Tetrapod Zoology

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

(on elasmosaurids)

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.

Comments

#1 C. M. Koseman
December 16, 2009

The four parallel tracks are pretty awesome. What could have caused them? Early attempts at farming by possible dinospohonts? Jurassic equivalents of the Dover footprints? The mind boggles.
#2 Darren Naish
December 16, 2009

After I’d written the article I had a crazy idea… rather than being tracks made by limbs, could those parallel channels have been made by four ‘ploughing’ marine reptiles moving in parallel? The sediment was apparently aerially exposed when the track was made, however, so this can’t work… hmmm.
#3 Sigmund
December 16, 2009

“The sediment was apparently aerially exposed when the track was made, however, so this can’t work… hmmm.”
The Jurassic Pterodactyl Red Arrow display team?
#4 Dartian
December 16, 2009

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.

In modern marine biota, for how long do such feeding traces typically last on the seafloor until they’re obliterated by currents? Hours, days, weeks, months?
#5 Bill
December 16, 2009

I would like to second Dartian’s question here – how did such a trace become fossilised? I would imagine that they would quickly become obliterated . Was this a possibly an ephemeral lake/lagoon and might this help in identifying the species that caused the traces?
#6 John Conway
December 16, 2009

Really awesome post Darren.

The first thing I thought when looking at the four parallel grooves was that something was being dragged. If it was on a sand bar, I guess it could be a large theropod dragging… something. Its own tracks could have been obliterated by the drag marks.

If the trace was made underwater… I dunno. Do aquatic predators drag around their meals?
#7 Greg Laden
December 16, 2009

I have to admit that I don’t find trace fossils – the vast majority of which are footprints – that interesting.

I have often lived in regions with snow, and this quite possibly enhances one’s interest in trace fossils. I saw some very interesting marks on thing snow covering fresh ice the other day which consisted of the landing marks of a small flock of Canad geese, for instance. They were actually rather humorous looking.
#8 Karl Zimmerman
December 16, 2009

Wow, the coolest ichnotaxa since I read about that mystery monodactyl theropod from the Jurassic of South America in the new Novas book.
#9 Anonymous
December 16, 2009

“After I’d written the article I had a crazy idea… rather than being tracks made by limbs, could those parallel channels have been made by four ‘ploughing’ marine reptiles moving in parallel? The sediment was apparently aerially exposed when the track was made, however, so this can’t work… hmmm.”

Maybe just one marine reptile plowing through the sediment (maybe going after a patch of burrowing marine invertebrates or something), then turning around and repeating the motion four more times.
#10 Mu
December 16, 2009

The four parallel grooves clearly are the feeding pattern of a hyrda-like four-necked plesiosaur. Good luck describing the neck posture on that one Darren.
#11 AnJaCo
December 16, 2009

If only one of the critters had plowed into a buried log and lost a tooth or two…

The four parallel tracks are very parallel. Little/no variation in the distance between them. Also no sign of other marks such as the footprints of the ‘dragger’. As Darren says, better photos would be helpful. Very strange. Filed under “WTF?”
#12 Sclerophanax
December 16, 2009

The first thing that came to my mind looking at the four exceedingly weird parallel tracks was that they were left by some kind of a machine, maybe a robotic rover. Proof of alien intelligence or time travel? Yeah, probably not, but it would be incredibly cool if we did one day find something like that in the fossil record.
#13 Hank Roberts
December 16, 2009

Chuckle. I imagined something _really_ big floating over the seafloor –neutrally buoyant– reaching down and briefly dragging one limb to turn or slow down, making four claw-marks.

Let’s see, that would scale to what, about the size of the Graf Zeppelin?
#14 Ranjit Suresh
December 16, 2009

Darren: You said it couldn’t have been a plesiosaur because it would have left a belly trace behind. But, is it possible that some plesiosaurs could have supported themselves at least partially upright at times, as an alternative means of getting about on land? How about pliosaurs or marine crocodilians? Or is that ruled out by their biomechanics? I’m thinking here, for instance, of the differences between the terrestrial abilities of otariids and phocids.
#15 Jerzy
December 16, 2009

So it was TRUE!!!

It’s classical story! Only I forgot the title. Paleontologists dig out, bit by bit, tracks of a predatory dinosaur running towards something. Then they help soldiers from the local military base fix a jeep, putting a band of cloth across one tyre. Then they see an explosion in the base. Then they uncover the dinosaur tracks further and see that they are coming towards fossilized tracks of a jeep with a band on a tyre. It’s all true!
#16 Jerzy
December 16, 2009

Somebody can help with the name of the story?

Seriously, they look like tracks of a quadruped, perhaps in shallow water making the body float above, or perhaps a turtle leaving few tracks of the carapace. I cannot imagine animal digging and moving so regularly up and down to make dents in the tracks.

I think one of you geek could figure out what marine animal has hind flippers much closer together than front flippers. Some turtle?
#17 Zach Miller
December 16, 2009

Extremely strange. I like the idea of plowing marine reptiles, though. And Darren, did you do the pliosaur picture? I really like it!
#18 Darren Naish
December 16, 2009

Thanks as always for comments.

