The ichthyosaurs of Holzmaden, Germany, are among the most exquisite fossils ever discovered. Many of their skeletons have been left in stunning, articulated detail, and the very outlines of their ichthyosaurian bodies were preserved by the bacteria which decomposed their carcasses after the marine reptiles had settled to the bottom of the Jurassic sea. It was specimens like these, which clearly showed the outlines of dorsal fins and tail flukes supported by downward-kinked tails, that caused paleontologists to see ichthyosaurs are swift, reptilian tuna mimics and not the slow, paddle-bearing lizards envisioned by 19th-century naturalists.
But what happened between the time the Holzmaden ichthyosaurs died and when they sank to the ocean bottom? Did the predators fall through the water column as soon as they died, or did they float until the gases from decomposition exploded their bodies and scattered their bones? That is what paleontologists Achim Reisdorf and co-authors consider in a new paper given one of the more evocative titles in recent memory, “Float, explode or sink: postmortem fate of lung-breathing marine vertebrates.”
The problem of how ichthyosaurs went to pieces is difficult to approach. For one thing, the marine reptiles died out tens of millions of years ago — ichthyosaurs swam the world’s oceans from 245 million to 90 million years ago, disappearing before the end-Cretaceous extinction rocked the world. Direct observation is right out. And ichthyosaurs don’t have any close living relatives among the reptiles, nor has any reptilian lineage converged on the ichthyosaur body plan.
Due to their roughly similar body plans, modern whales are often taken as ichthyosaur proxies. Of course, there are some significant differences between the streamlined marine predators. Among them is that whales have blubbery bodies, a condition that aids the cetaceans in staying buoyant, while ichthyosaurs lacked blubber. How ichthyosaurs stayed heavy enough to remain submerged but light enough to prevent themselves from sinking is a mystery, although some paleontologists have hypothesized that ichthyosaurs had bodies that were less dense than the surrounding seawater. This would have influenced what happened to the marine reptiles after death. Instead of sinking, a putrefying ichthyosaur carcass would float along the surface as gases from decomposition built up inside. Sooner or later, the marine reptile would explode and plummet to the bottom. When the ichthyosaur busted would determine how much of the skeleton would be preserved — specimens which popped earlier would be more intact than those that had drifted, dropping bones, for longer periods.
But Reisdorf and co-authors doubt that such putrid ruptures account for the specimens at Holzmaden, which initially inspired the explosion hypothesis decades ago. For one thing, a considerable amount of pressure would be required to inflate an ichthyosaur’s body to the point of popping. Based upon the pressure buildup recorded in human cadavers and dead goats, the researchers suggest, it seems doubtful that a dead ichthyosaur would have filled with enough gas to be in danger of bursting. Reisdorf and collaborators point to a different set of circumstances for the preservation of the Holzmaden ichthyosaurs.
A newly-deceased ichthyosaur probably sank rapidly. Despite ideas that these reptiles would have been relatively buoyant due to a decreased body density, Reisdorf and colleagues point out that most whales are actually negatively buoyant and rapidly sink after death. The same would have be true for ichthyosaurs. (In fact, right whales and sperm whales were often specifically targeted by whalers because these slow-swimming cetaceans contained such high levels of oil and fat that they were more likely to float than other whale species.) And while it would have been physically possible for a dead ichthyosaur to resurface after death – if the water were shallow enough and warm enough to promote the bacterial growth which would have generated gas buildup – the conditions in the Jurassic German seas were deep and cold enough that ichthyosaurs sank without ever coming back to the surface. There is no indication that these ichthyosaurs went down with a bang.
After reaching the bottom, the ichthyosaur carcass would have eventually been denuded of flesh. Bacteria probably did a lot of the dirty work — the sediments which preserve the ichthyosaurs indicate a low-oxygen environment where few creatures could survive. Once the skeleton of the reptile was exposed, relatively gentle currents disarticulated parts of the body without eroding the mud which began to encase the bones. Ribs and vertebrae were shifted while heavier bones mostly stayed in place. This vision of Mesozoic death, Reisdorf and collaborators write, could be termed an “ichthyosaur fall” — a scenario roughly analogous to what happens to modern whales when they sink to the deep sea.
