I have to admit that I don’t find trace fossils – the vast majority of which are footprints – that interesting. But some trace fossils are very neat and provide excellent information on behaviour and lifestyle. Examples include pterosaur take-off traces, the trackway of the little theropod that does an abrupt about-turn and runs back the way it came, and the Myotragus tracks that show how individuals had to stagger and battle through wet sediment in order to escape alive.
Particularly fun are giant mystery traces: the ones made by large animals doing unusual things… yet what these large animals were, and what they were doing, remains uncertain.
One of my favourite ‘giant mystery tracks’ has only recently come to attention. It was photographed by palaeontologist Tim Palmer in Middle Jurassic sediments from Madagascar and consists of four parallel ‘channels’, generally thought to have been made by the limbs of some very large animal as it dragged itself across an emergent sand-bar [the fossil is shown here, photo from here on the Palaentological Association site]. Dinosaur tracks are preserved on the same surface. The animal changed direction near the end of the preserved section, hence the kink you see in the ‘channels’. The obvious guess is that the track-maker was a giant turtle or (heaven forbid) a plesiosaur, but that can’t work as both animals would leave a belly trace. It’s also difficult to try and make an identification without better photos. You can read a bit more about this weird track on the Palaeontological Association site.
However, this is far from a one-off. Numerous gutter-like trace fossils are known from the sedimentary rocks of the Jurassic though, unlike the Madagascan example, the other ones I know of were definitely not made by limbs. Instead, they represent feeding traces of some sort, and they were made by jaw- or snout-tips.
We have Jörn Geister of the Geologisches Institut at the University of Bern to thank for his excellent paper on an assemblage of giant, gutter-like traces, all discovered in the Middle Jurassic Callovian Marl of Liesberg in Switzerland (Geister 1998). Similar traces are also known from the Upper Jurassic of Spain and have been given the ichnotaxonomic name Megaplanolites ibericus (Calvo et al. 1987). The Swiss traces – which we can be absolutely sure were produced by large vertebrates – were exposed on the sides of what used to be the Liesbergmüli clay pit. Sadly, the pit later became a regional garbage dump and is now filled up with domestic waste [image at top shows the exposed bedding plane at the former clay pit, with silhouette of a 12-m-long pliosaur added for scale. The gutters are obvious. The photo was taken in 1987. 'In 1998 the bedding plane shown on this photograph was completely covered by garbage' (Geister 1998, p. 106)].
Three kinds of traces
As you can see from the various pictures used here (all from Geister (1998)), the traces were parallel-sided grooves or gutters. Some were straight; others were sigmoidal and meandering They extended over a large area of exposed surface (20 x 200 m). Some were huge – as much as 9 m long and up to 60 cm in width – while others were smaller, with widths of about 15 cm. It must be concluded that the traces were made by animals pushing their jaws or snout-tips through the once-soft sediment, and this narrows down the possible trace-makers to a select list. More on the identification of the trace-makers in a minute.
Geister (1998) was able to identify three different kinds of gutter-like trace [adjacent image, from Geister (1998), shows schematic longitudinal and transverse sections of Type (a), (b) and (c) traces]. In one type – Trace (a) – the trace-maker gradually pushed its jaw/snout deeper and deeper into the sediment while creating the gutter, eventually reaching a depth of 30 cm. The animal then stopped ‘ploughing’ and pulled its jaw/snout straight out. These gutters tend to be straight. In a second type of gutter – Trace (a) – the animal started making the gutter, and immediately pushed down into the sediment to a depth of 20 cm. It then pushed its jaw/snout horizontally, cutting sideways into the sediment. After ploughing for a few metres, the animal then retracted its jaw/snout sideways (creating a wide depression in the gutter), and it then finished making the trace by pulling out vertically. In the third type of gutter – Trace (c) – the trace-maker began by slowly lowering its jaw/snout 20-30 cm deep into the sediment. It then continued ploughing at this depth, and usually moved to the left or right while doing this, thereby creating a curving trace. Type (c) traces were made by the largest trace-makers: animals with jaw or snout tips 45 or even 60 cm wide that continued ploughing for up to 9 m [a Type a trace shown below. A penknife 85 mm long is visible down at bottom left].
