Why Fur Coats?
My husband and I recently visited the Philip J. Currie Museum at Wembley in northwestern Alberta. The rationale for building this beautiful new facility was the Pachyrhinosaurus bonebed at nearby Pipestone Creek. This horned dinosaur is very interesting, known from bonebeds in Alberta and the north slope of Alaska. The museum did not display a model of the highly concentrated bone bed (up to 200 bones per square metre), nor a skeleton of Pachyrhinosaurus, nor a model of the creature (that I saw). They did however display different sizes of one kind of arm bone to illustrate that animals of all ages were overtaken by the same watery catastrophe.
What the museum did display was a number of carnivorous (theropod) dinosaurs in fur coasts. “Why fur?” you may ask. Well this is an interesting story, important to Philip Currie, for whom the museum is named. So, let us begin ….
If you are like me, you probably like to categorize things. Even youngsters love to arrange toys into cars, trucks, blocks, stuffed animals, dolls and so on. Scientists are no different. When faced with a whole bunch of artifacts, for example, they want to arrange them into categories so that they can begin to understand what they are seeing.
Take dinosaur fossil bones for example. It is obvious that not all dinosaurs are built the same way. Some are slim and trim (of various sizes), others are heavy with long necks, some have horns and huge ruffles and some, with beaky mouths seem moderately stream-lined. Obviously the first step is to group individuals with similar body plans together. As you well know from experience however, there are always a number of ways that one can designate basic categories in any collection. The dinosaur experts however have an additional objective. They want their categories to reflect lines of evolutionary descent.
Firstly, scientists have to assume that the dinosaurs did develop through an evolutionary process. They want to discover what the earliest specimens were like and where the process took them. In the past scientists made value judgments on these issues. Nowadays scientists rely on a mathematical process called cladistics. This is a technique for analysis of evolutionary relationships “that insists on recognizing natural groups only by newly evolved traits that their members uniquely share.” [members of the group share the same exclusive trait] (Nature March 23/17 p. 494) One problem with this approach however is that many characteristics are not exclusive to only one group. Many features are considered to have arisen several times by “convergence”, so they are not unique. So, does the new technique better demonstrate that evolution has occurred? Stay tuned.
In 1887 British Harry G. Seeley suggested that dinosaurs could be divided into lizard hipped specimens (with pubis bone pointing forward) and bird hipped dinosaurs in which the pubis bone points backward. The bird hipped dinosaurs had solid backbones (heavy) and the lizard hipped had backbones with hollow chambers (much lighter). Thus the dinosaurs were divided up into bird hipped plant eaters like duckbills, horned dinosaurs, and armoured dinosaurs all with solid vertebrae (backbones). The lizard hipped dinosaurs were divided up into the heavy long necked plant eating sauropods and the frisky meat-eating theropods like Allosaurus, Tyrannosaurus rex, and Albertosaurus. They all had chambered back bones. This seemed to work with both the old or new system of categorizing.
A German scientist Willi Hennig, in the 1960s, proposed that one can separate organisms into groups based on possession or lack thereof of any certain specialized condition. Then one looks at all those “haves” and separates them into “have” or “have not”, for a further specialized condition. For example, one could separate dinosaurs into “have”, or “have not”, for hollow backbones. Then one could look at the creatures with hollow backbones and separate them into heavy four footed creatures or bipedal streamlined creatures. One could separate those with solid backbones into those with four footed lifestyles or basically two footed specimens etc. These latter groups would then be separated into new categories.
At each branch (diversification), the condition of the common ancestor indicates that all descendants whatever their further specializations, at least share the characteristics of the common ancestor. Thus if all descendants have hollow back bones, for example, it means that the common ancestor also had hollow bones. Hennig believed that he had demonstrated that you do not need a lot of examples to figure out lines of descent. This technique (cladistics meaning branching) seems very mathematical in that computers are needed to calculate the most likely branching pattern (evolutionary tree). However one caveat on the procedure is that the scientists choose which features to include in the analysis and which ones to ignore.
