What is a Dinosaur?
Before we advance in our discussion on Dinosaurs, it is imperative to define what a Dinosaur is. Dinosaurs are unique creatures that ascended from the ashes left by the Permian Extinction. To fully answer this question, we must understand from where dinosaurs come. We often think of Dinosaurs as giants, but the truth is they came from animals that are smaller than a house cat (Thomas R. Holtz. 2007). In the beginning, dinosaurs were small, bipedal lizard-like animals. Coelophysis is the oldest dinosaur. But before Coelophysis, another group inhabited the earth, the archosaurs.
Archosaurs, the dinosaurian ancestor, were small with one adaptation, open hip joints with hind limbs positioned directly under their bodies. This adaptation would prove vital during the Jurassic when the Titanosaurs roamed the Earth. Without this very primitive but necessary anatomical adaptation, dinosaurs could not grow to the giant sizes that mesmerize us. Without open hip joints and legs positioned directly under their bodies, dinosaurs (such as the giant Sauropods and Giant Tyrant dinosaurs) could not assume an upright posture.
Open hip joint with hind legs positioned directly under the body with three or more sacral vertebrae fused to the pelvis is the first requirement to classify an animal as a dinosaur.
Above are images of Walter, Tyrannosaurus Rex located at the National Museum of Natural History in Washington, D.C. If you look at his skeleton, the open hip joint of this large theropod is visible. Without a specialized hip joint and hind legs directly underneath his body, Walter could not support his massive 6-ton weight.
Additionally, open hip joints provided yet another essential mechanism that aided dinosaurs, smooth locomotion and running. As bipedal locomotion organisms, we can relate to this efficient mode of movement. Without open hip joints connecting our legs to our pelvis, we would not be able to perform many feats that humans accomplish. Sports that require running, jumping, or aerial maneuvers we could not do without this vital anatomical modification.
When looking at dinosaurs, there is another anatomical adaptation they possess. When you look at a Theropod such as Allosaurus, we notice “Big Al’s” hands. This anatomic trait is of much importance to the success of the dinosaurs. Hands complete with digits on the manas (hand) and pes (feet) allow the dinosaurs to grasp and carry objects. Bipedalism freed up their forelimbs to perform other functions that aided their survival. The incredibly tiny arms of Tyrannosaurs Rex are often the punch line of many jokes, but even T-Rex could use his hands. While small compared to the rest of the muscular bodies, it’s speculated that he could lift 100 lbs. with his forelimbs. But more importantly, forelimbs unnecessary for locomotion proved to be a great asset to the success of the dinosaurs.
As habitual bipedal locomotion specialists, we appreciate our ability to have our hands free to perform other functions besides movement from one place to another. Just like the dinosaurs, the adaption of bipedal locomotion proves essential to our success as Homo sapiens.
The second requirement to classify an animal as a Dinosaur is hands complete with digits, both manus (hand) and pes (feet).
We often think of Dinosaurs as being extinct; they are not. Dinosaurs fly over our heads every day, Avialians known as birds. Birds are not just related to dinosaurs; birds are dinosaurs, Coelurosaurs, a branch of theropod dinosaurs. I will revisit this in a later post. For now, understand that birds are dinosaurs, because they are dinosaurs and possess a vital anatomical adaptation that all dinosaurs share.
The skull of an animal has many bones. We often think of the skull as one big bone, the head bone or cranium. However, a skull is a collection of many smaller bones that fuse together, forming the cranium. In dinosaurs, one fundamental skull bone is missing, the postfrontal bone. In the image below, labeled are the bones of a dinosaur skull. Notice that the frontal bone (pink) is present, but the postfrontal bone is missing.
Anapsids (line of descension for reptiles) and synapsids (line of descension for mammals, including humans) possess postfrontal cranial bones. Diapsids (the evolutionary line which contains the dinosaurs) lack postfrontal cranial bones. Ancestors of the dinosaurs, including the Archosaurs, retain this bone. This missing cranial bone in all dinosaurs present in dinosaurian ancestors assists their definition (Martin 2006).
All birds (heir of the dinosaurs) do not have postfrontal cranial bones. The postfrontal cranial bone fuse into the frontal bone during the later stages of embryonic development.
The third requirement needed to classify an animal as a dinosaur; is the absence of a postfrontal cranial bone.
The limb bones of dinosaurs, specifically if they are related to the appendicular skeleton (bones that attach or append to the axial skeleton). are vital because they provide information on how the dinosaur moved. In the posterior portion of a dinosaur, hind limbs are attached to the body by the pelvic girdle (hip socket) below the sacral vertebrae (fused vertebrae that attach the open hip joint to the spine).
Due to the evolutionary adaptation of an open hip socket, dinosaurs can support their weight directly over the hip joint. For this posture to be possible, dinosaurs require adaptions to facilitate this mode of locomotion. Attached to the hip joint by the acetabulum, the femur (thigh bone) terminates at the cnemial crest and the tibia. The cnemial crest is visible in the image above in Walter’s skeleton.
The tibia is key to two additional dinosaurian traits. The cnemial crest and the ascending process into the astragalus or anklebone. (Martin 2006). The astragalus (anklebone) and other small bones form a dinosaur’s tarsals (feet and toes). Dinosaurs walked with their metatarsals in contact with the ground. In other words, dinosaurs walked on thier toes. The digitigrade motion is visible in the image above. Walter walks on his toes, heals never touch the gound when walking. This digitigrade locomotin can also present in dogs.
Contrast this with human locomotion. Humans walk with a relaxing stance in which most of the body weight in on the heels in a plantigrade stance. Human can only achieve a digitigrade when wearing high-heeles shoes or tiptoe.
These adaptations relate directly to the evolution of dinosaurs from reptiles and archosaurs. Unlike reptiles which move with legs sprawled out to the side of the body like large lizards, dinosaurs walk upright with their legs directly underneath their bodies with digitigrade motion.
In addition to open hip joints and fused sacral vertebra, dinosuar locomotion requires the adaption of the cnemial crest, tibia, and astragalus to facilitate this upright smooth motion.
Finally, we come to the last requirement to classify an animal as a dinosaur; the femur that terminates into the cnemial crest, a tibia that ascends to the astragalus, and the astragalus which forms the metatarsals for digitigrade locomotion. Without these evolutionary adaptions, dinosaurs would not be able to support weight in an upward stance; dinosaurs would crawl in a sprawling posture like the reptiles that preceded them.
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As always, thanks for reading. Next time, I will introduce a newly discovered theropod, Meraxes Gigas.