This and this is really interesting.
Stegosaurs roamed the earth from the Middle Jurassic to the Late Cretaceous. They are related to ankylosaurs and form a larger grouping (the Thyreophora) with them in the order Ornithischia (named after the shape of the ischium). Stegosaurs are one of the more popular dinosaurs and have been used in movied such as “Fantasia” and “Jurassic Park II:The Lost World” One of the reasons, perhaps the only reason, for their popularity is the series of plates running down their back and ending in four massive spikes on thier tail (used to incredibly dramatic effect in Fantasia). Paleontologists have always been somewhat puzzled as to what these plates were for. There are four theories to explain these plates.
A study to be published in the upcoming issue of the journal Paleobiology examines these exlanations and concludes that number four is probably correct. The study is authored by Paidan, Main, Horner and de Ricqles.
They examined the histology of the plates and scutes of stegosaurs and their relatives (the Ankylosaurs and Scutellosaurus among others).
They ruled out the use of the plates as a defense mechanism because they are very thin and only lie along two rows on the back – which doesn’t provide that much protection (especially when the amount of energy required to grow them). They also ruled out the role of sexual selection because female stegosaurs had the plates too – so they couldn’t be the result of male-male competition or female choice.
This leaves thermoregulation and species identification as possible explanations. Stegosaur plates have large blood vessals leading into them, and, as mentioned above, are extremely thin. This has lead to the idea that they are involved with heat exchange with the environment (as are, for example, the ears of elephants and the flippers and tails of seals). From the press release:
As for heat exchange, one major reason earlier scientists proposed such a function for stegosaur plates is that these plates have large blood vessels piercing their interior, perhaps channels to carry blood to be cooled or heated. But it turns out that these “pipes” lead to dead ends, so their roles as major blood vessels are difficult to establish.
To probe the possibility that the plates and spikes were heat exchangers, the paleontologists looked at the evolution of these skin growths in the thyreophoran family, which included the stegosaurs. The team obtained fossils from a half-dozen different species of thyreophorans, ranging from the stegosaurs’ earliest ancestors – “armored” dinosaurs that lived 200 million years ago – to the first stegosaurs and related ankylosaurs – which had bony plates or scutes all over their bodies – to the last stegosaurs, which died out in the Early Cretaceous period more than 120 million years ago. All were plant eaters with formidable flat or erect plates on the neck, back and tail. The team sliced through about 10 fossil scutes to study their internal structure.
Added a few days later: Here is an example of one of the thin sections.
The earliest thyreophorans, such as the North American dinosaur Scutellosaurus, which measured about four feet from nose to tail, had small bony plates lying flat over their backs and tails, each with a slightly raised keel. These scutes, about a half-inch across, had an internal structure similar in some aspects to the much larger plates of the stegosaurs, yet were obviously useless in regulating the internal temperature of the animal, Main said. The same is true of the later Scelidosaurus, a 13-footer covered with larger scutes with bigger keels; the scutes had the same type of blood vasculature as stegosaur plates and spikes. Ankylosaurs, a sister group to the stegosaurs that survived into the late Cretaceous and went extinct with the rest of the dinosaurs 65 million years ago, had more diverse scutes and ossicles that nevertheless were plumbed in the same way as those on stegosaurs.
Based on this analysis, the team argued that it was unlikely that the larger plates that evolved in the stegosaur ancestors of Scutellosaurus and Scelidosaurus were used for heat exchange.
Below is a picture of the skeleton of a species related to stegosaurus – called Huayangosaurus. It was found in China and is from the Middle Jurassic. Note that it has small plates intermixed with spines. Paidan et al are argueing that becuase Huayangosaurus and Stegosaurus were related their plates and spikes would have come from a common ancestor and would serve roughly the same purpose. If that purpose was thermoregulation they would be a lot similar than what they are.
Padian and Main point out, too, that the horns or antlers of many living animals contain large vessels to supply blood needed for fast growth. None of these horns or antlers function as heat exchangers. A possible role of the large “pipes” in the scutes of stegosaurs and their ancestors was to carry the large blood supply needed for the fast growth that was thought to be typical of dinosaurs.
In addition, not all stegosaurs living at the end of the Jurassic had the big, flat plates of Stegosaurus stenops that most people associate with stegosaurs. Kentrosaurus of Africa and the Asian Huayangosaurus, which were about the same size as Stegosaurus, had mostly spikes with a few “dinky” plates, Main said. These spikes and small plates would have been useless for heat exchange.
“You get quite a large variety in the types of osteoderm arrangements in these animals, but they are not specialized in the way that one would expect if they were built specifically for a thermoregulatory function,” he said. “What it looks like is the scutes simply show hypertrophic growth of the keel region, it’s simply a modification of an already existing growth pattern.”
This leaves us with species identification as the explanation:
“There is a natural tendency that leads to elaborate displays for social group recognition, like the calls of birds,” Padian said. “This underscores the importance of behavior to evolution.”
And from National Geographic News:
“The skeletons of these dinosaurs [stegosaurs] below the neck are identical,” Horner said. He points to deer as a modern-day analog: While the skeletons of mule deer and white-tailed deer look very similar, the animals themselves have different colors, and their ears and tails are also different.
Horner says these deer attributes serve a similar function to that of the elaborate frills, crests, and back spikes found in dinosaurs. They allow mule and white-tailed deer to identify members of their own species.
Referring to the dinosaurs, Horner noted that “all of these big features on dinosaurs are very expensive” in terms of the energy required to grow them. He added that such features must therefore be extremely important.
At this point, three out of four hypothesis’ concerning the function of stegosaur plates have been eliminated. Some of the evidence suggests that the fourth (species recognition) may be correct. A note of caution, however:
De Ricqlès cautioned, however, that “an accessory role in thermoregulation cannot be ruled out for the Stegosaurus plates. Being so large, well vascularized (and available) they may have been inevitably exapted for such a function. This is so even if the primary explanation of their occurrence in an evolutionary context may be elsewhere: namely in some sort of ‘display’ (mate or species recognition), as suggested by the comparative, phylogenetic, context of plates development among Stegosauria.”
So how do we proceed? Let’s ask for a few outside opinions. What’s your advice William Dembski?:
But is the problem ignorance of the material causes needed to bring about biological complexity or an inherent inability of such causes to do so?
So, we should just infer intelligent design and call it a day.
Richard Dawkins what is your advice?
Science mines ignorance.
Mystery – that which we don’t yet know; that
which we don’t yet understand – is the mother lode
that scientists seek out. Mystics exult in mystery
and want it to stay mysterious. Scientists exult in
mystery for a very different reason: it gives them
something to do. Maybe we don’t understand yet,
but we’re working on it! Each mystery solved
opens up vistas of unsolved problems, and the scientist
eagerly moves in.
So we have two different opinions. One says stop, enjoy blissful ignorance and say God did it. The other says don’t despair, accept the unknown as a challenge and go forth to meet it.
To investigate further whether the elaborate horny displays of stegosaurs and other dinosaurs are involved in sexual displays, Padian is going to South Africa in May and June to measure skulls and bodies of African antelopes to look at the range of sexual dimorphism. Such studies have never been done on a full range of African bovids, he noted. Meanwhile, Main at Harvard is studying bone growth and skeletal mechanics in animals such as goats and emus to see how they change with age.
“We know more about growth in some dinosaurs than we do about growth in large living mammals,” Padian said.
Looks like Dawkins wins! A wonderful example of how science operates!
Filed under: Intelligent Design, Paleontology | Tagged: Stegosaurs | 3 Comments »