While these traces sound remarkable (well, I think they do), they perhaps aren’t so incredible when we compare them to the feeding traces made by modern-day aquatic tetrapods. Walruses, for example, can and do produce feeding traces on the sea-floor that are as much as 47 m long and 40 cm wide. The giant pits excavated by filter-feeding grey whales can be 25 m long and 1.5 m wide.
Who made the traces?
So: what made the traces? ‘Animals’ is the obvious answer, and ‘big animals’ at that. Furthermore: ‘big animals with pointed structures that can be pushed into, and through, sediment’. It seems inescapable that these animals were pushing their jaw-tips or snouts through the sediment, and – given the parallel-sided, gutter-like shape of the traces – we have to conclude that these were animals with relatively narrow jaw- or snout-tips. While it can’t be entirely ruled out that large fishes (remember that there were some very big fishes in the Jurassic seas of Europe) made some of the traces, ichthyosaurs and plesiosaurs, and perhaps thalattosuchian crocodilians, are the most likely candidates. Geister (1998) suggested that some of the smaller traces (like some of the Type (b) traces) might have been made by the narrow jaws of ichthyosaurs or by small plesiosaurs (the traces that terminated with deep cavities could not have been produced by blunt-snouted plesiosauroids, however), while giant pliosaurs like Liopleurodon – the only animals that are both in the right size range, and capable of sea-floor ploughing – might have made the gigantic Type (c) traces [hypothetical plesiosaur feeding bottom-behaviour depicted below, from Geister (1998). I don't think that long-necked plesiosaurs would stick their back ends out of the water as shown. I'd also don't think that a pliosaur would need to adopt the bent-necked posture shown here; the animal could just as well have kept its whole long axis at a diagonal to the substrate].
In recent years the idea that long-necked plesiosaurs were predators of benthic, rather than nectic or planktonic, prey has become popular. This is supported by evidence from stomach contents (McHenry et al. 2005), and it also seems to fit with what we know about plesiosaur neck flexibility and general morphology (a few comments here refer to Lez Noè’s thoughts on this issue). Perhaps, in these gutter-like feeding traces, we have direct evidence for this behaviour.
Additional support for a marine reptile origin for these feeding traces might come from cololites (the fossilised remains of gut contents). It’s well known that ichthyosaur cololites contains lots of cephalopod hooklets as well as fish scales and whatnot (e.g., Pollard 1968). Geister (1998) makes the point that what’s less well known is that ichthyosaur cololites also contain a lot of sand, suggesting that some of these animals ingested a lot of sediment, at least on occasion. The same is also argued to be true for plesiosaurs: again, cololites from these animals contain a lot of sand, gravel and stones, and this might show that these animals swallowed sediment while foraging (e.g., Zhuravlev 1943, Martill 1992). If the idea of ichthyosaurs or pliosaurs scooping up mouthfuls of sediment seems weird, remember that benthic feeding of this sort is not incompatible with the predation on pelagic prey that we’re more used to imagining: the animals concerned were apparently generalists, feeding at various levels within the water column [image below shows hypothetical 'ploughing' pliosaur].
Presumably, these animals were searching for what we call infaunal prey: that is, animals that are living buried in the sediment, like molluscs, worms and burrowing crustaceans. Broken mollusc shells were never discovered alongside the gutters, perhaps showing that molluscs were not being caught and eaten. This is, of course, negative evidence: Geister (1998) knew that broken shells were discovered alongside the feeding traces left by walruses, and therefore assumed that shell remains should be found alongside the Swiss traces if molluscs had been unearthed and eaten.
However, this assumes that the sea-floor surface preserved at Liesberg preserves all of the clutter present on the sea-floor just after the gutters had been created, and I think that this might be a questionable inference. It also assumes that – on discovering a mollusc in the sediment – the predator fed by extracting the soft parts and leaving the shell valves behind. This is what walruses do (they suck the mollusc body out and leave the shell behind), but it doesn’t follow that Mesozoic marine reptiles did likewise. It is, after all, conceivable that a predator like a pliosaur might have swallowed bivalves whole. This is (I believe) what crocodiles do with gastropod prey. Anyway, the many crustacean burrows preserved at Liesberg do indicate that these were the prey animals the trace-makers were searching for. Worth noting here is that some plesiosaurs (I’m thinking Pachycostasaurus from the Oxford Clay) have been interpreted as specialist bottom-feeders of crustaceans (Cruickshank et al. 1996).
