Everyone has encountered a T. rex in books, movies or museums. Besides its size and menacing jaws one of the most striking features of a T. rex is its diminutive arms. So why were the arms of T. rex so short? There have been many hypotheses including: 1) they had no use and were like a vestigial organ, 2) they were used to hold on to potential mates, 3) they were used to hold onto small prey while they ate, 4) they were used to help them get up when they fell down or 5) some combination of these explanations. Can these hypotheses be tested? Well, yes and no. Behavior traits are notoriously difficult, by not impossible, to assess with the fossil record. For example, see my post on extinct elephant behaviors deduced from fossil footprints (Preservation of Behavior: Fossilized Elephant Tracks from the Arabian Peninsula) or what we can learn from piles of fossil poo (Piles of Fossil Poo: Providing a Peak into the Past). What about the first hypothesis that the arms were useless? The assumption here is that over time the arms became useless and so atrophied to the point of being these tiny little nubs in comparison with the entire animal. In effect maybe there are just useless vestigial organs. This hypothesis can be tested and shown to be unlikely to be true. The bones of the arms show evidence of large muscle/tendon attachment points. Analyses of these arms then suggest that they were very strong even if they were very small. There is also the presence of two sharp claws at the end. These observations can be used to strongly support the hypotheses that the arms had some use even if we may not understand what that use may have been.
Rather than go on debating these speculative hypotheses, I want to look at a more fundamental question that I often ask my students: How do we know that T. rex had short arms?
Sounds like a simple question. After all, doesn’t every know that they had laughably small arms!? But ask yourself, how do you know this to be true? Have you ever seen a Tyrannosaurus rex in person? If you were not there to witness T. rex’s arms can how can you be sure they had short arms? Has anyone seen a T. rex in person? I don’t think so. If they lived 65 million years ago how can we be sure today that they had short arms?
So what is this evidence that we all find so compelling. It is found in nothing more than bones turned to rock. As philosopher Carol Cleland would say, there is your “smoking gun” evidence (see Origins Science and Misconceptions of Historical Science for a further discussion and references). The bones have left evidence or a “trace” of history that we can use to test our hypotheses about the lengths of the arms of T. rex and theropod carnosaurs in general.
Ok, we know because of the bones. So how many total bones do we have to support this conclusion? Well we have a lot of bones from T. rex specimens from many locations so there is quite of bit of fossil data. But you might be surprised to find out that there are very few complete sets of arm bones found. I haven’t been able to find firm numbers but it seems that there are fewer than 15 near complete skeletons of T. rex that have ever been found. Of those I am not sure how many of these even had the arms actually attached to the shoulder blades but I would be surprised if it were more than a couple do. The first was not found until 1989! and yet we have drawn a T rex with short arms long before this. Of course there have been hundreds of partial skeletons found and I am sure many arm bones have been found in the general vicinity of other T. rex bones. The smoking gun evidence is found in those few that are actually attached. But the observance of other small arm bones near other T. rex bones also adds to our conviction (or you could call it theory) that T. rex had short arms.
Now lets say you aren’t convinced. You might claim that we have simply missed a few bones because T. rex had more bones than other reptiles in their arms and so we don’t have the full picture. Finding a set of bones all connected or at least lying in a connected configuration ought to be enough evidence to dispel you of that thought, but we do have additional evidence that strongly supports these diminutive T. rex arms. That evidence comes from bones of many other species of dinosaurs that are similar to T. rex. It seems that there many other theropods which also had short arms. The fact that there are more than one species with short arms provides even greater support for the short arms of T. rex because it is far less likely that we have just missed some crucial bone in all of these fossils specimens.
So it is just some bones found in rock that are found next to a shoulder bone of a T. rex are the foundation of every image you have ever seen of a T. rex with short arms. The lesson here is not that we should doubt that T. rex actually had short arms but rather that the historical sciences can produce results for which we have great confidence. In this case we can have great confidence that we can “know” that these massive animals had tiny arms. We can’t test this theory by going back in time but we can test it with evidence that we can gather in the present and this evidence can yield a high degree of confidence in our conclusions about the past.
This is just one example of something we can know about the past by examination of data from the present. Let us look at another example of deriving conclusions about the distant past via the historical sciences and see if our study of dinosaurs will yield some more provocative conclusions than the lengths of a dinosaurs arms.
How do we know that some dinosaurs ate animals and some only ate plants? Can we tell what plants did they ate?
Teeth are one important clue because we can compare their teeth to teeth from various animals alive today. While this is fairly compelling evidence it is not quite smoking gun evidence in my opinion. The smoking gun is the combination of evidence from teeth and dinosaur coprolites (yeah, that’s dino-poop). When coprolites are found associated with dinosaurs with teeth that imply herbivory and those dino-droppings also support a herbivorous-only diet then you have beyond reasonable doubt-type evidence of their ancient diet.
