Southwest Arkansas has a lot of Cretaceous rocks. During that time, Arkansas was right at the border of the Western Interior Seaway. As such, one can find a lot of marine fossils from the dinosaur era, including mosasaurs, elasmosaurs, oysters by the millions, clams, shark teeth, and much more. You can also find shoreline fossils, such as thousands of dinosaur footprints and the occasional bone.
If one crosses the border into North Texas, one can find a lot of fossils that are similar to the southwest Arkansas marine fossils. There is a website, called txfossils.com, a Texas resident has put up showing lots of pictures of the fossils found in the area. The pictures are divided into several categories. The first category is for cephalopods, which are mostly ammonoids. They are listed as ammonites, but they are mostly goniatites from what I can tell, although I can not see the suture patterns (the septa, or walls, between body chambers) well enough and most of them to say for sure. Even though most people refer to the entire group as ammonites, ammonites comprise a single subgroup with the larger group called ammonoids known for complex body septa. Goniatites have a simpler wavy pattern. Nautiloids comprise the third group and are known for simple curves forming the septa.
The other categories include gastropods (snails), echinoids (sea urchins), bivalves (clams), coral and bryozoa, petrified wood plants, and bones from vertebrates. The vertebrate bones include a varitey of shark teeth and some teeth from Enchodus, the “fanged herring”. There are also what appear to be turtle shell fragments and some random bits of bone I cannot identify.
There are lots of pictures (the two shown here are from the website) which should give you some idea what you are looking at if you go fossil hunting in southwest Arkansas. Enjoy. As always, if you find any vertebrate material, please let me know. If you find invertebrate fossils you would like identified, your best bet is to contact Rene Shroat-Lewis at the University of Arkansas at Little Rock or Angela Chandler at the Arkansas Geological Survey.
National Geographic has a website called Phenomena, which is the home to a group of blogs, one of which is Laelops, a blog written by Bryan Switek devoted to paleontology, with a considerable emphasis on dinosaurs. Recently he wrote an essay on Lazarus taxa. That got me thinking about how species can appear to disappear and reappear in the fossil record and the strange and funny ways scientists talk about species, so this post will expand upon that topic and discuss ways that species can appear to come back from the dead, figuratively speaking.
What is a Species?
First off, it is important to understand what we mean by a species. Most people think of species using the biologic species concept, which requires a species to be reproductively isolated, the species is defined by who can breed with who. Unfortunately, there are a number of instances in which this doesn’t work. There are species that do not reproduce by having two individuals of different sexes breed, such as some parthenogenic frogs and fish, in which the females create viable eggs without the need of a male. Bacteria, which reproduce by simply dividing, can’t be called species this way, especially since they exchange DNA on a regular basis. Fertile hybrids screw up the biologic species concept as well and are much more common than most people realize. For paleontologists, the biggest problem is that we have no way of knowing how well fossil species could mate because fossils aren’t known for their breeding potential, being dead and all.
In fact, there are at least 26 different published concepts of what a species actually is. As a result, researchers have to choose the one most applicable to their research. Paleontologists frequently use what is known as the morphospecies concept. If it looks sufficiently different, we call it a different species. Are there problems with this? Yes, but we are limited by the data we have. We just have to keep in mind that paleospecies are not the same thing as modern species.
Gaps in the Record
The fossil record is quite good for many types of organisms, not so good for others. As a result, there are gaps in our knowledge. Sometimes a fossil can disappear from the fossil record and appear to go extinct only to reappear millions of years later. These species are called Lazarus taxa (a taxon, pl. taxa, is the name for a generic taxonomic unit, be it species, family, order, etc.). If one is familiar with the biblical story of Lazarus, the name makes sense. Lazarus died and was brought back from the dead. In a conceptually similar manner, taxonomists can bring a species back from “extinction” by finding more fossils, or, in some cases, living representatives.