Incidentally, feeding traces weren’t the only impressions preserved at Liesberg. Geister (1998, Plate 21) also figured a wide, shallow impression – about 4 m long – that appears to have been made by a large vertebrate rubbing its body against the sea floor. Perhaps this represents a site where a creature (an ichthyosaur or plesiosaur?) rubbed its sides against the sediment in order to remove dead skin (Geister 1998). Cetaceans do this at select spots on stony sea floors, and we’d certainly expect Mesozoic marine reptiles to engage in behaviour of this sort.
One final thing that has to be mentioned… if plesiosaurs and/or ichthyosaurs and/or thalattosuchians really were in the habit of ploughing through sediment in quest of infaunal prey, how were they finding these animals? Were they relying on smell, on touch, or on some other sense? The answer is, of course, that we don’t know. The presence of large bifurcating canals within the bones of the ichthyosaur rostrum has led some to speculate that ichthyosaurs had enhanced tactility in the snout, and it’s even been proposed that ichthyosaurs might have had electroreceptive abilities (that is, that they could detect the electrical signals given off by muscular activity in other animals). This is unashamed speculation for sure (the electroreception idea comes from a conference poster and abstract), but it’s at least conceivable that some extinct tetrapods had amazing and unexpectedly weird sensory skills, just as many modern ones do.
So, there we have it. I think the ‘feeding gutters’ preserved at Liesberg and documented by Geister (1998) are remarkable, and this article represents my attempt to make them better known. Most biologists are familiar with the idea that whales, walruses and rays act as ’tillers of the sea floor’* [image above shows a Grey whale scooping up sea-floor sediment for sieving, from here]. We now have evidence that behaviour of this sort wasn’t only a Cenozoic thing: it was also occurring in the Jurassic at least.
* A 1987 article from Scientific American provided my inspiration here: it was titled ‘Whales and walruses as tillers of the sea floor’ (Nelson & Johnson 1987).
For previous Tet Zoo articles on Jurassic marine life see…
- Tet Zoo picture of the day # 25
- SVPCA 2007: lepidosaurs, turtles, crocodilians, the plesiosaur research revolution continues
- Voracious snub-nosed robber
- The Cumnor monster mandible
- A life secretly devoted to fish-lizards
- At the 56th SVPCA – hello Dublin!
- The skin of ichthyosaurs
- Sea Dragons of Avalon: a 2009 seminar
- In which Bob Nicholls exceeds expectations and produces some jolly good artwork
- The world’s biggest ever fish: time to put out the trash
- Sea Dragons of Avalon, an Arthurian adventure (part I)
- An Arthurian adventure, part II: more fossil marine reptiles than are good for your health
Refs – –
Calvo, J. M., Gil, E. & Meléndez, G. 1987. Megaplanolites ibericus (ichnogen. et ichnosp. nov.), a new trace fossil from the Upper Jurassic (uppermost Oxfordian) of Bueña (Teruel Province, Iberian Chain, Spain). Palaeogeography, Palaeoclimatology, Palaeoecology 61, 199-204.
Cruickshank, A. R. I., Martill, D. M. & Noè, L. F. 1996. A pliosaur (Reptilia, Sauropterygia) exhibiting pachyostosis from the Middle Jurassic of England. Journal of the Geological Society, London 153, 873-879.
Geister, J. 1998. Lebensspuren made by marine reptiles and their prey in the Middle Jurassic (Callovian) of Liesberg, Switzerland. Facies 39, 105-124.
Martill, D. M. 1992. Pliosaur stomach contents from the Oxford Clay. Mercian Geologist 13, 37-42.
McHenry, C., Cook, A. G. & Wroe, S. 2005. Bottom-feeding plesiosaurs. Science 310, 75.
Nelson, C. H. & Johnson, K. R. 1987. Whales and walruses as tillers of the sea floor. Scientific American 256 (2), 74-81.
Pollard, J. E. 1968. The gastric contents of an ichthyosaur from the Lower Lias of Lyme Regis, Dorset. Palaeontology 11, 376-388.
Zhuravlev, K. I. 1943. Nakhodki ostatkov verkhnejurskikh reptiliy v Savel’evskom slancevom rudnike. Izvestiya Akademiya Nauk Soyusa SSR, Otdelenie biologich 5, 294-307.