There may have been “croc falls,” too. A paper by paleontologist Susan Beardmore and co-authors in the same issue of Palaeobiodiversity and Palaeoenvironments considered the marine Jurassic crocodyliform Steneosaurus, which is found in the same geologic formation as the ichthyosaurs. It seems that dead crocs went through a similar series of events between death and burial as the ichthyosaurs. When a Steneosaurus died, the animal apparently sank to the bottom very quickly and began to decompose. Currents swept away many of the smaller, lighter skeletal elements while leaving heavier parts — especially the skull — in place. This scenario may also explain why small specimens of the croc are so rare. Much like the often-disarticulated baby ichthyosaurs, small Steneosaurus may have been buffeted by currents until they were scattered over the seafloor.
The ichthyosaurs and crocs found in Germany’s Posidonienschiefer Formation were entombed in relatively low-oxygen environments where there were few organisms to scavenge the carcasses. But similar Mesozoic deadfalls in other places may have played a significant role in the formation of those ephemeral deep-sea communities which glom onto dead whales and other fleshy bonanzas. In 1994, around the time that marine biologists were taking an intense interest in the organisms which sprang up to deconstruct dead whales, paleontologist Jennifer Hogler considered the Mesozoic equivalents to whalefalls in a PALAIOS paper titled “Speculations on the role of marine reptile deadfalls in Mesozoic deep-sea paleoecology.”
Hogler noted that the question of what happened to marine reptiles after they sank to the ocean bottom had not received much study — more attention had traditionally been paid to ichthyosaurs, marine crocodiles, mosasaurs, and plesiosaurs as apex predators rather than worm food. But paleontologists may have been documenting evidence of Mesozoic deadfalls without knowing it. In situations of good preservation, fossils of possible scavengers have often been found around the bodies of marine reptiles. (In fact, pockets of gastropods and other possible scavengers are sometimes a sign that there are remains of a large marine reptile close by in the deposit.) These scavengers range in size from tiny, armored amoebas called foraminiferans to the tentacled, coil-shelled ammonites and cow sharks.
How different organisms utilized marine reptiles depended on the state of the carcass. When the dead marine reptiles were still covered in flesh, sharks and cephalopods probably picked at the body. Once denuded of soft parts, though, the reptile’s skeleton could have been a refuge for various encrusting and burrowing organisms (although, as far as I am aware, no one has yet found evidence of bone-burrowing snotworms among Mesozoic marine reptile skeletons). Fine-scale field investigations are required to further investigate this hypothesis, but Hogler made a reasonable case that marine reptile deadfalls may have been ecologic precursors to modern whalefalls. Perhaps some of the organisms which congregate to dismantle whales today are the descendants and relatives of creatures which used to greet the carcasses of mosasaurs, ichthyosaurs, and other Mesozoic sea dragons.
Beardmore, S., Orr, P., Manzocchi, T., & Furrer, H. (2012). Float or sink: modelling the taphonomic pathway of marine crocodiles (Mesoeucrocodylia, Thalattosuchia) during the death–burial interval Palaeobiodiversity and Palaeoenvironments DOI: 10.1007/s12549-011-0066-0
Hogler, J. 1994. Speculations on the role of marine reptile deadfalls in Mesozoic deep-sea paleoecology. PALAIOS. 9 (1), 42-47
Reisdorf, A., Bux, R., Wyler, D., Benecke, M., Klug, C., Maisch, M., Fornaro, P., & Wetzel, A. (2012). Float, explode or sink: postmortem fate of lung-breathing marine vertebrates Palaeobiodiversity and Palaeoenvironments DOI: 10.1007/s12549-011-0067-z