Lazarus taxa can be formed a variety of ways. There may simply not be any rocks of the right age where the organism lived to preserve fossils, either through erosion or never having been laid down in the first place. It could be that they simply haven’t been found yet. The organisms could have moved so they are no longer in the same spot. For instance, crinoids at one time lived throughout shallow marine areas and were quite abundant. Almost all of them died out at the end of the Paleozoic. One group survived, but are most common in deep water and are much less common in shallow marine areas, making their fossils after the Paleozoic few and far between.
The most commonly discussed Lazarus fossil is the coelacanth, a fish whose fossil record had ended at the end of the Mesozoic and was presumed extinct, until live ones were caught in the Indian Ocean near South Africa and Indonesia. They were another species which seems to have retreated to deep water refuges to avoid complete extinction.
Come out, We Know You’re There
A related concept is that of ghost lineages. For Lazarus taxa, we know the species was present, we just haven’t found them yet. Obviously, if coelacanths were alive in the Cretaceous and today, they must have lived in between as well. Sometimes the gaps get filled in by discoveries of new fossils, others are still waiting to be found. Ghost lineages cover any situation in which we know a taxon had to exist, but we have no evidence for it, so Lazarus taxa can be thought of as a subset of ghost lineages. For Lazarus taxa, it is that gap between one fossil range and a much later one. But there is also the situation wherein a taxon had to exist much earlier than for which we have a record; and it is in these situations ghost lineages are most commonly invoked.
Suppose we have two taxa that we think are closely related. They had to have then had a common ancestor which lived before either taxa, even if we have no evidence for it. Everyone knows they have a great-great-grandfather even if they don’t know who it is. In the same way, every species has to come from a preexisting species even if there are no fossils proving its existence. It is also devilishly hard to determine an ancestral species (so hard in fact that many researchers view the best course is to just assume we can never do so), so we may even have found it already and just not recognized it as such, but that is a topic for another day. Unlike your ancestors though, the fossil record for related species can be separated by tens of millions of years. This range in which we have no fossil records for the species that existed later in time and for the ancestor itself is referred to as a ghost lineage.
Will the Real Taxon Please Step Forward
Another weird fossil species concept is that of Elvis taxa. After Elvis died, huge numbers of impersonators appeared. They may look like Elvis, they may sound like Elvis, but they are not Elvis. Species can do the same thing. When one species dies out, they can sometimes be replaced by a species that fills the same niche and may even look superficially similar, but they are unrelated. When two or more species develop in the same, at least superficially, ways, we call that convergent evolution. The pangolin, anteater, aardvark, and echidna (the spiny anteater) are all similar with similar lifestyles, but none of them are closely related to any of the others. If we apply this convergence to the fossil record, we can sometimes find a species that goes extinct, but is replaced by another species that fills the same niche, thus impersonating the previous taxon. If the two taxa are separated by time, they might appear to be Lazarus taxa, but they aren’t really because they are not the same species.
Bring Out Your Dead
Then there are the zombie species. Zombies are taxa that have gone extinct and fossilized, but the rocks encasing the fossils gets eroded. Sometimes, the fossil can then get reburied in new sediment, which becomes lithified, forming a new sedimentary rock layer. The fossil then appears to be the younger age of the new rock even though it is in reality much older. This can happen most commonly with teeth because the enamel is very resistant to weathering. So if you find isolated teeth, they may not have actually originated in the rock in which you found them. You may have just found a zombie. A number of such zombies, or reworked fossils, have been found in the form of dinosaur teeth in the early Paleocene, after the mass extinction event that wiped out all the nonavian dinosaurs.
Finally, there is the Dead Clade Walking. These are species that are extinct, they just don’t know it yet. This can happen after a mass extinction in which most species die out, leaving a few stragglers, which limp along for a while, but eventually succumb. After the Permian mass extinction, anomodont therapsids, the most famous being Lystrosaurus, survived the extinction that killed so many others Unfortunately for them, the group did not thrive, instead dwindling down until they too went extinct (if one follows the dinosaur teeth, you will find discussion of a possible dinosaur dead clade walking example). Lystrosaurus itself did spectacularly well and survived well into the early Triassic. So well, in fact, that that some places even have a “Lystrosaurus zone” because of all the fossils of them in the layer. As a result, Lystrosaurus is often called a “survivor taxon”. Even so, it too dwindled and went into the west, went extinct. A better group that deserves the survivor label is the cynodonts. They were minor players in the Permian, but after the great extinction event, they prospered, taking over the world in the early Triassic, at least until they crashed in the late Triassic and were replaced by the dinosaurs.
