Sinking Ornithoscelidians: Sitting Ducks, Water Chevrotains & Ceratopsid Death Beds

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River Bottom Running Ankylosaurus created for this blog piece by Robin Liesens

Dinosaurs back in the water and it feels sooo good… There are probably fewer narratives, memes, and dialogues as storied as the on and off again love affair between dinosaurs and the water in both scientific and popular forums. From pre-renaissance snorkeling duckbills and bottom walking brontosaurs to fanciful water world assertions that all dinosaurs were aquatic to bona-fide actual aquatic dinosaurs such as Spinosaurus the concept of dinosaurs in the water is one that is not soon to leave us. Dinosaurs sloshing, mucking, wallowing, foraging, and hiding in the water is an evocative suggestion because such depictions are not without analogue in many modern terrestrial tetrapods and large mammals in particular.

It is a contention of mine and this piece that just because an animal is not explicitly aquatic does not negate the potential for water to still shape and inform significant aspects of its biology and anatomy.

credit Burian

The opening depiction of a bottom running Ankylosaurus, kindly provided by Robin Liesens (Dontknowwhattodraw94), represents what I view as a logical culmination of the ornithischian body plan with relation to water. Ankylosaurus was quite possibly a sinker not a floater. An evolutionary arms race in ornithischians that gave this branch of dinosaurs especially thick, dense skin to protect from both theropods and intraspecific combat; an osteoderm studded skin; lack of skeletal pneumaticity; thick bones.  What this creates is the distinct possibility that ankylosaurids and perhaps other "dense" ornithischians - especially among thyreophorans and marginocephalians - did not actually float in the water as humans, birds, and most terrestrial mammals do but would sink right to the bottom like hippos, tapirs, and water chevrotains.

Bottom punting animals have a rich history here at antediluvian salad: bottom punting Spinosaurus is the ultimate conclusion I reached regarding the most likely aquatic propulsion for that animal. A necessary caveat of a good bottom punter that utilizes body density (as opposed to changing/shifting lung volume like aquatic turtles & crocodiles) is that actual swimming ability decreases as punting ability - and therefore body density - increases. This creates the paradoxical situation in hippos where they can not actually swim - hippos can not cross deep bodies of water because they sink like a stone. The same situation may have occurred in several ornithischians with thick dermis - not necessarily because they were semi-aquatic like hippos - but because they had embarked on a social and defensive strategy of thick skin, armor, and dense non-pneumatic skeletons. Body density and therefore punting ability and the inferred possible loss of actual swimming ability is an interesting side effect of their particular defensive anatomical pedigree.

All of this discussion should be couched in the recent elucidation (not withstanding a rebuttal) of a revelatory linkage of theropods and ornithischians into ornithoscelida (Baron, 2017). One of the most interesting questions that this linkage of neo-theropods and ornithischians raises is "why would ornithischians have lost the highly pneumatic (and presumably air sac filled) skeletons that most likely was ancestral to both ornithischians and theropods?"

Several commentators on the Tetrapod  Zoology post on ornithoscelidia approach this question and suggest that the ability to bottom feed on aquatic vegetations is a potential reason. I agree, but I would embellish this reasoning with a more dire and immediate consequence: ornithoscelidians did not want to be sitting ducks.

Sitting Duck: A person or thing with no protection against an attack or other source of danger.

Theropods we know were full of air - it is no stretch to imagine that, when immersed in water, they would have floated like a buoy. Maybe not sitting so high in the water as modern ducks but possibly with much of the head, neck, and even some of the back out of the water. This buoyancy would have made theropods excellent patrollers, explorers, and navigators of aquatic environments. It also may have helped in dispersal situations and survivorship of catastrophic aquatic inundations such as tsunamis and the relatively frequent storms surges, tidal inundations, and hurricanes that we should expect in hot house climates.

The floatability in water made diving a little bit difficult but not impossible: for theropods wanting to get under the water they simply point their nose in the direction that they want to go in and enact rear propulsion from the legs and tail. One of my favorite observation recorded of opportunistic hunter/scavenger theropods is of giant petrels (not known for diving abilities) repeated going underwater to scavenge a dead wedded seal.

creditJohn Van Den Hoff & Kim Newberry 2006

This digression also allows me to finally get rid of a piece of art I did quite a while back. Some opportunistic Coelophysis diving to scavenge a dicynodont at the bottom of some recent flood waters. Take note of the opportunistic pterodactyloids buzzing in for dislodged scraps.

