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For Day 2 of Prehistoric Shark Week on paleoaerie, we are going to take a look at my personal favorite shark. In the late Cretaceous, it was called Scapanorhynchus, the spade snout. But its closest living relative is called Mitsukurina owstoni, also known as the goblin shark. The perfect shark for Halloween.
Scapanorhynchus means spade snout, so named for the elongated, flat snout, the same feature which got the modern shark named goblin. Most of them are small, less than one meter, but can get in excess of four meters. Spade snouts were some of the earliest sharks in Neoselachii, the modern sharks. One of the things this means is that they did not just have straight cartilaginous skeletons, they calcified most parts of the skeleton to reinforce the cartilage. They didn’t make true bone, but the calcium spicules provided more strength for the cartilage.
Goblin shark teeth are long and thin, looking like a mouth full of curved needles. But what most people are fascinated by is the amazing length to which they can protrude their jaws. Modern sharks have what is known as hyostylic jaws, meaning that the jaws are not directly connected to the skull. Instead, they are attached at the back of the jaws on an intermediary bone that allows the jaw to swing forward. All sharks can do this to an extent, but the goblin shark is expecially known for it.
The modern goblin sharks are generally only found in deep water. Its Cretaceous cousins, on the other hand, were widespread in shallow marine areas. Like many fish in the Cretaceous, they seem to have survived the mass extinction even at the end of the Mesozoic by going deep.
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.