Were you able to identify Monday’s fossil? Allie Valtakis was. Find out after the picture what it is. I hope it doesn’t make your weekend too crappy.
These two things are coprolites, otherwise known as fossil poop. Always a hit with kids when I show them in schools, but I always get the same questions. Do they smell? Will I get poop on my hands if I touch it? Most are tentatively reassured when I inform them that to be considered a coprolite, the poop has to be replaced with mineral. After a long period of time, there isn’t any actual poop left.
Coprolites can be quite informative. Coprolites preserve traces of what the animal that left it ate, so they can be useful for looking at the diet of prehistoric animals. Karen Chin, a curator at the Colorado University Museum at Boulder, is the leading expert on coprolites, particularly dinosaur coprolites. She found wood In some coprolites found in the Two Medicine Formation in Wyoming, which is unusual for two reasons. One, most coprolites are from carnivores, so herbivore coprolites are relatively rare. Secondly, most herbivores don’t eat wood except as a last resort when no other food sources are available. She was able to tentatively attribute these coprolites to the hadrosaur called Maiasaura (mainly due to the size and content of the coprolites, and the abundance of maiasaur bones in the area), making this the first dinosaur known to eat wood, as well as giving a unexpected perspective on the lifestyles of these “duck-billed” dinosaurs.
Probably the most famous coprolites known are also from the Two Medicine Formation and were also studied by Dr. Chin. They were uncommonly large and clearly from a carnivore. The only known carnivore from that formation big enough to create such a ponderous poop was Tyrannosaurus rex himself.
These coprolites told a fascinating story. The coprolites were readily identified as being from a carnivore due to elevated levels of phosphorus, which results from eating a high protein (i.e. meat) diet. The coprolites contained numerous bone chips, indicating that T. rex was not a dainty eater. T. rex had a massively built skull with powerful jaws, providing T. rex with the most powerful bite of any terrestrial animal. It put these jaws to use chomping through a carcass, bone and all. If one compares the thick, broad teeth of a tyrannosaur with the flatter, blade-like teeth and lighter skull of an allosaur, it is clear they had fundamentally different niches and eating styles.
There was bigger surprise found in the tyrannosaur coprolites. Dr. Chin found traces of undigested muscle. Obviously, it was not original muscle left in the coprolites, but mineralized remains. Why is this important? Modern reptiles have a slow metabolism. Food takes a long time to go through the digestive tract. As a result, digestion is phenomenally thorough. Crocodilians can take the enamel off teeth. Mammals, on the other hand, have notoriously inefficient digestive tracts. It is not uncommon to find recognizable bits left in the feces. Because of the elevated metabolism, food simply passes through too quickly for digestion to be complete. Meat is far easier to digest than plant matter, so carnivores, even mammalian carnivores, typically do a good job of digestion. To have traces of undigested muscle in the coprolite of a T. rex means that either the tyrannosaur was terribly sick with a bad case of the runs, or more likely, tyrannosaurs had short digestive times and a high metabolism to go along with it. It is possible to have thorough digestion with a high metabolism, but it is much harder to have incomplete digestion in a carnivore with a low metabolism.
Thus, coprolites not only tell us about the diet of extinct animals, they can also tell us about their physiology.
On the preservation side of things, one may ask how something as soft and squishy as a poop can fossilize. The answer to that is not easily. The vast majority of poops get washed away. But fecal material does have some advantages that help them get mineralized. As I stated earlier, carnivore feces is enriched in phosphorus. Phosphorus is an important nutrient, eagerly sought after by many organisms because it is not all that common in the environment, making it what is known as a limiting resource.
The other advantage is that feces is mostly made of bacteria, not really waste products. Our intestines are populated with microbes without which we can’t digest our food very well. The richer foods we eat, the more the microbes can grow and meat is a very rich food source.So why is having bacteria in the feces an advantage? Because the waste products they give off during their metabolic processes cause minerals to precipitate around them. Those bacteria are in a phenomenally rich food source in the poop, so they are growing like crazy, which means they are also precipitating minerals like crazy. In effect, they fossilize the poop while they are trying to eat it. If the poop can stay together, is not disturbed, and there is sufficient water around to allow the continued growth of the microbes, you will get a coprolite. The problem with this of course, is that poops are rarely left alone. Other animals eat them, dung beetles carry them off, they get stepped on and spread about, and rain washes them away.
If you have a kid interested in learning more about coprolites, I recommend the book Dino Dung, by Karen Chin. The book is written for elementary school kids, but is packed with a lot of good information on the study of coprolites and provides a great introduction to the study of fossil poop.
Were you able to figure out what last Monday’s fossil was? It is on display at Mid-America Museum in Hot Springs, AR. Ordinarily this would have been posted last Friday, but real life intervened. Apologies for that. Part of what happened was that when I posted the original picture last Monday, I thought I understood the background behind the fossil. It turns out that new research was published in 2013 that changed a lot of the more detailed interpretations. It didn’t change anything of importance to anyone not obsessed with details, but it sent me on a three day search for answers.
