placoderm called Entelognathus was published in 2013. This fossil was of great interest because it showed the structure of the jaws in great detail. Entelognathus had a jaw that looked very much like an osteichthyan. This fossil was 419 million years old, so it was likely too late to be ancestral to either chondrichthyans or osteichthyans, but old enough to be very close to the ancestral form. What Entelognathus tells us is that the bones forming the jaws in placoderms was already like those seen in modern bony fish, indicating that sharks would have started out with bone, but lost it during their early evolution. Of course, since sharks don’t have bone, this was hard to demonstrate on sharks, so the question was still unsettled.
Outside of the actual jaws themselves, there are all the other support structures around the jaw, such as the hyoid bone. Many of these structures are formed from the next few gill arches (earlier jawless fish had at least seven gill arches, so using a few to make the jaws and throat still leaves plenty for gills). The skeletal structures in bony fish that support the gill arches form a fairly simple chevron, or wide v shape, whereas the sharks have a slightly more complicated structure. But if finding fossils of shark skeletal structures is rare, finding one with tiny gill arch supports still in position is almost impossible. There is where Ozarcus comes in, because it is here that the almost impossible becomes reality. Once the researchers were able to see inside the fossil with the synchotron, they were able to see preserved gill arch supports in position. That position resembled the simple chevron shape of modern osteichthyans.
Thus, between Entelognathus and Ozarcus, we can confidently assess the development of jaws as having started in placoderms with primitive, osteichthyan jaws. We have evidence of the bony origins of the jaws from Entelognathus and evidence from the gill arches from Ozarcus, so we now have plenty of evidence to strongly support the claim that the chondrichthyans are not in the evolutionary pathway to modern jaws at all. They are an offshoot that actually lost bone to form a more flexible skeleton for some reason. Whether it was mechanical advantage for their lifestyle, such as increasing flexibility, or a reduction in mineral storage, who can say.
This new view of jaw evolution demonstrates that the common view of sharks as being evolutionary relics is wrong. Far from being primitive, they evolved throughout the millenia to the modern sharks we see today, changing the shape and development of their jaws and throat as they evolved from the primitive condition seen in early bony fish into the efficient predators of today. But then, they have been separated from the lineage that led through bony fish up through the early tetrapods all the way to us for over 400 million years. Why would anyone think that in all that time, they stayed still evolutionarily? Even if they look much more similar to their ancestors than we do, they have evolved too. Nothing stays still.