Now, that this is all said and done, I want to do this Fossil Fact as a reward for your patience (and, in some cases, impatience) for the first post in two months of school and growing up. The topic for today, at the suggestion for my new friend, Ian Garofalo, is the so-called "Dino-Bird Theory" here in "pop culture" of Paleontology. ;) The theory describes the relationships between our flying neighbors the birds, and theropods. So, let's get right into it! :)
Dinosaurs Alive? A Look at the Relationship between Birds and Dinosaurs:
What's that up in the sky? It's a plane! No, it's a Bird! No, it's a......Dinosaur?! That's what the latest research on theropod dinosaurs seem to tell us. Now, I assume that you all look a bit incredulous, and, believe me, that is truly understandable. The relationship between birds and Dinosaurs is clear in the genetics, molecular biology and anatomy of many coelurosaurs. The implications indicate that a change in taxonomy is needed, or is it? That depends on how you could classify a bird and a dinosaur.
What is a Dinosaur?
Now, recall that a Dinosaur is a unique type of Archosauria ("ruling reptile") with the ability to walk either upright, bipedally or quadropedally, thanks to their hip design (The Saurischians have their pubis pointed backwards, with it, the illium and the ischium creating a ball-and-socket joint with the femur, while the Ornithischians have a much similar design yet, the pubis and the ischium pointing forward, still creating that ball-and-socket joint with the femur, much like the Saurischians, or Theropods and Sauropods.). The Dinosaurs are also unique in many respects, depending on the stage of their diversity and evolution.
For examply, the Triassic saw many early carnivores like Eoraptor, Herrarasaurus, Coelophysis and Liliensternus, share many of the same features as the archosaurs they co-existed with, yet, some of the primitive features of a "true" theropod; the ankle joints weren't as pronounced as in later taxa. They had features such as fore-arms with the primitive five-digits of their predecessors, the brain wasn't as developed, et cetera.
The Fossil record indicates that as many families progressed in their evolution, many limbs were merely shadows of their former selves, meaning they're vestigial. (Vestigial simply means that limbs or body parts that once had a use in our bodies, no longer carry on that function. i.e. the human appendix.) Specifically digits in the most of the larger theropod species going from five, to four, to three, and in the case of the Tyrannosauridae, two. (Actually, later Tyrannosaurs in the Cretaceous, such as Tyrannosaurus rex, Tarbosaurus bataar, Albertosaurus sarcophagus, Daspletosaurus, et cetera actually have three digits; one is merely small and vestigial, giving the appearance of two, though that's not necessarily incorrect either.)
What is a Bird?
Birds, or Aves, is a class of endotherms (animals that have to consume other organisms, in order to generate and maintain a constant body temperature, commonly referred to as "warm-blooded") that lay hard-shelled amniotic eggs (Thus, making them one of a myriad of other animals classified as "Amniotes.") that evolved during the mid to Late Jurassic period.
There are approximately 9000 species globally. The fact that they are the descendants of the Dinosauria is well-established among the majority of Paleontologists. However, they have evolved specializations for flight and a "unique 'one way' breathing system." The bones are light, yet strong. They have powerful flight muscles, and a skeleton in which many bones are fused or lost. And, the most defining feature--Feathers!
Now, granted, there are many bird species that are unable to fly, meaning that flight shouldn't be the defining characteristic of this clade to differentiate between birds and theropods. Early fossil birds, such as Archaeopteryx, lack the sternal keel where the flight muscles attach, that is also missing in the group of flight-less birds known as the ratites (the group that includes ostriches, emus, and rhea).
The Dinosaur-Bird Connection
Previously, I listed some of the characteristics of the two groups, but nothing truly defining of each group. This is the problem with the current taxonomic system (although, recent changes in the system have labeled Birds as a specialized sub-group of theropod dinosaurs).
Now, why the change? As you have been reading this post, I can only assume you have noticed that the line between the two clade has blurred significantly. The criteria for this change will be discussed in this section, in addition to the theory on Avian Origins, or, more popularly, known as the "Dino-Bird Theory."
Take the common domestic chicken (Gallus gallus domesticus), for example. The domestic chicken is hardly built for high-performance flight. Yet, the skeleton for Gallus gallus domesticus are clearly present. The sternum bears the aforementioned prominent sternal keel where the flight
The furcula, a fused clavicle (collar bone), serves as a brace during the flight stroke; in the chicken it's appearance is a Y-shaped bone in front of the sternum (breast bone). The same clavicle in non-avian Dromaeosaurids, which provided a brace for the shoulder girdle while holding prey.
In order to reduce their weight, birds have highly shortened tails compared to extinct taxa like Archaeopteryx, whose tail contained at least 15 vertebrae in it's tail. Living birds lack teeth in the bill unlike their Mesozoic relatives.
Finally, there's the "most obvious anatomical feature" of all birds- Feathers! Of course, recent exquisite specimens from China revealed that certain genera had feathers or primitive "proto-feathers." Feathers are simply highly modified scales which are important for several reasons:
- Soft Down Feathers trap still air close to the surface of the body thermally insulating the animal.
- Contour Feathers establish the smooth contours of the animal's body.
- "Flight Feathers" form the aerodynamic surfaces of the wings and tail.
Waggoner , Ben. "UCMP Web Lift To Taxa". University of California at Berkeley. 7/4/2010
"Bird". Wikipedia. 7/4/2010