Diving For Coins Coelophysis by Duane Nash

What is good for the goose is not necessarily good for the gander. While sitting high in the water may have been a benefit for wide ranging, opportunistic theropods sitting high up in the water for prey animals like ornithischians may have been fatal. And when we look at the earliest ornithischians they were not gigantic, heavily armed, or especially refined cursors. Their niche may have best been approximated by large rodents, small forest dwelling deer and antelope, and water chevrotains (Hyemosuchus aquaticus). It is water chevrotains I want to pay special attention to as here is a small herbivore that utilizes the water to good effect to literally vanish from predators. In the clip below the predator happens to be a modern predatory theropod , the crowned eagle. Also wonderfully narrated by Mr. Honey Badger himself:

Water chevrotains are noted for a thick, dense padding of skin along the rump and around the neck. Skin is actually the largest organ in the body and plays a little heralded but profound role in buoyancy as I discussed in bottom punting Spinosaurus. Readers should note that two other bottom punting specialists - tapirs and hippos - have thick skin and sink to the bottom.

It is quite possible that the earliest ornithischians quickly and resolutely diminished their system of air sacs and pneumatic condition - that in conjunction with thick and sometimes armored skin - allowed them to sit lower in the water. Analogous to water chevrotains and other small mammals that utilize the aquatic medium for concealment these earliest ornithischians set in motion a trend of aquatic concealment that - to greater or lesser degrees - likely persisted throughout their tenure with ornithopods like Thescelosaurus. There is no way to tell at this point where various ornithischians sat in the water - if they achieved true negative density like water chevrotains or if they just sat with more of their body submerged and that was good enough. But the point remains that such animals would have benefited cryptically by sitting lower in the water and dispensing with air sacs.

credit Duane Nash Thescelosaurus hiding underwater from topside threat

But if we assume that the baseline condition for ornithischians was to sit low in the water, when we add density in the form of scutes, osteoderms, thick skeletons, and heavy skulls then we have the likely potential for true bottom-punting, negative buoyancy situations to develop. Among both marginocephalians and thyreophornas we see many likely candidates that may have trended towards  negative buoyancy in these animals.

Water Sports by Duane Nash

For the most part I think bottom punting would have served these animals just fine. They still likely moved with grace and efficiency through bodies of water - they just did not swim in the strictest sense of the word. Moving through relatively shallow streams, rivers, ponds, sloughs, estuaries less than 5 meters deep or so not a problem and waters of this depth would have been what these animals came across in their day to day existence.  Indeed a ceratopsid could easily have been prone to flee into the water when chased by a theropod. The theropod in grave danger from a bottom punting ceratopsid as it floats vulnerable at the surface, its belly exposed to horn thrusts.

However in deeper waters that they could not get up to the surface in that they found themselves in trouble. I am specifically alluding to the ubiquity of ankylosaurid skeletons recovered from oceanic sediments - were these animals taken by sudden intrusions of sea water that put them in waters too deep to kick off the bottom to the surface in? And then you have the infamous ceratopsid mass bone beds, most notably from Centrosaurus that most likely document tremendous inundations of the ocean onto land. Researchers have struggled to explain why Centrosaurus - and pretty much nothing else but Centrosaurus - succumbed to these inundations. The wikipedia web page on the Hilda mega-bonebed summarizes a lot of the work on this topic.

The ubiquity of ceratopsids in these death assemblages is potentially explained by a negative buoyancy for these animals. All the theropods and hadrosaurs in the environment would float away. Ceratopsids were doomed in deep water.

