Home » Posts tagged 'fish'
Tag Archives: fish
Three years ago, a ten year old boy was visiting a monastery in Colombia. Being a curious boy, he looked around at his surroundings. He could have done like others have done for centuries and not paid that much attention to the stones upon which he walked, but he didn’t. He noticed a curious fossil fish in one of the flagstones. Most people, if they noticed it at all, would have simply given it a passing nod of interest. He, on the other hand, took a picture of it and sent it to the local Paleontology Research Center to see if they knew what it was.
Firstly, I amazed they even had a local paleontology research center, most places don’t. Secondly, it is amazing that the boy took the time to bring it to their attention. Thirdly, it is amazing that someone there noticed what they had and brought it to the attention of the needed experts. All these amazing, unusual occurrences have resulted in an article in the January 31 edition of the Journal of Systematic Paleontology detailing the new fossil species discovered by that boy. Sadly, no one knows how to contact him to let him know about the publication. The researchers have his name and email, but have apparently been unable to contact him to give him his copy of the paper about his fish.
The fish he discovered was named Candelarhynchus padillai, after the Monastery of LaCandelaria near Ráquira, Colombia, where it was found. The stone for the flagstone came from a nearby quarry. According to the authors of the paper, the rocks in the quarry corresponding to the flagstones were “fossiliferous, finely laminated, light to dark grey, indurated mudstones ofthe lower-middle Tuonian San Rafael Formation…” The rock strata also contained numerous plankton, ammonites, clams, and crabs; so quite a rich fauna. The Turonian is 89.8-93.9 Mya, according to the latest GSA time scale, so we are talking roughly 92 Mya.
The fossil is excellently preserved, with slabs containing both part and counterpart, meaning that when they split the slab, pieces and impressions were left on both sides. The whole body can be seen, with nice detail around the head, as well as impressions of the soft tissue portions of the fins. At 27 cm (just over one foot), it is a decent-sized fish. It’s a thin fish, with a long skull full of tiny, conical teeth. It was clearly a fast-swimming predator, and likely prey for a lot of larger species.
The reports on the fish said that it does not have any living relatives. That is true, in a way, but also not. The specific family the phylogenetic analysis placed it in is Dercetidae, an extinct family that all died out in the Cretaceous. However, if we look a bit broader, it is in the Order Aulopiformes. This order is mostly known for a variety of mostly deep water fish known as lizard fishes, which is why all the news reports of this find have said Candelarynchus was a “lizard fish.” Even though it is in the same Order, it is not in the same family as any of the modern lizard fish.
But the title of this post mentioned Arkansas and I have thus far not done so. Vernygora reports that current analyses of fossil aulopiforms include three main families: the Dercetidae, Halecidae, and the Enchodontidae. One of the most prominent Cretaceous fish from Arkansas is Enchodus, commonly called the “saber-toothed herring.”
This is a terrible name because Enchodus has nothing to do with herrings. It was at one time considered part of Salmoniformes, making it closer to salmon. However, more recent analyses have consistently placed it in Aulopiformes, specifically within the Enchodontidae, making it closer to lizard fish. This makes a good deal of sense to me because, if you add the fangs from a payara, commonly known as the vampire fish, onto a lizard fish, you have pretty good idea of what Enchodus was like.
Fossil lizard fish then were quite abundant in the Late Cretaceous in both worldwide range and diversity. They may not be the most recognized fish today, but they have a long history and make for great fossils that can be found in a lot of places, including southwestern Arkansas.
Welcome to Day 4 of Paleo-Animal Fest, celebrating the creatures populating the Arkansas seas during the Cretaceous. Today we are going to look at a fish that has survived for an amazingly long time. They first appeared in the Late Cretaceous and have survived to the present day, still thriving. You can find them in many freshwater lakes and rivers, especially brackish and hypoxic (low oxygen) waters, even into marine waters on the occasion. They are a tough predator in many ways, from their durability in the fossil record to their physical defenses and their intimidating jaws. I am of course talking about gars.
Gars are piscivorous, meaning they eat other fish. The most common description of them is “voracious predator.” They are known for their tooth-filled jaws, scales of armor, and their fight. Their typical mode of attack is a lightning-quick sideways bite. Gar fishermen are often called “not right in the head.”
