National Geographic

When Pseudo-Crocs Walked Tall

Fossil tracks – the clear imprints of living creatures – have often sparked the imagination of those who have found them. When, in 1802, a young boy found footprints in stone on his family’s South Hadley, Massachusetts farm some of the tracks wound up as a doorstop. Visitors joked that the family must have raised some hearty chickens if the bird-like footprints were imprinted on solid rock, while religious authorities suspected that the tracks might have been left by Noah’s raven when it landed after the Noachian Deluge. Later, the Amherst naturalist and theologian Edward Hitchcock would accumulate a massive store of similar tracks found throughout the Connecticut Valley. He suspected that many of the three-toed tracks were made by enormous birds not unlike the moa of New Zealand, though it would later be discovered that they were made by dinosaurs and other contemporary creatures.

Enigmatic tracks piqued the curiosity of authorities on the other side of the Atlantic, as well. In 1827 the Church of Scotland minister Henry Duncan was given a set of strange tracks which had been found near his home in the vicinity of Annandale. Duncan used them to decorate his summerhouse, but he also recognized that they might be of paleontological significance. Hence he contacted the Oxford naturalist William Buckland about the tracks, and, while Buckland was at first skeptical, a cast of the trackway (see above) convinced him that the find was authentic. The question was what kind of animal had made the tracks.

Buckland knew that the tracks had come from the New Red Sandstone deposits, meaning that – as far as potential trackmakers were concerned – the animal must have lived before the giant Megalosaurus and Iguanodon which had so recently been discovered. Buckland did not just rely on geology to solve the mystery, though. Using a small crocodile and three species of tortoises Buckland set up a show for his colleagues in which he persuaded the reptiles to walk across sand, clay, and soft piecrust to see if their tracks matched those on Duncan’s slab. Among those in attendance was John Murray, the man who would eventually publish Charles Darwin’s On the Origin of Species, and he later described the show to a friend:

It was really a glorious sight to behold all the philosophers, flour-besmeared, working away with tucked-up sleeves. Their exertions, I am happy to say, were at length crowned with success; a proper consistency of paste was attained, and the animals walked over the course in a very satisfactory manner; insomuch that many who came to scoff returned rather better disposed towards believing.

Based upon this actualistic experiment, Buckland thought that the tortoises provided the best match for the prints. The animals represented by the tracks would have been a bit quicker and more active than the poor little tortoises he put through the experiment, and when it came time to publicly announce the discovery Buckland did not show a shred of doubt that the tracks had been made by extremely ancient tortoises.

Soon after Buckland’s experiment, similar, but larger, tracks given the name Chirotherium began turning up in various fossil sites. Again, naturalists were presented with a mystery, although Buckland felt that some sort of enormous amphibian was the best candidate. This identification was backed up by the discovery of “Labyrinthodon” (now called Mastodonsaurus) in rock of about the same age, and so it seemed naturally that the two were connected. By the 1840′s it became traditional to restore the animal as an enormous, squat frog with a wicked smile standing in a line of Chirotherium tracks, and even as “Labyrinthodon” was revised to have a more salamander-like shape the connection between the tracks and the bones remained until around the beginning of the 20th century.

But “Labyrinthodon” did not actually make the tracks which had for so long been attributed to it. The tracks originally got their name – roughly meaning “hand beast” – because of their resemblance to a human hand, with four forward-pointing digits and a “thumb.” Now, with a more complete fossil record than was available to Buckland and his peers, we can recognize that this track shape is most consistent with the anatomy of terrestrial relatives of crocodiles called rauisuchians. These animals were among the dominant terrestrial predators during the Triassic. Compared to other tracks Buckland deemed to be made by tortoises, Duncan’s tracks may have been made by similar animals, as well, making them records of the heyday of extinct lineages most closely related to living crocodiles.

Even with today’s more extensive petrified menagerie to compare against fossil footprints, however, determining the exact identity of a trackmaker is a tricky task, and this is made all the more difficult by the fact that deposits which preserve trackways often lack skeletal remains. It is often possible to narrow down the general type of animal which made a track based upon the organisms known to exist in the area at the time, but, unless a creature literally dies in its tracks, attributing footprints to specific trackmakers is something treated with great caution (hence why tracks are given their own binomial names instead of simply being attributed to a known species).

Despite these difficulties, however, fossilized tracks can be extremely informative parts of the fossil record. Each footprint is a bit of behavior preserved in the fossil record and contains clues about soft tissues, biomechanics, and paleoecology. Placed in the right context, trackways may even record the timing of certain evolutionary events, and in a paper published earlier this year paleontologists Kevin Padian, Cheng Li, and Julie Pchelnikova used footprints made by a Late Triassic pseudo-croc to investigate the timing of an important evolutionary change among crocodiles, dinosaurs, and their kin.

Alan Charig's conception of the three types of archosaur posture: A) sprawling, B) a semi-sprawled "high walk" posture, C) erect posture. From Padian et al., 2010.

