Leonard Finkelman writes...
The child gazed upward, slack jawed, half aware that his legs weren’t working. Even if his legs did work, he couldn’t be sure whether they would carry him away from the terrible subject of his gaze or, counter-intuitively, towards it. All things considered, the traffic jam of information between the child’s brain and his muscles was probably for the best. He was forced to indulge the rare opportunity to gawk. The world faded, Cheshire cat-like, leaving only a smile made of dinosaur fossils.
I was the child in this scene during my first visit to the American Museum of Natural History in 1984, but the story is not uniquely mine. October 2015 marked the centennial anniversary of the American Museum of Natural History's public display of AMNH 5027, better known as the museum's Tyrannosaurus rex specimen. For over 100 years now, the scene described above (or one very much like it) has played itself out on an almost-daily basis as new dinosaur enthusiasts have come face-to-skull with one of palaeontology's biggest stars.
When my colleagues and I made plans to launch Extinct, there was never any question in my mind about what my first contribution would be. Of course I'd be writing about T. rex, and AMNH 5027 in particular. Doing so will give me the opportunity to explore one of the hot topics in philosophy of biology--evolutionary novelty--but I'll admit to an ulterior motive. I want to write an ode to my first true love.
AMNH 5027 was first discovered in 1908 by Barnum Brown near McCone County, Montana. Rocks from the Hell Creek Formation yielded 48% of a complete skeleton, missing the four limbs and about half of the tail. When the American Museum's curators developed plans to mount their T. rex display, they decided to combine the AMNH 5027 material with fossils from a specimen described in 1905--now designated CM 9380--to create a composite skeleton.  Even after combining material from both specimens, however, the composite skeleton was still only half complete. In order to complete the skeleton for display, the curators filled in the skeleton's gaps by scaling up homologous bones from Allosaurus fragilis, which was then the only other well-known predatory dinosaur of comparable size. 
Ninety-nine years later, the woman who is now my fiancee would, in the course of our first date, ask me what I would do on the day before the end of all existence. I told her that I would go to the American Museum, throw all caution to the wind, and finally give a hug to AMNH 5027. She then asked a question that I've been asked many, many times before: why the love for T. rex?
Such an unintentionally philosophical question! What, after all, is so unique about T. rex that I would favor that species of dinosaur over other species of dinosaurs? What are the differentiae that distinguish between my beloved dinosaur and less-beloved dinosaurs? I offered a smile and a shrug in response. After I spent years writing a dissertation about the species problem, a longer answer would have seemed disingenuous.
Philosophy aside, common perception does recognize one distinguishing feature of T. rex: its stubby two-fingered forelimbs. As my then-date-now-fiancee noted: it's ironic that the thing I so want to hug is also the thing that would be least capable of reciprocating the gesture.
An even greater irony is that AMNH 5027, the world's first and likely most famous T. rex specimen, was not originally restored with those forelimbs. The American Museum's initial point of reference for completing the composite AMNH 5027 skeleton, A. fragilis, has three fingers on each forelimb, and so AMNH 5027 was introduced to the public with a total of six fingers. The museum amputated one finger from each hand in 1927, after it became clear that A. fragilis was not a particularly close relative of T. rex and that closer relatives (such as Albertosaurus sarcophagus) had two-fingered forelimbs. So those stubby little arms can't be the defining features of T. rex because the first restoration of T. rex didn't have them! 
Nevertheless, the popular imagination's image of T. rex is dominated by those arms. One of the most famous and influential papers in philosophy of biology--"The Spandrels of San Marco and the Panglossian Paradigm" by Stephen J. Gould and Richard C. Lewontin--even uses T. rex arms as a punchline. Without falling too far into my speculative psychology armchair, I would imagine this is because the arms are just so weird: we don't know how they evolved or what function, if any, they could have served. T. rex arms are, in a word, novel.