OK so let’s try a little exercise. The bird hipped dinosaurs have solid backbones and are vegetarian. The lizard hipped dinosaurs have chambered backbones and some are vegetarian and some are meat eaters. The problem comes when scientists look for an ancestor of birds among the dinosaurs (a long-standing dream of scientists). Birds have bird hips and chambered backbones. They do not fit either of the above categories of dinosaur. Nevertheless specialists look for ancestors of birds among the lizard hips. But what about feathers and the wrong kind of hip? Some like Deinonychus and Velociraptor actually exhibit bird hips. So their backward facing pubis is something that does not fit the overall group of lizard hips. How did this bone with attached muscles get turned around?
Since the 1980s, when cladistics first became commonly used by many taxonomists, comparisons of bird and dinosaur skeletons convinced most of them that birds are an advanced form of “maniraptoran coelurosauran dinosaur.” Maniraptorans are defined as “hand grabbers” with generally long arms. These are a subcategory of “coelurosaur” theropods that include Allosaurus, Compsognathes, Ornitholestes, Tyrannosaurus and maniraptorans like Deinonychus and Troodon. Evidently the coleurosaurs include the bulk of the theropods.
In 1996 a small fossil skeleton was found in China. [Such a specimen, in a death pose, is displayed in the museum.] The scientists called it a bird because it had some kind of smudge around the outside. What was that smudge made of? Examination under the microscope indicated that these were hollow fibres. Were these “primitive feathers” or hairs (polar bears have hollow fur), or what? The Chinese specialists named the creature Sinosauropteryx and considered it to be a bird. Later Philip Currie from Alberta, declared that this was a compsognathid dinosaur with “protofeathers”. The term “protofeather” suggests that these structures were in the process of developing into true feathers. This is quite a leading assumption! These proto whatevers are “more primitive than any type of feather on modern birds – even the down of baby chicks – the fluff seems to represent an intermediate stage between true reptile scales and modern bird feathers!” (Thomas Holtz Jr. 2007 Dinosaurs. Random House Children’s Books. p. 114)
Nevertheless in the maniraptorans, some specimens like Caudipterix (oviraptoran) and Sinornithosaurus (like Deinonychus) have been found with true feathers on arms, tails and maybe legs. Therefore “because compsognathids and maniraptorans have some sort of feather structure, scientists recognized that their common ancestor would have had protofeathers too. So if the common ancestor of birds and compognathids had protofeathers, then the tyrant dinosaurs were descendants of the same fuzzy common ancestor.” (Holtz p. 119) The whole objective of the exercise is to include birds within the dinosaur category.
And so, based on cladistics, the meat-eating theropod dinosaurs in the Wembley museum are dressed up in most improbable looking fur coats. If the common ancestor had feathers/fur, then they should have too. But is this conclusion reasonable? Philip Currie and Eva Koppelhus wrote: “Skin impressions have been found associated with at least one specimen of Gorgosaurus [formerly Albertosaurus] libratus and one specimen of Daspletosaurus torosus. The skin is similar in texture to that of the hadrosaurids [duck bill dinosaurs], although the individual tubercles are on average smaller and shallower. Patches of skin impressions recovered so far from these and other tyrannosaurs come from the flanks, hind legs and feet of mature individuals.” (Dinosaur Provincial Park. 2005. Indiana University Press. p. 385) Although they did not observe any feathers, but only scales, Currie and Koppelhus try not to rule out protofeathers altogether. They suggest that these might have occurred on other parts of the body or on young individuals.
So are the fur coats on the theropod dinosaurs justified? One has to accept certain arguments to think so. Firstly one must assume that the dinosaurs are all related through lines of descent. Secondly one must agree that this system of cladistics adequately depicts those lines of descent. But a recent article in Nature proposed an alternative quite different separation of the dinosaurs into groups. (Matthew Baron et al. 2017 March 23 pp. 501-506.) A strenuous rebuttal, by a long list of experts, was published on-line at Nature on Nov. 2/17. Apparently both computer generated trees are equally reasonable from a statistical point of view. The point is that it depends upon the choice of characteristics included in the analysis what the resulting lines of descent are like.
The characteristics of the common ancestors (if any) could all be different depending on the branching pattern chosen and thus the conclusions as to what features these creatures would have shared could be different too. Therefore, it may not be too many years before the fur coats come off these dinosaur models! In the meantime, always ask on what basis were any scientific conclusions made. Arguments may initially appear impressive, but on further analysis, they may appear dubious indeed.
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