Yesterday was Darwin’s birthday. So instead of trying to shoehorn some sort of Valentine’s Day themed post for the week or an article about Darwin and his life and the importance of evolutionary theory, I thought I would briefly discuss a few of the most common Darwinian myths I have heard. For most people, it seems, it is accurate to say You Don’t Know Darwin, or Evolution, or Darwinian Evolution.
1. I don’t believe in evolution.
Yes, you do. You just don’t know it because you’ve been lied to by people who don’t understand evolution either and are threatened by it. But before we get into that, let’s please dispense with the term “belief”. Belief requires faith with no evidence. Since there are mountain-loads of evidence for evolution, don’t believe in it. Accept the evidence all around you. Once you understand what evolution is, you will agree that you have to be brain damaged not to accept it is true. Here is the big secret. Here is the definition of evolution.
Change over time.
Let me hear you say it!
Change over time.
I can’t hear you!
Change Over Time!
That’s it. See? Not so painful. You’d have to be an idiot not to understand that things change. If nothing ever changed, we would have no history books and people could never complain about the “good old days” when students were better (yes, people have complained that today’s students are worse than the previous generation for literally over 3000 years, one can only assume that either the ancient Greeks were God-like brilliant or people are biased).
What’s that, you say? That’s not what evolution means? Well, yes, it really is. But you want to talk about biological evolution. Some people think that simply saying change over time is overly simplistic and doesn’t really describe biological evolution. Ok, then. Here is a better definition of biological evolution. Ready?
Descent with Modification
Seriously, that’s it. Children are different from their parents. Now, unless you are going to argue that you are exactly the same as your parents, that everyone is in fact a clone, you are an evolutionist. Congratulations.
Oh alright. You may have heard that individuals don’t evolve, only populations, or even species. What that means is that one does not evolve over the course of one’s own lifetime. For most organisms, that is true. Of course, if you are a plant, which has what is termed modular growth, that is not strictly true. Plants can reproduce through one of two methods. They can reproduce through seeds, or they can reproduce through vegetative growth. In vegetative growth, the plants can send out tendrils (many people might call such tendrils “roots”). Those tendrils can grow horizontally through the soil and then spring up to grow what appears to be a new plant. The new plant is often called a clone, thus some people refer to this as clonal growth. Cottonwoods and sumac are great examples of this, most of them you see are actually clones grown this way. I say clones, but that does not necessarily mean they are genetically identical. If a mutation occurs at some point in one of the cells in that root tip, it can get passed along through the continued growth of that root so that the clone is indeed slightly genetically different. Considering that some of these plants can grow vegetatively for thousands of years through thousands of “clones”, a fair bit of genetic diversity can occur from one end to the other. I mentioned earlier that this is called modular growth. It gets that name because mutations that occur at the root tips affect all growth after that point, but do not affect the part of the plant before that point. Different parts of the plant are effectively separated from each other genetically and, to a point, physiologically. This is why you can grow new plants from cuttings. If the plant didn’t have modular growth, you couldn’t do this. Just imagine cutting an arm off of a person and trying to grow a new body from the arm. Animals, like us, do not generally have modular growth (unless you are a starfish, or planaria, or…).
Many people prefer a definition of biological evolution that takes populations into account. Thus, you will find this definition in many places.
A change in gene frequency in a population over time.