Gars can be found in many places within North America, but their fossils can be found all over the world. The vast majority of the fossils have been identified as Lepisosteus, which includes the longnose, shortnose, spotted, and Florida gar. However, most of their fossils are isolated scales, which makes it difficult to impossible to tell what type of gar it is from. So I am going to go with most people’s favorite gar, Atractosteus spatula, the alligator gar (pictured above). It is the biggest one reaching almost 3 meters. Another impressive armored, ancient fish that is still around is the sturgeon, which can get a lot bigger, but are nowhere near as impressive in the teeth department.
There are not a lot of skeletons of gars with heads and tails, but there are a lot of body pieces covered in scales. Gar scales are thick, rhomboid-shaped ganoid scales, meaning they are covered in what is effectively enamel. The scales form an excellent armor, making handling them hard on the hands. They are so tough and dense, in fact, that the scales have been used as arrowheads and make even CT scans on gars hard to impossible to get decent views. On the plus side, this results in them having excellent preservational potential and can be found quite commonly. The scales make the fossils really stand out and readily identifiable to at least the group Lepisosteiformes.
By far, the most complete and detailed description of gars ever published is by Lance Grande, the universally acknowledged leading world expert on fossil fish, called “An empirical synthetic pattern study of gars (Lepisosteiformes) and closely related species, based mostly on skeletal anatomy. The resurrection of Holostei.” Special publication 6 of the American Society of Ichthyologists and Herpetologists, published in 2010. This is a massive tome, amassing almost 900 pages of detailed observation on gars. This book is a companion to a similar volume he did on bowfins. I can honestly say I have never seen a more thorough job on any group such as this in my life. Every time I look at it, I think wow, all this on just gars? This would make any scientist proud to have one of these capping their life’s work and this doesn’t even begin to touch the work put out by Grande. I am in awe.
It’s Friday, time for the answer to Monday’s mystery fossil. Were you able to identify it?
These fossils are from a fish called Enchodus, the “saber-fanged herring.” Teeth of Enchodus are commonly found in the Cretaceous-aged rocks in southwestern Arkansas, especially near Malvern and Arkadelphia in the Arkadelphia and Marlbrook Marl Formations, up into the Paleocene rocks of the Midway Group a bit farther north. In other places you can even find them in rocks of Eocene age, although you will have much better luck in the Cretaceous rocks. At this time, southern Arkansas was shallow to coastal marine. Go to the Bahamas, imagine Enchodus, mosasaurs, plesiosaurs, and plenty of sharks in the water around the islands and you would have a good picture of the landscape back then. They were abundant at the end of the Mesozoic Era and survived the asteroid impact that rang the death knell for many animals, including the non-avian dinosaurs. But they never regained their prominence as a key member of the marine ecosystem, eventually dying out completely in the Eocene sometime around 40 million years ago (the Eocene lasted from 55 to 34 million years ago).
The fish reached sizes over 1.5 meters, which makes them on the large side, but not really big, considering there were mosasaurs in the same waters that surpassed 10 meters. Still, with fangs longer than 5 cm, they would not have been fun to tangle with. They were clearly effective predators on smaller fish and possibly soft animals like squid. At the same time, fossils have been found showing they were themselves prey for larger predators, such as sharks, the above-mentioned mosasaurs, plesiosaurs, and even flightless seabirds such as Baptornis. Baptornis was a toothed, predatory bird, but as it only reached 1 meter or so, it would have only been able to hunt young Enchodus. So like many of us today, Baptornis was always up for a good fish fry.
Enchodus is often called the “saber-fanged herring,” although it is unrelated to herrings. So what then was it? Herrings are what is known as forage fish, meaning they are mostly prey items of larger fish and other animals. Most of the fish called herrings are in the family Clupeidae in the groups Clupeiformes, which includes such well-known fish as sardines and anchovies. Enchodus, on the other hand, has been placed into the group Salmoniformes, which includes trout, char, and of course, salmon. When one typically thinks of trout and salmon, one doesn’t think of bait fish, they think of the fish that eat the bait fish. Thus, Enchodus would better better described as a fanged salmon (they were a bit large to call them fanged trout).
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.