The Swiss traces do raise a lot of questions, and questions that I don’t know the answers to. For gutter-like structures to be preserved in the sediment, they must be infilled by softer sediment (it must be softer, otherwise it wouldn’t become differentially eroded away). This must happen fairly soon after the creation of the gutters, as (presumably) the structures would have been deformed or filled in.

This raises the question of how long such structures persist in the sediment (comments 4 and 5): I have no idea on that, but obviously it can be highly variable and is very dependent on local currents, sedimentation rates, and so on. I did a quick google search, and found that marks made by dredging vessels were still present on the seafloor seven years (!!) after creation, so it seems possible from this that feeding traces could well be preserved if the right kind of sediment was dumped on top.

And, on ‘preservation’: the complete and near-perfect preservation of a whole bedding plane covered in burrows and feeding traces suggests rapid deposition of a new, softer (finer-grained?) sediment, and I would guess that some event brought an influx of new sediment to the local system. But I’m no sedimentologist.

One palaeontologist suggested to me today that the traces might not be feeding traces at all, but ‘flute casts’: that is, scour marks made by currents. I have no idea whether this might be true, but I find it hard to believe. The traces that Geister described are not parallel (in fact, they meander all over the place and are sometimes at strong angles to each other), and they look too complex and regular to be caused by currents. But I’d be interested in further comments on this, I didn’t pay much attention during sedimentology classes.

And, yeah, I did the pliosaur cartoon, thanks. The proportions are bullshit (forelimbs longer than hindlimbs?) but I did it in a hurry.
#19 Erich Fitzgerald
December 16, 2009

Look out next Monday (21 December; London time zone) on the Linnean Society website for new research related to Cenozoic aspects of this blog post.
#20 Hank Roberts
December 16, 2009

The story you’re thinking of is Clarke’s “Time’s Arrow”

http://arthur-clarke-fansite.blogspot.com/2007/04/times-arrow-finally-clarkian-time.html
#21 Nathan Myers
December 16, 2009

The four parallel tracks were clearly made by the claws of an immense beast being dragged, backwards, into the water, obviously by an even more immense beast. We see precisely that phenomenon, with huge parallel gouges in the earth, illustrated in the film, “Sky Captain and the World of Tomorrow”, in a scene set on Totenkopf’s island. In that instance the skeleton of the manus responsible for the gouges remains to offer mute confirmation of the interpretation.

If you haven’t seen Sky Captain yet, what are you waiting for?
#22 John Harshman
December 16, 2009

Darren:

Thank you very, very much for the illustration of the plesiosaur as dabbling duck analog. And of course they stuck their tails in the air. That’s how the neontological model does it, and so that’s how the plesiosaur has to do it. And besides, it allows you to reach the bottom at a greater depth.

Is that really the currently popular model? If so, great. I’m in favor of anything with ducks in it.
#23 Alton Dooley
December 16, 2009

[from Darren: sorry, delayed by spam-filter]

Brian Beatty and I suggested this benthic feeding behavior in a paper earlier this year, based on rib morphology and lower jaw and cranial injuries. Here’s a link to the paper (it’s open access-scroll down to Jeffersoniana 20 for the link):

http://www.vmnh.net/index.cfm/topic/free-downloads

And a summary (and reconstruction) is at:

http://web.me.com/dooleyclan/Site_2/Blog/Entries/2009/8/19_Left-handed_whale_died_a_painful_death.html

We did not, however, find obvious feeding traces at the site. A turbidity flow is one way you could bury and preserve the traces. There might be other ways, as well.

One thing to note with the parallel traces is that gray whale traces tend to be parallel; I suspect they’re aligned with the current (easier for the whale to stay on course, maybe?).

Another point, with the bent neck in the pliosaur. Gray whales (and humpbacks, which occasionally feed on the seafloor) avoid this by feeding out of one side of the mouth. You can see the gray whale doing that in the photo you included. They are strongly handed, and always use the same side of the mouth for feeding (right-handedness is more common); the baleen plates are shorter on the feeding side from abrasion. I wonder if plesiosaurs and/or pliosaurs show heavier tooth wear on one side? Has anyone looked?
#24 Kristoffer
December 17, 2009

Very interesting indeed. Here in Norway we have a lake called Seljordsvatnet, it is the “norwegian Loch Ness” one might say. During one expedition in 2004 similar tracks to these were found on the bottom. At the time they baffled the research team, as they had no good suggestion as to what had made them. It was also recorded some kind of “clicking/humming” noise in the lake, like the sound of echolocation made by whales.
#25 Dartian
December 17, 2009

John H.

Thank you very, very much for the illustration of the plesiosaur as dabbling duck analog. And of course they stuck their tails in the air. That’s how the neontological model does it, and so that’s how the plesiosaur has to do it. And besides, it allows you to reach the bottom at a greater depth.

In some languages (e.g., Danish, Norwegian, Swedish, and Finnish) plesiosaurs are actually called ‘swan lizards’.
#26 johannes
December 17, 2009

> Who made the giant Jurassic sea-floor gutters?