In this definition, evolution is restricted to changes that affect the DNA throughout a population. Ok, fine. But what does that really mean to a nongeneticist? It means that populations change over time in a way that those changes can be passed on to offspring. This is different than, say, changes in height and weight through strictly dietary changes. Just because Americans eat more and are thus typically taller and fatter than people in most other countries does not mean we have evolved to be taller and fatter, it just means we eat too much. It shouldn’t take a genius to realize this is true. A great example of evolutionary change in humans is our wisdom teeth, otherwise known as our third molars. Does it make any sense to anyone that we were created with jaws too small to fit all of our teeth so that we wind up having to pull some out? No, that’s ridiculous. The reason that our jaws are shrinking is that we have switched from eating tough, raw foods to softer, cooked, and processed foods that are easier to digest and we no longer have to chew as much. Some people are now being born who never have wisdom teeth. Eventually no one will have wisdom teeth and orthodontists will be very sad as a good chunk of their income will be lost to evolution.
But what really defines a population? It should be clear by now that biology does not lend itself to neat little boxes. Biology is messy (if it stinks, it is probably chemistry, but that’s another discussion). Typically, a population is defined as a set of individuals capable of interbreeding. This is very much like the biological species concept (BSC). The difference is that a species can be divided into multiple populations because not every member of a species has access to every other member. If something gets in the way, you get separate populations of the same species. And here we have a problem. What is a species? Most people have heard about the BSC. Unfortunately, it doesn’t work for a lot of organisms. It doesn’t work for plants, who hybridize at the drop of a hat and can grow vegetatively anyway. It doesn’t work for bacteria, or parthenogenic species who only need females to reproduce, or animals that can be cut up like sponges and starfish and planaria, etc. The last time I counted, there were 26 different definitions of a species. The idea that a species is the only “natural” unit in taxonomy is a myth. Even species are not natural. Researchers use the definition that is most applicable to their research. For instance, paleontologists can’t possibly use the BSC. It is really hard to get fossils to breed. Some might even say impossible. As a result, paleontologists are stuck with what is called the morphospeces concept. If it looks sufficiently different, it’s a new species. This means of course that you can’t realistically compare modern and fossil species because they don’t mean the same thing.
This is a really long-winded way of saying that it is much better to talk about evolutionarily discrete lineages, rather than populations or species and why I prefer sticking with the “descent with modification” definition of biological evolution. If that seems harder to deal with, biology is messy. Get used to it. But just because it is messy doesn’t mean it’s wrong. Life is usually messy. Just ask any parent. If you want absolutes and certainty, go talk to a physicist. Biologists have to deal with the real world in all its chaotic mess. I envy physicists, I really do. They have it easy. Yes, physics is easy. Biology is hard.
Ok, that was a lot. But if you still say you don’t believe in evolution, you are deluding yourself. The other myths can be dealt with much more succinctly.
2. You have to be atheist to believe in evolution. Darwin was an atheist until he converted on his deathbed.
I hear that a lot, but seriously? Are you seriously going to sit there and tell me these guys are atheists? Ok, maybe the last one, but this is an issue of pitting one faith against another, so please pardon the joke.
If you can’t trust that the Popes are devoutly religious, you have serious issues. But say you are one of those people that say the Popes may be religious, but they are going to Hell because they aren’t your sect of religion. Ok. Dr. Francis Collins is the head of the National Institute of Health. He led the Human Genome Project. He also happens to be an outspoken Evangelical Christian and has written extensively on why evolution does not conflict with Christianity. But Dr. Collins is a scientist, what does he know? What about Pat Robertson, leader of the 700 Club? Surely we can all agree that if Pat Robertson, of all people, does not think that evolution conflicts with Christianity, we can agree that you do not need to be an atheist to accept evolution.
Ok, you say that Pat Robertson is crazy. I won’t argue with you. But what about Billy Graham? If there is anyone more respected in the Evangelical Christian community, I don’t know them. Billy Graham has no problem with evolution. He is clearly not an atheist.
What about Darwin being an atheist? No, he wasn’t. He actually thought about going into the seminary to become a minister, but decided against it to pursue his academic interests. He didn’t seriously begin questioning his faith until his ten-year-old daughter died. After that, he lost faith in any sort of benevolent deity and he never recanted. The story that he converted to Christianity and denounced his views on evolution on his death bed is complete fiction. It was made up by someone who wasn’t even there. Why Lady Hope made up this story, I can’t say, but it is definitely a fraud. The important point here is that at no time did Darwin ever think that evolution conflicted with the Bible.
