From Punting to Tromping

I grew up with submerged sauropods. In the outdated library books I constantly stuffed my elementary school backpack with, the likes of “Brontosaurus” and other gigantic dinosaurs were almost always lazing about in warm Jurassic swamps. Such bulky animals were too large to have walked on land without crushing their own limbs like jelly, the books said, and sauropods even had their noses on top of their heads to facilitate their subaqueous lifestyle. Zdeněk Burian’s waterlogged Brachiosauruswaterlogged Brachiosaurus looked perfectly at home.

And there was more to the argument than incredulity. A curious trackway found the Early Cretaceous rock of Texas recorded the steps of an enormous dinosaur that had apparently dragged its buoyed bulk along an ancient waterway.

A crew working with legendary fossil hunter Roland T. Bird found the traces while scouting sauropod tracks to lay down behind the American Museum of Natural History’s iconic “Brontosaurus” skeleton. Known to the locals long before Bird’s arrival, the numerous sauropod tracks in the vicinity of the Paluxy River were geologically too young to have been made by the New York City museum’s Jurassic sauropod, but prehistoric potholes were nevertheless about the right size and shape for such a sauropod. Among them were a sequence of horseshoe-shaped forefoot prints that surely belonged to a big sauropod, but mysteriously lacked any trace of the hindfeet.

[R.T. Bird’s crew uncovers a different set of sauropod tracks near Glen Rose, Texas.]

As he and his crew uncovered the tracks during the 1940 field season, Bird wrote in his journal that “I still had hanging over my head a mind-boggling load: forty tons of dinosaur stumping about back at the Mayan Ranch, tromping down the mud on his great feet, and what did he do with the balance of his great body? And how did he balance it? And why?” Sauropods doing handstands required impossible gymnastics.

An isolated impression of the dinosaur’s rear foot seemed to resolve the quandary.  As Bird’s team worked back along the trackway they discovered a partial hindfoot impression just before the impressions of the forefeet changed direction. From the evidence, Bird imagined that:

The big fellow had been peacefully dog-paddling along, with his great body afloat, kicking himself forward by walking on the bottom here in the shallows with his front feet. Deciding to turn back to deep water or onto land, he had kicked his body into an S turn with one push of a hind foot!

Sauropods were often seen more as waders than swimmers, but, at the time, the trackway made sense. The long-necked giants were thought to be far too large to cope with life on land, and the water was the herbivores’ only refuge from the enormous, tearing theropods that had evolved to feed upon them. The trackway evidence was so compelling that even as I gained access to newer books that usually, and rightly, showed Apatosaurus and kin on land rather than in the water, I was still captivated by the image of a bobbing “brontosaur” artist John Sibbick had restored for a 1993 Natural History Museum guide to dinosaurs.

The trouble with trackways is that they’re not always what they seem. Even though tracks are reflections of prehistoric behaviors, the state of the sediment also affects how that behavior becomes preserved. In the case of Bird’s swimming sauropod, an updated understanding of anatomy and sedimentology offer a more prosaic scene of a sauropod tromping about on land.

Bird’s mission was to find tracks for the AMNH’s “Brontosaurus.” This dinosaur, properly known as Apatosaurus, lived roughly 150 million years ago. Whatever sauropod made “swim” tracks, however, lived much later. The Texas site is only about 113 million years old, recording a time after a major changeover in prehistoric North America’s sauropods. Apatosaurus and many of its enormous neighbors were diplodocids, but by the time of the Paluxy tracks this style of sauropod had disappeared and given way to another sort called titanosaurs.

It can be hard to tell just by looking at them, but diplodocids and titanosaurs differed in two ways that helped sink Bird’s swim track story. Whereas Apatosaurus and other diplodocids had a center a mass centered towards their hindlimbs, titanosaurs had a center of mass closer to their middle. Since weight has quite a bit to do with how footprints are preserved, this means that diplodocids were more likely to leave deeper rear footprints and titanosaurs were more likely to leave deeper front footprints.

Then there’s the pattern of the tracks themselves. Tracks and biomechanical studies have shown that diplodocids made “narrow gauge” tracks with their foot impressions arranged relatively close to the midline of the body. Titanosaurs, by contrast, made comparatively “wide gauge” trackways, and the Paluxy traces match the titanosaur pattern.

[Jurassic Park played it safe by showing both standing and wading Brachiosaurus.]

Anatomy and the dinosaur timeline match a titanosaur – perhaps a dinosaur similar to Sauroposeidon – but there’s one more piece to all this. Even with the front-wheel drive locomotion style of a titanosaur, it’s puzzling why Bird’s trackway only has one edge of a hindfoot impression. The explanation is that the tracks don’t represent the perfectly-preserved impressions the dinosaur left right at the surface, but are undertracks.

When that sauropod walked across the Cretaceous ground, the weight of the animal pushed impressions into the damp layers of sediment beneath the surface. Since the sauropod in question was a titanosaur, this meant that traces of the front feet were better preserved than the rear feet, left only as a partial footprint and an additional disturbance that can just barely be seen. The sauropod wasn’t punting along a lakebottom, but simply walking unhurried.

Other “manus-only” sauropod tracks have been found at various  sites around the world, from Korea to Spain. Similar combinations of anatomy and sediment created these structures. There are swim tracks for other dinosaurs – especially theropods, not so aquaphobic as traditionally thought – but no solid cases for sauropods.

Did dinosaurs such as Diplodocus and Sauroposeidon ever take dips in Mesozoic waterways? Probably. There’s no reason to think that they assiduously avoided water at all costs. But, contrary to what I was taught so long ago, no one has uncovered the fossilized moments of sauropods frolicking in Jurassic swamps or Cretaceous lakes. For a time sauropods were thought to be so dependent on the water that paleontologists asked when, if ever, the giants came onto land. Now we’re left with the contrary question, wondering how such titanic animals could have ever swum.

References:

Bird, R. 1985. Bones for Barnum Brown. Fort Worth: Texas Christian University Press. pp. 160-161

Falkingham, P., Bates, K., Margetts, L., Manning, P. 2011. Simulating sauropod manus-only trackway formation using finite-element analysis. Biology Letters. 7,1: 142-145

Farlow, J. 1992. Sauropod tracks and trackmakers: Integrating the ichnological and skeletal records. Zubia. 10: 89-138

Gardom, T., and Milner, A. 1993. The Book of Dinosaurs: The Natural History Museum Guide. Rocklin: Prima Publishing. pp. 20-21

Henderson, D. 2006. Burly gaits: Centers of mass, stability, and the trackways of sauropod dinosaurs. Journal of Vertebrate Paleontology. 26, 4: 907-921

Lee, Y., Huh, M. 2002. Manus-only sauropod tracks in the Uhangri Formation (Upper Cretaceous), Korea and their paleobiological implications. Journal of Paleontology. 76, 3: 58564

Lockley, M. and Hunt, A. 1995. Dinosaur Tracks. New York: Columbia University Press. pp: 189-191

Vila, B., Oms, O., Galobart, A. 2005. Manus-only titanosaurid trackway from Fumanya (Maastrichtian, Pyrenees): further evidence for an underprint origin. Lethaia. 38: 211-218

Wilson, J., Carrano, M. 1999. Titanosaurs and the origin of “wide-gauge” trackways: A biomechanical and systematic perspective on sauropod locomotion. Paleobiology. 25, 2: 252-267