Is there any independent evidence pointing us in the direction of thick skin in ceratopsids? By golly yes there is. Before the world fell in the love with the preternaturally adorable Psittacosaurus soft tissue restoration there was some other work on Psittacosaurus dermis (Lingham-Solia, 2008). What was revealed in the study is that the Psittacosaurus revealed a cross section cut out that had a remarkable density of 40 collagen layers and a skin thickness of .8 inches - which does not sound like a lot but is pretty astonishing for such a small animal. Unfortunately the author behind this work used these multiple layers of collagen to argue that not only was Psittacosaurus not "feathered" - which it wasn't of course - but that the feather impressions recorded on theropods are actually layers of collagen peeling off. The study was pirated by B.A.N.D.I.T. notions and what could have been a perfectly interesting document on exceptional skin thickness got turned into something that it shouldn't have. But let's not throw the baby out with the bath water here. The paper still shows a startling level of skin thickness - among the highest recorded in any vertebrate.

Again, let me hammer that point home, "multiple layers of collagenous fibres in excess of 25, among the highest recorded in vertebrates". There was an arms race in the Mesozoic, or more like it there was a skin race. Prey species attempted to evolve the thickest most durable hide they could to thwart those pesky theropods who more than kept pace in evolving forms more adept at sawing or crushing through thick and armored hides.

There has been some research into this topic of ceratopsid swimming and buoyancy (Henderson, 2014). The work concluded that ceratopsids sat in the water with their heads submerged. On the other hand hadrosaurids were much better natural swimmers with their head above water, hence the lack of hadrosaurids succumbing to such oceanic intrusions. This study points us in the right direction but it left out two things; 1) It did not account or even mention skin - a particularly dense skin may have sunk ceratopsids right to the bottom; 2) Pretty much any large tetrapod that lives in and around bodies of water has at least some capacity to move through bodies of water. Ceratopsids living on coastal Laramidia - especially bordering the western interior sea - would have have come across water all the time - lagoons, estuaries, swamps, tidal channels, rivers. It is absolutely non-sensical that an animal that comes across bodies of water daily has no way to efficiently move through them. Long time readers know that this is a constant theme I iterate on this blog: animals have to make sense. They have to reasonably move through their environment; eat; protect themselves; mate. When you conduct research that points you in the other direction - towards a maladaptive animal ill-equipped to deal with day to day encounters (such as bodies of water) - there is probably something wrong with your scenario. Such an animal, so maladapted to it's environment, would be selected for extinction and not leave a fossil record. Whenever you see a paleontologist utter the phrase "failed evolutionary experiments" run away quickly.

Now some have taken the mantra "most every tetrapod can swim" a little too literally:  I would add the caveat: "most every tetrapod can swim and/or bottom punt". Sinking ceratopsids right to the bottom would at least allow for movement through >most< bodies of water. For a large animal that can bottom punt it can still go through potentially fairly deep bodies of water, at least most bodies of water it will encounter on a daily basis. The problem is when it encounters bodies of water too deep to kick off of the bottom of to reach the surface. Large deep rivers, lakes, and extensions of the ocean can act as barriers to such animals. Or a catastrophic inundation of the ocean. That would be a big problem for negatively buoyant animals.

Bottom punting ceratopsids offers a potential explanation for why these animals were selectively killed while no other dinosaurs suffered comparable losses during large oceanic intrusions. It is not that ceratopsids could not move through water - they could quite well as a bottom punter - it is that in deep water they sank like a stone. It must have been horrific for them when the sea took over the land.

And this speculation is where it gets really cool. Because once you have a geographic barrier thwarting travel - in this case deep water - then you have a potential cause for speciation events. And we all know how wild with diversity ceratopsids on Laramidia got...

Papers

Baron MG, Norman DB, Barret PM (2017) A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature March 23, 2017

Henderson, D. 2014. Duck Soup: The floating fates of hadrosaurs and ceratopsians at Dinosaur Provincial Park, in Eberth, D. and Evans, D. (eds). Hadrosaurs. Bloomington: Indiana University Press. pp. 459-466

Lingham-Soliar, Theagarten. 2008. A unique cross-section through the skin of the dinosaur Psittacosaurus from China showing a complex fibre architecture. RSC Proceedings of Biological Sciences 2008 April 7. 275(1636) 775-780. online

Van Den Hoff, J & Newberry, K. 2006. Southern giant petrels diving on Macronectus giganteus diving on submerged carrion. Marine Ornithology 34: 61-64. online


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