Novelty has become a central and controversial concept in evolutionary theory over the past several decades. As all undergraduate students of biology learn, natural selection--the only proven cause of evolution--can only work on variations that are already present in a living population. Darwin himself suggested that "descent with modification" would be a more accurate and descriptive term than "evolution." How, then, can novel features evolve? Arthur (2000) has gone so far as to suggest that the true origin of species is really the origin of evolutionary novelties.
It might seem at first glance that I'm guilty of equivocation: the word "novel" means one thing in the sentence, "T. rex arms are novel," and something else in the sentence, "novelty is a controversial concept in evolutionary theory." In the first sentence, "novel" means weird. In the second sentence, "novelty" can mean a number of different things. Indeed, Pigliucci (2008) and Brigandt & Love (2012) distinguish several different uses of the term "novelty" in the philosophy of biology literature, including developmental, functional, and morphological senses of the word (among others). Following John Wilkins, however, I think that philosophers of biology generally do have something like the weirdness attributable to T. rex arms in mind when they talk about all these various kinds of novelty. To see why, let's consider each kind of evolutionary novelty in turn, and how the arms of T. rex might be considered novel in each context.
A morphological novelty is a structure found in a species that is not present in any modified form in ancestral species. Allosaurus fragilis is not an ancestor of Tyrannosaurus rex, but the story of AMNH 5027's arms shows how this kind of novelty works in principle. Given knowledge of the ancestral species, one wouldn't have enough information to predict a morphological novelty in the descendant species, just as the American Museum's curators couldn't predict the structure of T. rex arms from knowledge of A. fragilis arms. Put in another, even less technical way: a morphological novelty is a structure that an observer of evolution wouldn't expect before it appears.
A functional novelty is a structure found in a species that allows organisms in the species to perform new functions that couldn't be performed by organisms in ancestral species. Krauss & Robinson (2013) argue that T. rex arms might have been a functional novelty that allowed T. rex to hunt prey using a strategy (purportedly) favored by modern hooligans: cow-tipping. As it turns out, the arms of a fully-grown T. rex would have been about as high off the ground as the back of a fully-grown Triceratops horridus, and those arms would have been strong enough to absorb the force of a T. rex running into a T. horridus at full speed. The probable ancestor of T. rex, Daspletosaurus torosus, didn't have arms capable of Triceratops tipping, and so the evolution of T. rex arms gave the species a new means of hunting. An observer of D. torosus wouldn't expect that its descendants would behave like bored rural human teenagers. 
A developmental novelty is a structure found in a species that is a new result of the modification of developmental processes in ancestral species. In recent years, the novelty of T. rex arms has been most commonly cast in this light. Lockley et al (2008), for example, argue that T. rex arms evolved as a result of (probably genetic) linkage: among the relatives of T. rex, there seems to be a strong correlation between larger head size and smaller arm size, and there are clear uses for a large head full of big teeth in an environment full of oversized prey. Brigandt & Love (2010) argue that the field of evolutionary developmental biology has provided significant insights of this kind into the evolution of novelties: since the physical expression of information encoded in some genes influences the expression of information encoded in other genes, changes in the development of one structure can have unexpected results in the development of other structures. One wouldn't necessarily expect the development of a T. rex head to have an effect on the size of T. rex arms, but if the two features are genetically linked in some way (for example, if the relevant genes are on the same chromosome), that might nevertheless be the case.
At the risk of oversimplifying, it's significant that weirdness, which I take to be the defeat of reasonable expectations, is a factor in all these descriptions of evolutionary novelty. Given a particular context--morphological, functional, or developmental--a learned observer would infer certain conclusions given available information. One would expect that the ancestors of organisms with three fingers on each hand will have three fingers on each hand, or that the ancestors of organisms that engage in typical chase-and-bite predatory behavior won't employ cow-tipping strategies, or that growing a bigger head doesn't have to shrink the size of an organism's arms. The emergence of novelties renders these reasonable inferences invalid, just as the observation of something truly weird defies one's expectations. 
And if one thing can be said of AMNH 5027 after one hundred years, it's that the dang thing defies all expectations. There is nothing in our modern heaven or earth to prepare someone for that first encounter with the American Museum's tyrant king. And maybe that, in the end, explains my love for that T. rex. Its weird presence shows that the world is a surprising and more interesting place.  If that isn't worth embracing, then what is?