But we can learn so much more from these coprolites than just the general herbivorous behavior of these giants. With these dinosaur dropping we can ask what specific kinds of plants they ate. To find out, cross sections of plant-eating dino-poops are made. Pieces of stems and leaves often pass right through the gut and pollen and spores of plants are especially well-preserved. The anatomy of these plant parts and structures of these spores can be examined in detail and the types of plants determined very reliably. In this case plant-eating dinosaurs from the Triassic and Jurassic Periods are shown to have eaten ferns, tree-ferns, lycopods, and primitive forms of conifers. What is missing in this list? Yes, flowering plants. Today’s the most dominant plant type on earth are flowering plants which include most trees and the grasses. No large herbivore today could possibly eat vegetation and not get at least some flowering plant pollen in them much less any other portions of a flowering plant. Yet, there have been many dinosaur coprolites from this time period that have been examined from all over the world and they lack pollen and flowering plant parts. This is very striking given that there are cellular features that are unique to flowering plants which could easily be seen in these specimens if they were present. Surely if flowering plant parts and pollen in any coprolites were present, microscope examination would reveal these features.
How do scientists interpret this coprolite evidence?
These fossilized coprolites tell us that some dinosaurs only ate plants and that globally what the vast majority of all plant-eating dinosaurs had to eat during the Triassic and Jurassic Periods were non-flowering plants most of which are extinct today. How confident are scientists that most dinosaurs had nothing to eat by a fern and conifer diet? I would say that their confidence level approaches that of their confidence level that T. rex had short arms and that some dinosaurs ate plants and some ate meat.
The bottom line is that the preponderance of evidence regarding the arms of T. rex and the diet of dinosaurs produces a compelling pictures of the life of past organisms. These images of life are also supported by many other independent lines of evidence that I don’t have time to explore here today.
Young Earth Creationists, Dinosaurs and Historical Data
If you were to visit Ken Ham’s Creation Museum in Kentucky you would find dinosaurs of various kinds on display there including plant and meat eaters and a model of T. rex with his short arms. Ken Ham is fond of asking people who think the world is old: “Were you there?” He frequently suggests that we can’t be confident about things from the distant past because we were not witnesses and only the Bible is a witness to these events. So I ask you, why does he have such confidence that T. rex had short arms? Was he or anyone else alive and reported that they saw a T. rex with short arms? Does the Bible provide us with a physical description of T. rex? No! So the evidence that T. rex has short arms comes from observations of fossils and the assumptions about those fossils (eg. they actually represent parts of formerly living things and that bones next to other bones represent their order of attachment in the real organism). Ken Ham and AIG have published numerous books with recreations of dinosaurs and even discuss their behaviors all based solely on bones of stone found in the ground. He clearly accepts the evidence and interpretations of this evidence derived via the historical sciences without qualification. But what happened to that origins science vs operational science distinction that Ham and others of so fond of espousing? (see HERE for more about this distinction). Origins science is the label YECs like to slap on any conclusion of historical science that they deem contradictory to their interpretation of scripture. But how do they know when something is or isn’t under the purview of what they call origins science? To me re-creating the shape and size of an animal now extinct and never spoken of in the Bible sounds a whole like what they want to call origins science but since the don’t have a problem with these particular interpretations of historical science they don’t call them into question.
I’m really can’t blame Ham and company for accepting that T. rex had short arm as the evidence, though not abundant, is very strong. It is not unreasonable at all that they accept this but then what are we to make of the diets of the plant eaters? I am sure young earth creationists would readily accept the observation of coprolites full of bones as evidence that T. rex was a carnivore but then what do they do with the coprolites of dinosaurs from the Triassic and Jurassic ages that contain 100% ferns, lycopods and conifers and 0% flowering plants. Maybe they could dismiss a single coprolite as a dinosaur browsing in a bog full of ferns and confers. But when all coprolites from a single geological period all lack flowering plant pollen and plant parts and those coprolites are found associated with multiple species of dinosaurs in multiple locations on earth, it become very difficult to deny that the food that dinosaurs had available to eat did not include flowering plants. Someone might claim that dinosaurs found in these geological periods just didn’t like flowering plants and avoided them. This is very unlikely and would clearly be an ad-hoc hypothesis. Today all large herbivores very much prefer flowering plant leaves like grass over pine needles and ferns, but even if they tried to avoid all leaves and stems of flowering plants they should have still eaten pollen grains inadvertently since they are everywhere in the environment. When you combine the lack of flowering plants in dino poop along with the lack of flowering plant pollen and plant parts in the rocks that the same dinosaurs skeletons are found preserved in this becomes the smoking gun evidence for the theory that flowering plants did not even exist at the time that the dinosaurs lived in the Triassic and most of the Jurassic Periods.
****A side note here: For YECs, the dinosaurs preserved in the fossil record were preserved during a Global flood and they generally view those dinosaurs as running to escape the flood which is their explanation for why dinosaur fossils are not found in the lower portions of the geological column. There are millions and probably many billions of preserved dino-poops in the same rock layers that the dinosaur bones are found. Since these dinosaurs were presumably running around trying to avoid this chaotic flood the food they ate would have been whatever they could find which should have been a random mixture of the plants alive at the time of the flood. This makes it even more unlikely that they could have avoided eating flowering plants and that we would observe such a distinct pattern in their coprolites.****
What do YECs do with this data. They generally ignore it but when forced to confront it they deny that flowering plants were not present and prefer to say “How do you know, where you there?” In addition it is likely they would suggest that historical scientists don’t have the right worldview and so can’t interpret the data properly. They put models of dinosaurs in their museum surrounded by fruit trees and print books with dinosaurs helping people pluck fruit from trees. At the end of the day, they are sure that T. rex has short arms based on a couple of bones but they are more than willing to ignore hundreds of pieces of data that all point to the lack of flowering plants in most dinosaur diets. There seems to a picking and choosing what historical science data they want to accept.