3. Evolution says that we evolved from monkeys, which can’t be true because A) I’m not a monkey, eww; and B) monkeys still exist.
Tell me, do most mothers die in childbirth? Then why would anyone think that a species has to go extinct when a new species arises? The reason that people think this is because they still have this view of the Great Chain of Being,” which was an old Christian view that everything had its place in the universal order. Rocks were at the bottom, then plants, then lowly animals, on up to humans being the most important mortal thing in all of Creation, topped only by the Heavenly Hosts, Jesus, and God Himself (that bit always confused me, if God is male, then there should be a female God, so where is She? Nevermind, I digress, that’s a whole other discussion.) Anyway, with this view in their head, people naturally assume that evolution works the same way. One species should naturally transform somehow into a new species.
Except it doesn’t work that way. Not all individuals of a species have to evolve together. If that species is divided into separate populations, or evolutionary discrete lineages, each population could evolve into a separate species. The original, or parent species, never has to change at all. Take the peripheral isolates concept. In this case, there is a species that has a broad range. At the edges of the range, members of the species find themselves in a different environment from the members of the species in the center of the range. The population exposed to the new environment will evolve in response to that environment, but the population in the center of the population never has to change. Thus, you have two or more species evolving from an original species that is still present.
But what about humans evolving from monkeys? No, that isn’t technically true either. Again, using the family analogy, let’s say your parents had siblings. Your aunts and uncles had kids of their own. You are related to your cousins through your parents. Pretty straightforward, right? Now replace everyone with species. You and all your cousins would be individual evolutionarily discrete lineages, you all have your own evolutionary path. Now, say that you represent all humans and your cousins represent all the species of monkeys. You aren’t a monkey, neither are your parents. your cousins, on the other hand, are (sorry, cuz). You (meaning all humans) share a common ancestor (your parents, or the ancestral species of humans) with your cousins (all the monkeys).
4. Evolution says that the earth is really, really old.
No, evolution has nothing to do with how old the earth is. The geologic time scale was actually put together by people correlating different rock units based on their relative position. Using the Law of Superposition, the oldest rocks were at the bottom, with the youngest rocks on top. Examining the rocks from place to place, they were able to line up different rock units into a long column. But it was all relative. They had no idea how old the rocks were. Finding the age of the earth didn’t happen until physicists discovered radioactivity. Some very smart physicists figured out that they could use the rate of radioactive decay to date rocks. All paleontologists and evolutionists did was say Thank You! So if you don’t like the age of the earth being over 4.5 billion years old, go talk to the physicists, it has nothing to do with evolution. Of course, when you do talk to them, you will have to deal with the fact that they have tested the theories quite well. We know they work because if they didn’t, we would not have nuclear bombs, nuclear power plants, x-ray machines, and a whole host of other things that work because of our understanding of radioactivity.
5. How does evolution explain the origin of life?
Easy, it doesn’t. Evolution only works on life that already exists. If you want to complain about the origins of life, go talk to a chemist. The origins of life is a chemical and physics problem, it has nothing to do with evolution.
5.Darwin invented evolution.
The last one for today might surprise people the most. Darwin did not invent, discover, or in any way introduce evolution. People knew that organisms change long before that. Joseph Buffon discussed the mutability (change) of species and that they had common ancestors in the 1700s. What Darwin did was provide a plausible mechanism for how evolution worked. Darwin provided evidence for natural selection. Of course, Darwinian evolution via natural selection is not the only mechanism. There is gene flow, which involves new material being introduced by immigrants into a population, and genetic drift, which is simple, boring, old random chance. But that is a topic for another day.
This is just a quick post to point you to a review by Darren Naish. Darren has done a fair bit of research on pterosaurs and Mesozoic birds. He also spends a good amount of his time writing for the general public with several good books out. The reason I am saying this is to make it clear that his opinion on this book is far more important and knowledgeable than mine.