 Scott Hartman notes some important differences between AMNH 5027 and CM 9380. As he says, these differences might be the result of individual variation within the species T. rex. Two other possibilities are perhaps more interesting. The first alternative is that the two specimens may actually be representatives of different species. Some commentators have therefore dubbed AMNH 5027 Tyrannosaurus "x." The second alternative is that the two specimens may represent different sexes. There is evidence suggesting sexual dimorphism in T. rex, and some of the differences attributed to dimorphism can be observed between AMNH 5027 and CM 9380. See P. Larson 2008.
 I'll note an ambiguity here: the name "AMNH 5027" is used less technically to designate the full composite skeleton on display in the American Museum, but it more specifically refers to the particular fossil material found in 1908. Unless noted otherwise, I use the name in the less technical sense.
 As it turns out, the American Museum was originally right, if only accidentally. The forelimbs of T. rex remained speculative until 1988, when amateur fossil collector Cathy Wankel found a T. rex skeleton with its associated left forelimb. Published descriptions of the forelimb note the presence of a vestigial third finger that would have been covered by flesh. So T. rex really did have three fingers! See Lipkin & Carpenter 2008.
 Krauss & Robinson themselves admit that the evidence of T. rex's proclivity for Triceratops tipping is purely circumstantial, but I think we can all agree that the very possibility of this behavior is nothing less than delightful from the perspective of everyone other than the Triceratops.
 I'll admit that T. rex arms might not technically qualify as evolutionary novelties by any of these definitions. We would, after all, infer from the ancestors of T. rex that T. rex ought to have relatively small arms with some number of fingers, and that it would use those arms for some kind of predatory behavior, and that the size of those arms would be influenced by development from genetic mechanisms. But these inferences are all quite vague, and none of them predict the specific observations given. In any event, these examples are offered more in the spirit of illustration than in an attempt of proof.
 I must note that all of these are also true of my fiancee, who holds one significant edge over AMNH 5027: she can and does hug back.
Arthur, Wallace. (2000). Intraspecific Variation in Developmental Characters: The Origin of Evolutionary Novelties. American Zoologist 40(5), 811-818.
Brigandt, I., & Love, A. C. (2010). Evolutionary novelty and the evo-devo synthesis: field notes. Evolutionary Biology, 37(2-3), 93-99.
Brigandt, I., & Love, A. C. (2012). Conceptualizing evolutionary novelty: Moving beyond definitional debates. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 318(6), 417-427.
Krauss, D. A., & Robinson, J. M. (2013). The Biomechanics of a Plausible Hunting Strategy for Tyrannosaurus rex. In Parrish, J.M., Molnar, R.E., Currie, P.J., and Koppelus, E.A. (eds.), Tyrannosaurid Paleobiology, 251-264.
Larson, N.L. (2008) One Hundred Years of Tyrannosaurus rex: The Skeletons. In Larson, P. and Carpenter, K. (eds.), Tyrannosaurus rex, The Tyrant King, 1-56.
Larson, P. (2008) Variation and Sexual Dimorphism in Tyrannosaurus rex. In Larson, P. and Carpenter, K. (eds.), Tyrannosaurus rex, The Tyrant King, 103-128.
Lipkin, C. and Carpenter, K. (2008) Looking Again at the Forelimb of Tyrannosaurus rex. In Larson, P. and Carpenter, K. (eds.), Tyrannosaurus rex, The Tyrant King, 167-192.
Lockley, M., Kuhihara, R., & Mitchell, L. (2008). Why Tyrannosaurus rex had puny arms: an integral morphodynamic solution to a simple puzzle in theropod paleobiology. In Larson, P. and Carpenter, K. (eds.), Tyrannosaurus rex, The Tyrant King, 130-164.
Pigliucci, M. (2008). What, if anything, is an evolutionary novelty? Philosophy of Science, 75(5), 887-898.