The Anatomy Of A Horned Dinosaur

ANATOMY is the study of the parts that make up the whole organism, plant or animal. Anatomy is a very old subject, probably about as old as human curiosity. Anatomists today often use light microscopes or electron microscopes and tend to call themselves cell biologists, structural biologists, electron microscopists, or anything else but anatomists in order to sound modern. Anatomy is vitally important for doctors and veterinarians, and it is the first subject that students training for the healing professions study.

When paleontologists think about anatomy, we are usually thinking of skeletal anatomy. We sometimes have to remind ourselves that there are other body systems, most of which are what we call "soft parts." Soft parts themselves rarely fossilize, although under favorable circumstances impressions of them may. We have no direct knowledge of the stomach, intestines, reproductive or urinary organs, heart, or lungs of horned dinosaurs. However, we are confident that these organs were once present because all land-living vertebrates have them.1 It is an interesting exercise to try to infer what these organs may have been like.

Muscles are another matter of concern to the paleontologist. It is a naive thought that paleontologists routinely reconstruct muscle patterns on fossils, for this is actually a difficult enterprise. It is true that some muscle scars are prominent. For instance, in reptiles a major muscle sweeps off the tail (cauda) and inserts on the inner surface of the thighbone (femur), where it leaves a major scar. The caudifemoralis muscle may be seen in dissection of an alligator or lizard; it is an important muscle that pulls the thigh backward during walking or running. The scar for this muscle can almost always be recognized on a dinosaur femur. Yet for every muscle that leaves a definite scar, there are four or five muscles that leave no scar, either because they are too small or because their area of attachment to the bone is too diffuse.

Some muscles are found in virtually all tetrapods, for instance the well-known biceps and triceps muscles of the upper arm, or the gastroc-nemius muscles of the calf. We feel pretty confident that horned dinosaurs were no exception. Other important muscles familiar to us mammals are completely absent in both alligators and birds, as well as in other reptiles, and so were almost certainly absent in dinosaurs. The gluteal (rump) muscles are an example. Therefore we cannot rely on muscle scars alone to do a major muscle reconstruction, and we cannot rely on our knowledge of mammalian anatomy, either human or veterinary.

Is there no basis at all for reconstructing muscles in dinosaurs? Of course there is. The best sources for learning muscles are crocodiles and birds. Together they constitute the extant phylogenetic bracket for dinosaurs; that is, they are the living animals most closely related to dino-saurs.2 Crocodiles (including alligators and caimans) are the closest living reptilian relatives of dinosaurs, and birds appear to be direct descendants of small meat-eating dinosaurs (to some scientists, they are dinosaurs). Thus alligators and ostriches (or even chickens and turkeys) are extremely useful organisms to dissect. It is surprising how few published accounts there are of detailed muscular anatomy. The reconstruction of dinosaur muscles is a very sophisticated and intellectually demanding undertaking. Anyone who wants to reconstruct muscles on a dinosaur must first spend many hours dissecting to understand how muscles work. It is always appropriate to inquire after what animals a reconstruction is modeled. Any comprehensive muscle reconstruction does not merely flow from understanding the dinosaur's skeleton but encompasses significant conceptual inferences, which may or may not be justified. For instance, if mammalian muscles are applied to dinosaur skeletons, as was sometimes done prior to 1950, significant errors may be introduced. Any muscle reconstruction necessarily imposes something from the present onto the past.

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