The daily lives of fossil reptiles

The Mesozoic and Cenozoic deposits of Southern England have long been a rich source of fossil reptiles. Past finds of great historical importance include some of the earliest known examples of dinosaurs, ichthyosaurs and pterosaurs. Fossil material, including new species, continues to be revealed, mainly at rapidly eroding coastal sites. All these reptiles would have been active participants in their local ecosystems, whether on land or in the sea. Much information about the roles they played and their interactions with other organisms can be gleaned from their skeletal anatomy and from comparison with living relatives such as crocodiles. What this article is concerned with, however, is evidence of specific incidents in the lives, and deaths, of individual reptiles; tiny snapshots of opportunities, mishaps and the daily drudge of staying alive. These add more detail and colour to our knowledge of the lifestyles of these long-vanished animals. This evidence will be provided by four selected terrestrial and marine deposits from southern England, spanning the last quarter of a billion years of Earth history (Fig. 1).

Trace fossils in a desert world – the Triassic Otter Sandstone

Rocks dating from the Triassic period, laid down between approximately 250 and 200 million years ago, are magnificently exposed along the coast of East Devon, representing the western part of the ‘Jurassic Coast’ World Heritage Site. These rocks are non-marine, having been laid down by wind and rivers in the arid interior of the supercontinent of Pangaea, which had formed from the melding together of all the Earth’s previously separated continents (Fig. 2). They are generally very poorly fossiliferous, with the exception of the Middle Triassic Otter Sandstone, seen between the coastal towns of Budleigh Salterton and Sidmouth. This was deposited by life-supporting braided rivers running through what was otherwise a barren desert environment.

Fig. 1. Geological map showing locations of deposits discussed in the text. (1) Triassic Otter
Sandstone of South Devon; (2) Jurassic Lower Lias of the Somerset (a) and Dorset (b) coasts; (3)
Cretaceous Wealden beds of the Isle of Wight (IOW on map); and (4) Paleogene Hamstead beds
of the Isle of Wight.

The Otter Sandstone fossils are uncommon and generally fragmentary, but record the presence of plants, arthropods, fish and amphibians. In particular, diverse reptiles populated the local landscape. They comprised herbivores, such as the small lizard-like procolophonid in Fig. 3 (left), with stubby plant-crushing teeth, and pig-sized, and somewhat pig-shaped, rhynchosaurs (Fig. 2). Undoubtedly predatory archosaurs (relatives of crocodiles and dinosaurs), with razor-sharp serrated teeth, included the probably rat-sized creature in Fig. 3 (right) and no-nonsense killers several metres in length.

When these rocks were deposited about 245 million years ago, the Earth’s biota was recovering from the devastating end-Permian mass extinction, which took place less than ten million years earlier and was probably caused by volcanism. The Otter Sandstone reptiles lived several million years before the earliest mammals and dinosaurs, and almost a hundred million years before the first birds took to the air.

As well as bones, the Otter Sandstone yields trace fossils, and these provide additional insights into the lives of the Triassic reptiles. Trace fossils are remains not of the organisms themselves, but structures left in the sediment as a result of their activities while alive.

Fig. 2. Herbivorous reptiles contemplate a map of their Triassic world. Top left: the rhynchosaur
Fodonyx; bottom left: the therapsid (‘mammal-like reptile’) Lystrosaurus. Both reached about a
metre in length.

The walkway beneath Connaught Gardens along Sidmouth Seafront is backed by a vertical red sandstone cliff. Scattered among ‘regular’ bedding features in the cliff-face are some more intriguing structures. These have been interpreted by researcher Ramues Gallois as being fossil burrows in what would originally have been fairly soft sand, deposited by migrating river channels. Certainly some, such as that in Fig. 4, show what appears to be an inclined entrance tunnel leading to a larger chamber (oval in cross-section in the cliff) in which an animal could have sheltered from predators or the searing desert heat.

There is evidence from their bone structure that procolophonids, such as the one in Fig. 3 may have been burrowers, but these pet lizard-sized creatures would have been far too small to make the substantially-sized Sidmouth excavations. Fairly similar burrow-like structures from South Africa contain the skeletons of therapsids (‘mammal-like reptiles’; see Fig. 2), which were quite widespread during the Triassic. These could, therefore, have constructed the Otter Sandstone burrows. The only problem is that no therapsid bones or teeth have ever been found in these deposits. They may turn up one day, or alternatively the burrows were constructed by some other creatures, such as the chunky, and evidently common, herbivorous rhynchosaurs, which had a stout ‘beak’ and claws suitable for digging up roots, and perhaps also digging holes.