What we are looking at here is a foot print of a sauropod. Sauropods were herbivorous, long-necked dinosaurs and were the biggest animals to ever walk the earth, some of them possibly massing 50-80 tons and stretching well over 30 m (100 feet). We can’t say exactly which one made this particular footprint, but we can take a pretty good guess. If you guessed Sauroposeidon, or Astrodon, or Pleurocoelus, or Paluxysaurus, or Astrophocaudia, or Cedarosaurus, you are at least partially correct. These are all titanosaurs, a subgroup of sauropods. But which one we call it is more problematic. It is usually almost impossible to tell exactly which species made a particular track and in this case, it gets even harder because there isn’t a lot of agreement over which names are even valid.
Before we get into that morass, what is a titanosaur anyway? Titanosaurs have been in the news recently with the discovery of Dreadnoughtus. Most people are familiar with Diplodocus and Brachiosaurus, the two iconic sauropods. These two dinosaurs are the best known representatives of the two main groups of sauropods, with many species in each group. Diplodocus had shorter front legs than back legs and was relatively thin with a long, whip-like tail. It’s head was small and elongate, with simple, peg-like teeth in the front of the jaws. Brachiosaurus had longer legs in front than in back and was stockier, with a shorter, stubbier tail. It’s head was larger, with spoon-shaped teeth. Titanosaurs had front legs that were roughly the same length as the back legs, with a relatively whip-like tail like Diplodocus, although not thought to be as long. The heads looked like Brachiosaurus, but more elongate. Some had teeth like Diplodocus, some like Brachiosaurus. Basically, if you try to envision an intermediate form between Brachiosaurus and Diplodocus, you would wind up with something that looked like a titanosaur, which is rather interesting because all the studies trying to figure out their relationships place titanosaurs as much more closely related to brachiosaurs than to diplodocids. In fact, titanosaurs likely evolved from early brachiosaurids, which means that all the characteristics that make them look sort of like diplodocids are examples of convergent evolution, if the hypotheses about their relationships are correct.
What’s in a name?
Now that we know basically what we are looking at, what do we call the one which may have made this track? That is an excellent question. Two different trackways have been found in Arkansas, both in a commercial quarry in Howard County. They were fantastic finds, with thousands of tracks (5-10,000 tracks in the first trackway alone), placing them among the biggest dinosaur trackways ever found. Unfortunately, other than a few tracks that were spared, they no longer exist as they were destroyed by the quarry operations. That is a sad loss for paleontology, but in defense of the quarry owners, the tracks were found on private land and the owners had no legal requirement to tell anyone about them at all, they are running a business after all. They allowed scientists to study the trackways and in the case of the second trackway, they approached scientists at the University of Arkansas at Fayetteville about the tracks on their own initiative, giving them the opportunity to study the tracks before they were destroyed. As a result, careful maps were drawn, some tracks were removed and others were saved as casts. So the trackways themselves may be gone, but the knowledge of them is still with us and in the public domain.
The tracks were initially described as being from either Astrodon or Pleurocoelus, based on the fact that fossils from these dinosaurs have been identified in Oklahoma in rock units called the Antlers Formation, which is correlated with the Trinity Formation in southwest Arkansas. However, some researchers have concluded that the material upon which these names are based can not be reliably distinguished from any other titanosaur, so the names are what is called nomen dubium, literally dubious names. Pleuocoelus became what is commonly referred to as a junk taxon, which are used as a waste basket for material not identifiable as something else. In this case, when people found bits of a titanosaur in the southern United States they couldn’t identify, they said, it’s um…uh…Pleurocoelus? Pleurocoelus! Yeah! That’s the ticket! In 2013, Michael D’Emic published his research in which he found that part of the material identified as Pleurocoelus are really from two different sauropods called Cedarosaurus and Astrocaudia, and other parts are from a Texan sauropod called Paluxysaurus, leaving other bits unidentifiable as anything other than indeterminant titanosaur. Additionally, he found that Paluxysaurus was simply a juvenile form of Sauroposeidon, a giant sauropod known from four huge cervical (neck) vertebrae found in Oklahoma. So in conclusion, what can we say about the tracks? They were made by a titanosaurid sauropod.
Life’s a Beach
The first trackway was found by Jeff Pittman in 1983 while he was working in the quarry for his master’s degree at Southern Methodist University (SMU). The second set was found in 2011 by quarry workers, who brought it to the attention of Stephen Boss, a geologist at the University of Arkansas at Fayetteville. The tracks in the first trackway were 12-24″ across and were interpreted as being from from adult sauropods. The other trackway was more diverse, with tridactyl (three-toed) footprints attributed to the giant carnivorous dinosaur Acrocanthosaurus, as well as tetradactyl (four-toed) tracks which may have been made by a crocodilian of some sort. The pictures below are of the first trackway, taken by David Gillette, and can be found at his site discussing Seismosaurus.
