Smithsonian.com

The reconstructed skeleton of "Brontosaurus" from W.D. Matthew's 1915 book Dinosaurs.

Blog long enough, and it will happen eventually—someone else will get to that fascinating topic you had planned to write about before you do. I had intended to write about the rejected idea that sauropod dinosaurs gave live birth—a hypothesis popularized by paleontologist Robert Bakker in his 1986 book The Dinosaur Heresies—but zooblogger par excellence Darren Naish just wrote an in-depth summary of the idea and why it’s wrong.

In Bakker’s view, sauropods must have given birth to live young because the babies would have been too big to have been laid as eggs. The large size of the passageway in sauropod hips seemed in accord with the hypothesis. Without confirmed sauropod eggs, nests, or babies to test the idea, the notion was within the realm of possibility. But, as Darren points out, the subsequent discovery of scores of sauropod eggs and nests have shown that Bakker was wrong. Diplodocus and its allies started off as runts hatched from eggs in nests that also harbored about 10 siblings or so, and they quickly closed the size gap through rapid growth.

Bakker wasn’t the first paleontologist to come up with the idea that sauropods gave birth to live young. Decades earlier, when sauropods were cast as swamp-dwelling sluggards, William Diller Matthew proposed a similar notion for reasons exactly the opposite of Bakker’s. As also covered by Darren, Bakker posited his version of the idea under the theoretical architecture of sauropods as active, “hot-blooded” land dwellers, whereas Matthew saw live birth as a possible adaptation to a life spent wading through water.

Matthew tucked a brief summary of his idea into a footnote to the 1915 guidebook Dinosaurs: With Special Reference to the American Museum Collections. After casting sauropods as “spending their lives entirely in shallow water, partly immersed” and “unable to emerge entirely upon dry land,” Matthew included a note of dissent from the marine reptile expert Samuel Wendell Williston saying “I cannot agree with this view—the animals must have laid their eggs upon land—for the reason that reptile eggs cannot hatch in water.”

But Matthew disagreed. “[W]ith deference to Williston’s high authority,” Matthew replied, “I may note that there is no evidence that the Sauropoda were egg-laying reptiles. They, or some of them, may have been viviparous like the Ichthyosaurus.” What Matthew was referring to here were exquisitely preserved specimens of ichthyosaurs found in Germany preserved with near-term embryos peeking out of their mothers’ bodies. These fossils have sometimes been cast as mother ichthyosaurs that died in the act of childbirth, but it is more likely that the developing babies were pushed out of their mother’s bodies as gases built up during the process of decomposition. Either way, their presence confirmed that at least some ancient aquatic reptiles had independently evolved the ability to give live birth in the water, and with no known sauropod eggs, it was reasonable to suggest that sauropods might have evolved a similar reproductive technique.

Matthew and Bakker were both wrong about sauropod reproduction, but for very different reasons. Their views on sauropods could scarcely have been more different. That is what I find fascinating—how a simple hypothesis could so readily be accommodated into two very distinct theoretical perspectives of dinosaur lives. I wonder what other instances there might be of two paleontologists casting the same idea in very different ways.