Fig. 3. Small reptiles from the Triassic Otter Sandstone of the Devon coast. Skull of herbivorous, lizard-like procolophonid on left (large eye socket at top); upper jaw of predatory archosaur on right.

Moving on to a different type of trace fossil, in the 1830s, some very unusual footprints were observed in Triassic rocks in Germany. They bore a rather spooky resemblance to human hand prints, so much so that, in 1835, they were christened ‘Chirotherium’ by Johann Jakob Kaup, from the Greek for ‘hand beast’. Similar prints of approximately the same age were subsequently recognised elsewhere in the world, including, recently, in the Otter Sandstone of Devon (Fig. 5).

What sort of creature was the ‘hand beast’? First off, it is evident that the resemblance of the prints to human hands is superficial only and the distinctive ‘thumb’ was a curved toe lying on the outside, rather than inside, of the foot. But that was comparatively little help in establishing the maker of the prints. Johann Kaup conjectured that it might have been an enormous marsupial. The famous British palaeontologist, Richard Owen, in 1842, thought instead that it was more likely to be a large amphibian, remains of which had at least been found in British Triassic rocks. It is now known that the chirothere printmakers were almost certainly archosaurian reptiles, belonging to a currently rather untidy taxonomic grab-bag known as ‘rauisuchians’. Rauisuchians, teeth and bones of which can be found in the Otter Sandstone, were terrestrial predators that were probably occasionally bipedal and would have looked superficially like theropod dinosaurs, although were more closely related to crocodiles. At up to five metres or so in length, they were certainly apex predators in the mid-Triassic landscape. They died out at the end of the Triassic period, as the true dinosaurs were in the ascendancy.

Fig. 4. Presumed reptile burrow in Otter Sandstone cliff at Sidmouth. (The carved message that is just discernible on the burrow chamber was probably not produced by a burrowing reptile.)

Footprints are found at several levels near the top of the Otter Sandstone, mostly in red mudstone bands that were probably deposited in wide and very shallow lakes. One such layer can be seen in favourable conditions on the foreshore both sides of Sidmouth, at sites two kilometres apart, so was of wide extent. Judging from the density of the exposed footprints (often several per square metre), there are probably millions of them just in this one thin layer around or beneath Sidmouth town.

How many animals or separate visits does this particular footprint layer represent? Since the prints show a range of sizes, they were not produced by a single very busy individual; rather, it seems that these creatures were gregarious. It is not known, however, what timespan the thin layer represents – it could have been merely days, a season or many years, so it is not possible to establish how many were present at any one particular time.

What brought these animals to these sites? Perhaps they gathered to drink, although if this was the main purpose, one might expect the footprints to be concentrated around the margins of the water bodies rather than throughout them. Another possibility is that they were there to feed. Occasionally found in close proximity to the footprints are small clusters of broken bone. These seem all to derive from temnospondyl amphibians, which would have resembled overgrown armour-plated newts, reaching several metres in length (Fig. 6). On close examination, some of these bone fragments show grooves that could well have been produced by serrated archosaur teeth; others show an unusually finely pitted surface similar to the acid wear seen on bones that have passed through the digestive tracts of modern-day carnivores. It seems, therefore, that they could be the defecated or regurgitated remains of rauisuchian meals.

Fig. 5. Two chirotheriid footprints exposed on the shore at Sidmouth, Devon (bottom left and right of image), with distinctive backward-projecting fifth digit (the ‘thumb’) indicated on one of them. Speculative restoration of rauisuchian print-maker superimposed at top. Width of hammer head is 22cm.

The problem with feeding as an explanation for gatherings of archosaurs is that the temnospondyl fragments evidently belonged to creatures consumed and digested elsewhere. Apart from occasional shed archosaur teeth that had become blunted from over-use, the lake sediments the prints are preserved in yield no other animal fossils, and certainly no traces of unconsumed temnospondyls. The immediate environment therefore apparently lacked any organisms that could plausibly have been prey. The water may have been rendered inhospitable due to elevated salinity or extreme temperature fluctuations. Rather than feeding or drinking, then, perhaps the animals took to the water as a means of cooling down from the effects of the ferocious desert sun or even removing ectoparasites.

There is evidence from chirotheriid track sites preserved in Germany that groups of these archosaurs undertook lengthy, and presumably seasonal, migrations along the margins of a long-vanished sea. It could be that the Sidmouth animals were doing the same, conceivably even to or from America or continental Europe, which were at that time all part of the same continuous landmass. Perhaps what is now the Devon coast represented a way-station on a gargantuan desert trek.

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