This can’t be answered without breaking the Dresden Agreement .
#27 Mokele
December 17, 2009

Someone mentioned it earlier, but I think it deserves another mention: if these are feeding tracks, could there be teeth burried in the muck at the edges? Some teeth would undoubtedly be lost during such feeding, and while some might have been ingested and digested, some might have fallen out and been fossilized. Unfortunately, that would mean destroying the traces as you try to find teeth in them…
#28 David Marjanović
December 17, 2009

Thank you very, very much for the illustration of the plesiosaur as dabbling duck analog. And of course they stuck their tails in the air. That’s how the neontological model does it, and so that’s how the plesiosaur has to do it.

Except that ducks have much lower density than plesiosaurs. A plesiosaur sticking that far out of the water would just fall in.
#29 Valentin Fischer
December 18, 2009

All the “-cast” sedimentary figures (load cast, flute cast, scour marks) are usually made by small pebbles forcefully clashed and dragged onto soft sediments by strong waves… These traces are like 0.5-3 cm wide and 5-15 cm long, but relatively quite deep, around 1-5 cm…

If those large traces are “mega flute casts”, made by 10-30 cm boulders, any geologist would be able to identify storm wave sedimentary structures like tempestites, or large bouma sequences, or tsunamites on the above bed. But you mentionned some of these were found in a clay pit… which sounds more like a quieter/deeper palaeoenvironnement.

My 2 cents
#30 Douglas McClean
December 18, 2009

What does “nectic” mean? Google doesn’t seem to know, at least on the first page of results.
#31 Mark Evans
December 18, 2009

These are impressive trace fossils, but I’m still unsure as to the trace maker.

Even the largest Liopleurodon skull known only has a rostrum around 150 mm in width, about half the size of what would be needed to make most of the traces. The newly-unveiled Weymouth Bay pliosaur would have had a rostrum of the right width. However, this is just about the largest known pliosaur skull, at around 2.25 m in estimated length.

I have been wondering about large filter feeding fish. Even if Leedsichthys is too large (somewhere between 10 and 15 m in length), there were other Middle/Late Jurassic filter feeding pachycormids such as Asthenocormus and Martillichthys.
#32 David Marjanović
December 18, 2009

What does “nectic” mean?

Try “nektonic” — opposite of “planktonic”, means, able to swim around on its own. Basically vertebrates and squid, and presumably belemnites. I don’t know if jellyfish count.
#33 arachnophile
December 23, 2009

I am overjoyed that some of the more learned readers had the same thought that I did. I wondered about the 4 paralel lines and multiple individuals. As stated there are so many problems with that but I guess I still don’t understand why it is not, in fact, 5 furrows. Shouldn’t it be a jaw and 4 limbs we are seeing evidence of?

I have to state that, at this point I have ingested more than a bit of egg nog and may have missed an obvious part of the article that refers to this.

As always I’m a reader who is a “wannabe” in your world. I have to state this because the comments are always intimidating as hell! Many of your readers are obviously pros.
#34 Kris
January 3, 2010

Better late than never…

This makes me think of mosasaurs… specifically Globidens. One would think that it would have done some bottom trolling from time to time given its diet. So, maybe there are mosasaur-made trace fossils, very much like the ones from the Jurassic, that are out there to be discovered.
#35 drhook
January 25, 2010

Maybe a giant species of mollusc or crustacean. A horseshoe crab leaves similiar tracks. My bet would be something with a hard undershell with protrusions, forward movement aided by current or legs whose tracks might be washed away quite quickly.
#36 Phil gutters
August 15, 2010

It’s a very interesting subject. I think long ago there must have been creatures like giant turtles that would migrate, thus creating huge paths along the sea bed.
#37 David Marjanović
August 16, 2010

Maybe a giant species of mollusc or crustacean. A horseshoe crab leaves similiar tracks. My bet would be something with a hard undershell with protrusions

Then why haven’t we found body fossils of such a beast?

I think long ago there must have been creatures like giant turtles that would migrate, thus creating huge paths along the sea bed.

Why would they migrate along the seafloor and not higher up in the water?
#38 Zach Miller
August 16, 2010

Because they’re giant LAND turtles, David. They could hold their breath a reeeeally long time.
#39 jason
November 13, 2010

Gravity: the sea “floor” was tipped at an angle at the time the “tracks” were made, and some rocks rolled downhill.
#40 Darren Naish
November 13, 2010

No. The tracks aren’t all straight (some are curved or meandering), don’t occur in parallel (some are perpendicular to others), and the sediment on the bit of seafloor where the tracks are preserved wouldn’t have piled up to equal depth throughout the section if the seafloor was tilted. Furthermore, in some of the tracks the track-maker pushed sideways into the wall of the gutter it was creating, or probed into the ‘end’ of the gutter, leaving an overhang. And if rolling rocks were responsible, why wouldn’t they be left in situ?
#41 Jim Sweeney
November 14, 2010

Re: the four parallel “tracks” and your recent comment, “let’s get things straight and admit up front that Godzilla is not a real animal, nor was it ever.”
Time to reconsider, perhaps?
Just saying…