Looking at the diversity of living and extinct archosaurs – the large group of reptiles which includes dinosaurs, pterosaurs, crocodiles, and a motley assortment of closely-related prehistoric lineages – three different kinds of posture are readily apparent. There were archosaurs that sprawled like lizards, species with a semi-erect limb posture similar to what crocodiles use while walking on land, and others which carried their legs straight beneath their bodies. It is very nice and neat, and, as articulated by paleontologist Alan Charig in the 1970′s, it seems to imply that the straight-limbed lineages had gone through the earlier sprawling and semi-sprawling steps.

As pointed out by Padian and co-authors, however, the different kinds of posture seen among these animals is better understood as different points along a continuum of possibilities rather than as a series of distinct anatomical arrangements. This relatively recent (1990′s) shift in understanding has replaced the old three-step model, but Padian and colleagues propose a further separation between posture and the pattern of movement used by different animals. Words like “sprawled” and “erect” describe posture, the anatomical framework, but we also need to be more flexible in describing the range of motion those postures allow. Crocodiles, for example, have a sprawling posture at rest but straighten their legs to do a “high walk” when moving on land. In order to understand how posture and different ways of moving evolved, we need to go back to the fossil record to take another look at the archosaurs, and the authors of the new study use Triassic tracks designated Apatopus to make their case.

As with many other trace fossils, just what created the Apatopus tracks has been difficult to determine. When it was named by Donald Baird in 1957 it was proposed as the tracks of a phytosaur, a group of the gharial-like creatures which occupied the semi-aquatic ambush predator niche millions of years before true crocodiles did. In order to reassess this identification, Padian and colleagues compared the tracks to the skeletons of a variety of prehistoric reptiles which would have lived during the time the tracks were made, including phytosaurs, “armadillodile” aetosaurs, and creatures such as Ornithosuchus which carried their legs beneath their bodies. Using previously-published reconstructions and Photoshop, the skeletons of these animals were compared to the tracks to see which animals would be the best fit for the short Apatopus trackway originally described by Baird.

The known skeleton of the phytosaur Parasuchus hislopi superimposed over the Apatopus tracks. From Padian et al., 2010.

Determining the best fit ultimately came down to choosing between two sets of close candidates. The aetosaurs Aetosaurus and Stagonolepis, when scaled to the right size, had approximately the right proportions to fit the tracks, but a few things were off. In order to be a perfect fit, the limbs of the aetosaurs would have to have been held directly underneath their limb girdles close to the midline of the body. There is no indication that they did so. Furthermore, the fourth toe of the hindfoot of the trackmaker was the longest while the third toe of the aetosaurs were the longest, weakening the correspondence between the bones and the tracks. Overall, the phytosaurs were a better fit. The species Parasuchus hislopi corresponded well to the pattern of the tracks, and, unlike the aetosaurs, its fourth toe was the longest (a correction of a mistake Sankar Chatterjee made in his description of the animal). This does not mean that Parasuchus was the trackmaker, but – since it is one of the few known phytosaurs with nearly complete hands and feet – it is representative of the kind of animal which left the tracks.

Phytosaurus have often been reconstructed with sprawling postures which would have required them to rotate their limbs as they moved, yet the Apatopus tracks were made by an animal moving its limbs in a front-to-back manner. This means that the animal was using a high walk similar to that employed by living crocodiles, and therefore demonstrates the importance of considering as posture and gait as separate things.

Even though it had previously argued that phytosaurs were incapable of this kind of locomotion, the Apatopus tracks suggest otherwise. This means that the ability to do the high walk might have been a shared trait in the last common ancestor of phytosaurs and other groups of archosaurs more closely related to crocodiles than dinosaurs (the pseudosuchians). Perhaps the earliest archosaurs even counted the high walk in their locomotor repertoire, underscoring the fact that the posture and mode of locomotion in the ornithosuchia – the group which contains dinosaurs, pterosaurs, and their close relatives – involved a unique shift towards a posture in which the limbs were held under the body with a restriction to front-to-back motion of the limbs. The significance of this is that the anatomy of the already specialized pseudosuchians – such as the phytosaurs – were uniquely modified and cannot be looked at as simply intermediate stages in the evolution of the posture seen in creatures like dinosaurs. The pseudosuchians were a widely varied group of animals which must be appreciated on their own terms, and further studies of enigmatic tracks may provide paleontologists with a more comprehensive view of the pattern of their evolution.

References:

Davidson, J. 2008. A History of Paleontology Illustration. Indiana University Press: Bloomington. pp. 55-64

PADIAN, K., LI, C., & PCHELNIKOVA, J. (2010). The trackmaker of (Late Triassic, North America): implications for the evolution of archosaur stance and gait Palaeontology, 53 (1), 175-189 DOI: 10.1111/j.1475-4983.2009.00924.x

Rudwick, M. 2009. Worlds Before Adam. University of Chicago Press: Chicago. pp. 151-153

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