Matthew P. Martyniuk’s Beasts of Antiquity: Stem-Birds in the Solnhofen Limestone is a book on the pterosaurs, archaeopterygians, and a dinosaur that, as the name suggests, have been found in the Solnhofen Limestone. This formation is well known because one of the most famous fossils of all time was found there, that being Archaeopteryx, a fossil that has been used to clearly demonstrate the link between dinosaurs and birds. If this sort of book is interesting to you, read Naish’s review.
It is the unfortunate fact of life that volunteer efforts are all too often derailed by other pursuits. Such is the case for last week’s Mystery Monday fossil. Nevertheless, the answer shall be forthcoming. If you have been paying attention to the Facebook feed, you will know that the fossil presented last Monday was identified. Were you able to figure it out?
This is a large, very well preserved piece of tabulate coral. Corals are colonial species that are very important in modern ecosystems. A fourth of all ocean species live within these reefs. They form the backbone of reefs that are among the richer areas of biodiversity on the planet. Billions of dollars each year are pumped into local economies across the world.
What we think of as coral is mostly the calcareous homes they form, within which the animals live. The actual animal is a tiny animal in the Phylum Cnidaria. Cnidarians are soft-bodied animals, the best known of which are the jellyfish and sea anemone. Cnidarians take two general forms. Medusae are free-floating forms like the jellyfish. Coral and sea anemones are polyps, mostly stationary, or “sessile”, forms that remain in place their entire lives. Corals, like other cnidarians, are predatory, catching their prey with tentacles armed with nematocysts, cells containing potent poisons to immobilize or kill their prey. Of course, since corals are tiny creatures themselves, they prey on even tinier prey. The tentacles surround an opening which serves as both mouth and anus, basically making the animal a living, carnivorous sack. This is not the only way corals get food though. Most modern corals also have a symbiotic relationship with single-celled algae called zooxanthellae, which provide essential nutrients for the coral in which they live. Unfortunately, when the coral gets too stressed from increasing temperatures or other causes, they tend to respond by evicting the zooxanthellae. Because the zooxanthellae are what gives corals their bright colors, this is known as coral bleaching.
While there are several different kinds of coral, most of the coral people are familiar with are the stony corals, or Scleractinia, because these are the ones that build the reefs. They are part of the larger group of corals known as Hexacorallia (at least, if you are talking to modern biologists, paleontologists often restrict Hexacorallia to scleractinians), known for often having the individual coral homes partially divided with six partitions, or septa (although you may be hard pressed to identify the three axes forming the six partitions even if they are present in that number).
The scleractinians have only been around since the Mesozoic however. They did not build the coral reefs of the later Paleozoic Era. That distinction goes to the rugose, or horn, corals and the tabulate corals, such as the example above. Tabulate corals are known for the corals being aligned in horizontal stacks. The image above should really be rotated 90 degrees to get the life position. This stacking always reminds me of apartment building, particularly cheap tenement housing, or wire mesh. According to phylogenetic studies on modern corals, it appears that the earliest scleractinians did not have zooanthellae, the symbiotic relationship evolving later, so it seems likely tabulate corals didn’t either. Tabulate corals appeared in the Ordovician Period roughly 450 million years ago. They started dying out in the Permian and finally succumbed to extinction at the end of the Permian period 252 million years ago, along with most other life on the planet. However, it is a bit misleading to say they went extinct. It is thought that the modern scleractinians that arose in the early Triassic are descended from tabulate corals, so they appear to have evolved, rather than just died out.
If you want to find corals such as this in Arkansas, one need only travel anywhere in most of the northern part of the state. The Ozark Mountains are predominantly formed from shallow marine Paleozoic rocks. Anywhere you find limestone in the Ozarks, keep your eyes peeled for samples of this type of coral. They are invertebrates, so as long as you are not collecting in a National Forest or private property without the owner’s permission, you are free to collect them.