The rocks in which both of the trackways were found is in what is called the DeQueen Limestone, a subunit of the Trinity Formation. These rocks were laid down in the Early Cretaceous about 115 to 120 million years ago. At the time, the shore of the Gulf Coast went through Arkansas, so much of southwest Arkansas was underwater. The DeQueen Limestome has thin layers of sandy limestone, many of which are quite fossiliferous, with oyster shells in abundance. There are also layers of limy clay and gypsum, indicating the air was fairly hot and dry. Stephen Boss likens the environment at the time to be similar to the Persian Gulf of today. So what we have is the coast of a very warm shallow sea. The dinosaurs appear to have been using the area as part of a migratory pathway. So while no bones of these dinosaurs have been found in Arkansas yet, we know they were here, so keep an eye out when you are fossil-hunting in southwest Arkansas. Who knows, you might find something bigger than you imagine.
It is commonplace to hear people say they do not accept evolution because they don’t see how some of the changes could have taken place. It’s just too complicated they say. What use is half a wing, they ask. As it turns out, the usefulness of half a wing, even a featherless baby wing, has been demonstrated, so that argument is out. Another transition people have difficulty with is the transition from water to land. Regardless of the fact that we have a good bit of fossil evidence demonstrating the transition, many people think that fins and feet are so radically different that they don’t see how it could have happened. Recent research has demonstrated in several ways that this transition is not nearly as hard as people might think, which may explain why it has happened multiple times in the history of life. For a truly bizarre history, one can look at the evolution of the elephants. Before they were elephants, they went from water to land back to the water, then back to the land.
So what tells us it really isn’t that hard? Let me introduce you to the robot salamander. This robot and its predecessors were designed to test different models of neural circuits involved in locomotion. What they found is that the same movements of the limb and torso allowed both swimming and walking. The only difference was the amount of resistance placed on the feet. Obviously, the ground supplied much more resistance to the limb motion than did the water. This caused a change in the neural signal, causing it to slow down and become stronger to account for the change in muscle power needed and the reduced speed of the movement. It was the same signal from the brain, it activated the same motor pathways. In other words, fish already had the neural pathways to be good salamanders.
Still, there are all the changes needed in the musculature and bones that surely had to be problematic. Research that has only recently been published indicates this isn’t hard either. Bichirs are a type of fish that regularly flops about on land and has true lungs. Emily Standen wanted to see what would happen if a bichir was raised on land and not free-swimming in the water. What they found was that the bichir changed how they crawled about, adopting a pattern that was more efficient. They held their heads higher off the ground and brought their fins closer to the body. More than simple behavioral changes, their skeletons changed as well. The supporting their pectoral fins changed subtly in ways that bore similarities with fossil of the earliest “fishapods”. It should also be noted that these experiments were on juvenile bichirs who were less than 70 days old and only lasted for eight months. This is not a lot of time to see differences.
I want to be clear that the bichir experiments do not show evolution of the fish. Evolution does not occur within a single individual. What we see here are epigenetic changes, not involving changes in the DNA. Epigenetic changes demonstrate developmental plasticity, the range over which a species can adapt to new environments without needing genetic alterations. But we now know that epigenetic changes can be passed on to the next generation in processes that are still only dimly understood. Unless these changes become incorporated into the DNA, they will fade if taken out of the environment that is producing the selection for that change. But eventually, these sorts of epigenetic changes can lead to real DNA changes that will lock in the change for all further descendants. In other words, what these experiments demonstrate is that fish already had the necessary developmental plasticity to evolve adaptations to land.
As I stated, these sorts of changes have to be incorporated into the DNA, but surely that requires a lot of changes? There have to be a lot of genes that have to be changed radically, right? Turns out, no. The switch is rather simple and it doesn’t even involve changes to protein-producing genes. All it takes is a change in the regulation of those genes. Change the developmental timing, change the amount of a protein here or there, and you turn a fin into a limb. Fish and terrestrial animals use exactly the same genes to make fins and limbs. They just change how they use them. This is why you occasionally get people born with webbed hands and feet. It can even cause polydactyly, having more than the normal number of fingers or toes.
A study published in 2012 looked at the regulation of hox genes, the genes involved in controlling the shape of our bodies, how our limbs are made, how many fingers we have, that sort of thing. All animals have them, they just vary in how many and how they are used. Renata Freitas and her associates took the control sequence for Hoxd13 from a mouse and put it into a zebrafish. The only thing this did was cause the Hoxd13 gene to be overexpressed. This caused the fish to have reduced fin tissue and the growth of cartilage forming what can best be described as a rudimentary limb. Just for emphasis, let’s say that again, simply changing the amount of protein created from this one gene turned a fin into a rudimentary limb. In other words, fish already had all the genes needed to make limbs for terrestrial locomotion.
So, we’ve seen that we have a lot of fossils documenting the shift from water to land. We’ve seen that fish already had the needed neural wiring to walk, the developmental plasticity to get started, and all the genes necessary. You can even see the transition showing up in the nerves that supply the human arm called the brachial plexus, the bane of medical students everywhere, which seems bizarre and nonsensical, until one looks at it from an evolutionary perspective. Then it all makes sense. But that is a topic for another day. All the transition really took was a prolonged stimulus that provided a selective advantage for walking around and limbs developed naturally from what was already pre-existing and working fine in the environment in which they evolved. Amazing how much change can be accomplished simply by a change in venue and a little push in the right direction.