Dinosaur Snowball Fights: Recently recovered dinosaur fossils from high paleolatitudes rekindle debates of dinosaur thermophysiology, migration, and extinction.
David R. Pettersson
Department of Geology and Geography
Denison University, Granville, Ohio 43023
Fossilized dinosaur skeletal remains have recently been recovered from locations corresponding to high paleolatitudes. These sites include northern Alaska, northwest Yukon, the Northwest Territories, Spitzbergen, Siberia, southeastern Australia, Antarctica, New Zealand, and Patagonia. The paleogeographically isolated nature of southeastern Australia, ~106 million years ago (Ma), in combination with an analysis of recovered dinosaur fossils and associated biota of similar age from southern Victoria, Australia, present several novel insights which include: i) southeast Australia served as an ecological refuge during the Cretaceous, allowing some amphibious labyrinthodont and allosaurid dinosaur species to survive for millions of years after they had disappeared elsewhere; ii) Hypsilophodontids, typically having large eyes and cranial capacities, were preadapted for survival in the winters of high paleolatitudes and associated low temperatures and months of perpetual darkness; iii) as a consequence of their advantageous predisposition, the hypsilophodontids radiated into several endemic species, filling the presumably vacant niche of Cretaceous polar winters. The pan-paleopolar distribution of dinosaurs suggests that a global decline of solar insolation and temperature were not the only factors responsible for the Cretaceous-Tertiary (K/T) extinction event. Lastly, these recent discoveries add momentum to the shifting public perception of dinosaurs, away from being sluggish, dim-witted, ectothermic behemoths, set into motion by the work of Bakker and Ostrum ca. 1970.
Hot-Blooded, Intelligent Dinosaurs and the Pendulum of Scientific Thought
Prior to 1970, it was generally accepted among paleontologists that dinosaurs were sluggish, cold-blooded reptiles having low intelligence. This perception of dinosaurs quickly changed in the early 1970's, when paleontologists John Ostrom and Bob Bakker presented much compelling evidence supporting the case for warm-blooded, active, and intelligent dinosaurs. Since this shift in understanding of dinosaur behavior, intelligence, and thermophysiology, much additional evidence has been presented that supports the belief that dinosaurs were active, intelligent, and warm-blooded reptiles. For example, paleontologist Jack Horner argues that Maiasaur practiced more "advanced" parenting behaviors, characteristic of modern reptiles and birds. He claims that Maiasaur parents tended to their egg clutches and would subsequently care for newborns by watching over their nest and providing nutrition through the regurgitation of food into the newborns' mouths. Horner provides a few points of evidence for his claims: (1) Maiasaur nests have been found covered with fossilized vegetation (like modern crocodiles, he proposes, decomposing vegetation provided incubation heat for egg clutches). (2) Cartilage-deficient Maiasaur bone articulations have been reported (which, he suggests, indicates that hatchlings were initially immobile and would require food to be delivered by parents). (3) Partially-digested berries proximal to Maiasaur nesting sites have been reported (suggesting parental regurgitation of food for hatchlings). (4) Coprolites containing digested berries have been reported (again, suggesting parental provision of food for hatchlings). (5) Reconstructed (fleshed-out) hatchlings that are "cute" (i.e., short snout, big eyes and head), in appearance have been reported (which fits Horner?s hypothesis that newborn/hatchling "cuteness" is correlative with, and conducive to parental care).
Digressing momentarily, there seems to be a pattern in the history of science whereby a revolutionary ideology will spawn a great deal of new research that attempts to incorporate that ideology. This pattern is analogous to the swinging of a pendulum: a new idea initiates research and before long, scientists are attempting to incorporate that new idea wherever they possibly can, simply because of its popularity. The scientific community eventually recognizes the problematic pattern, and the pendulum slows and popular ideology begins to swing in the other direction, perhaps toward an older ideology.
Take as an example the situation in the field of art history during the 1940's. German-American art historian, Aby Warburg, had developed the idea of iconology by observing that some renaissance works had incorporated in them intentionally-hidden symbols which gave new meaning to the works. The art history community was turned upside-down by Warburg's observations and before long, art historians were inappropriately applying Warburg's iconology to pieces from all periods and artists. The community eventually realized the mistake and subsequently a great deal of research was discounted. Similar patterns can be observed in the field of geology. For instance, soon after the introduction of the idea of Milankovitch cycles, it was quite popular among stratigraphers to correlate sedimentation sequences with these periodic orbital cycles. Recently, however, it has become less popular to make such correlations as the idea has gone somewhat, "out of fashion."
Upon reviewing Jack Horner's proposal of a behaviorally-advanced Maiasaur, I get the feeling that the search for evidence that fulfills the idea of intelligent and evolutionarily-advanced dinosaurs has been unnecessarily over-extended. First, I must concede that the construction of vegetation-covered nests implies Maiasaur possessed some degree of intelligence. However, this evidence does not support post-hatching care in the slightest. On the contrary, it seems more reasonable that the vegetation-covered nests provided sufficient heat (from the decomposition process) and shelter that allowed the eggs to be lain and left to develop without further parental involvement.
Addressing the cartilage-deficient bone evidence and coprolite specimens, I cannot rightfully dispute Horner's observations. However, in drawing inferences from these observations, Horner makes several unsubstantiated assumptions. For example, it is quite possible that the cartilage in Horner's Maiasaur hatchling fossils deteriorated before the specimens were preserved. Similarly, the connection between a coprolite or a fossilized mass of partially-digested berry remains and a particular species, let alone a particular specimen, is loaded with assumptions.
I don't really know where to stand with regard to Horner's "cuteness" hypothesis. I recognize the importance of remaining open to new ideas. However, I would have to see a sufficient number of modern instances of juvenile "cuteness" at work before I would begin to consider baby Maiasaur "cuteness" as a point of evidence for parental care. Okay, to be honest?I think it is preposterous to correlate such a subjective characteristic as "cuteness" with a definite behavior trait, such as parental care. Where does one begin in deciding how "cuteness" is defined by any given animal species (let alone how humans define human "cuteness").
I do not wish to imply that I don't accept the idea of parental care among dinosaurs. Indeed, there is a mounting pile of evidence for such behavior. Of note is the seemingly intended arrangements of dinosaur egg nests at specific distances from each other, as preserved in the Gobi desert. Most convincing are the fossil assemblages of nests and nearby adults of the same species, in Montana, which could easily be reconstructed as parents watching over their clutches.
However, these and Horner's observations are circumstantial evidence only. There remains no indisputable evidence for parental care. In conclusion, I wish to put forth a warning to those studying dinosaurs. At the time being, the image of warm-blooded, active, and intelligent dinosaurs is quite en vogue within the scientific community. Researchers need to remain objective in the midst of the relatively recent upheaval of ideas regarding dinosaurs and the hot-blooded hype and hoopla which have swept through this field of study. Realize these are hypotheses only--definitive evidence remains absent. And, consider the patterns in the history of science discussed above--it is quite possible that the pendulum of popular ideas has swung beyond the realm of objective science.