Mesozoic deadfall communities - an ancient ecology

This post is about Mesozoic deadfall communities - think whalefalls, but marine reptiles!

The dead bodies of large aquatic animals have a big impact on the areas they settle in, to the point where they have created entire communities of creatures that solely depend on them in the modern day. These kinds of communities appear to have started in the Early Cretaceous. 

The most prominent identifier of these communities are Osedax worms, also called bone-eating worms or zombie worms. Osedax is a genus of tube worn which bores into bone using bacteria, rather than something like teeth, siphoning lipids and nutrients from the bones, leaving distinct marks that vary between species but are consistent in general morphology. Osedax have been shown to increase the short-term biodiversity of whalefalls (Lucas et al., 2017); as they break down the skeleton they create homes and resources for many other animals until they extract everything they can from the skeleton and it falls apart, being so effective at this that it's been suggested that their evolution may have negatively impacted fossil preservation (Danise and Higgs, 2015). Osedax have proven not to be picky when it comes to what kind of skeletons they inhabit, even latching on to cow bones that were placed on the seafloor as part of an experiment (Jones et al., 2007). Although currently best known today as whale specialists, their borings have been found on the bones of fish (Rouse et al., 2011), seabirds (Kiel, Kahl, and Goedert, 2011), and marine reptiles (Danise and Higgs, 2015).

The oldest confirmed traces of Osedax can be found on a plesiosaur humerus, pliosaur tooth, turtle rib, and shell plate, all discovered in Cenomanian deposits from the UK (Danise and Higgs, 2015). The presence of these marks implies the existence of entire deep sea communities. It has been suggested that these early deadfalls spurred the evolution of the first modern sulfophilic and hydrothermal vent ecosystems, although it has since been revealed to be a lot more complicated, and to vary by group (Georgieva et al., 2021).

The dead bodies of mosasaurs and plesiosaurs, of which we have specimens that bear Osedax borings (Jamison-Todd et al., 2024), would have been rich in lipids and other nutrients, fostering communities potentially made up of entirely different groups of weirdos (Holger, 1994). The KPG extinction should have completely wiped these communities out, but at least some of these creatures pulled through. The fact that Osedax fed on turtle bones is important, as it shows how these communities may have survived the end-Mesozoic extinction. When plesiosaurs, mosasaurs, and all the other major marine reptiles went extinct they relied on marine turtles and fish until the evolution of whales (Danise and Higgs, 2015). Modern experiments have shown that extant whalefall communities full took advantage of alligator remains (McClain et al., 2019).

This transitional period is in fact documented with examples of bone-eating limpets found on the shells of Eocene turtles (Marshall, 1994). Oligocene fossils show these communities forming on whale skeletons once nutrients were sufficient enough. 

Osedax currently speed up the removal of marine carcasses immensely, but they were not always there. There was a long time where that niche wasn’t filled at all, where those specialist communities did not exist. Instead, these fossils show a different and unique course of events that happened to marine reptiles after they died.

One Ophthalmosaurus skeleton, recovered from the UK Sandsfoot Formation tells a very interesting story. Marks on the bones come from scavenging by fishes, grazing by mollusks and echinoderms, and most interestingly, the anchors of reef-building organisms (Danise, Twitchett, and Matts, 2014). Another ichthyosaur, a Stenopterygius from the German Posidonia Shale, is surrounded by a wide collection of organisms including inoceramids, scallops, brittle stars, ammonites, and crinoids (Maxwell et al., 2022). These ichthyosaurs died in a shallow, off-shore area, and reached the floor seemingly near-intact. After death, their bodies were defleshed by fish. Cow shark teeth have been reported from the Jurassic (Serafini et al., 2024), while the teeth of Xampylodon and Notidanodon sharks have repeatedly been found in association with marine reptile remains. Isopods also would've aided in eating these carcasses, as shown by a specimen of Pachyrhizodus found covered in Brunnaega isopods from the Early Cretaceous of Australia (Wilson, Paterson, and Kear, 2011).

Specimens from the Rosso Ammonitico Veronese (RAV) of Northeastern Italy are littered with rhyncholites, calcified beak tips, were found associated with many carcasses, belonging to nautiloids. These rhyncholites may have been shed while feeding. These rhyncholites are far more prevalent than shark teeth in that deposit, suggesting that nautiloids were the primary scavengers of those deadfalls (Serafini et al., 2024)!

RAV specimens also suggest that belemnites may have spawned around the carcass; adults mated and laid eggs around these carcasses and then died, the larvae would then live around the deadfall for some time. An unpublished Clidastes specimen from the Campanian Pierre Shale was found surrounded by many Baculites and Holoscpahites juveniles (Ancient Hydrocarbon Seeps, 2022).

Once the bodies were stripped, microbes covered the skeleton; some extracted nutrients directly from the bones, while others gained energy from the sunlight. These microbial mats attracted invertebrates, which grazed freely. Urchins left star-shaped traces on the fossils. It is here where things get interesting, since they diverge from modern whalefalls.

Modern whalefalls are broken up into stages, starting off with the “mobile scavenger phase”, which consists of scavenging from fish and sharks (it seems that if this scavenging stage didn’t occur in ichthyosaurs, the body would bloat and return to the surface). This is followed by the “enrichment opportunist stage”, where animals feed on what is left behind. Mollusks, echinoderms, and crustaceans settle down and feed on the decomposing scraps or the ever-growing microbe communities. In whales, this grazing stage is followed by the “sulfophilic stage.” The sulfophilic stage is when microbes break down the lipids in the skeleton, which creates sulfide and methane; many modern organisms depend on this phase and the energy it produces. There is no evidence of this stage in these ichthyosaur skeletons; the skeletons immediately jump to the "reef stage", where colonies of reef building organisms anchored onto the skeleton, forming a proper reef. A single skeleton likely supported several generations of reef animals. 

RAV specimens show evidence of crinoid larvae embedding themselves in the actual skeleton, while Posidonia specimens, like the one above, are surrounded by reef-builders (Serafini et al., 2024).

This stage is very poorly documented in whalefalls, sometimes not even recognized; many whalefalls end with the skeleton being buried, never seen again. In general, the deadfall communities of the Jurassic (and Triassic) would not have been specialists, or even very diverse, judging by the lack of these bacteria and their communities, but the effects of that dead individual would be felt for way longer. One can only imagine the things that would appear on the skeleton of a Pliosaurus or Leedsichthys.

These deadfall reefs seem to have persisted through the entire Triassic and Jurassic, but once Osedax evolved it was game over.

Osedax's emergence occurs right alongside the earliest chemosynthetic communities associated with fossils. These fossils are of provannid gastropods and lucinid bivalves, found associated with a pair of plesiosaur skeletons (Kaim et al., 2008) as well as a leatherback turtle carapace (Jenkins et al., 2017) in Japanese rocks from the Middle Cretaceous. Provannid snails are specialists of fleeting ecosystems; they thrive in hydrothermal vents, cold seeps, and most relevantly, whalefalls. The plesiosaur’s bones show evidence of microbial mats made up of sulfide-producing bacteria quite similar to those that inhabit modern whalefalls, which would have been prime grazing patches for provannid snails. The animals associated with these two skeletons also reveal that these Mesozoic deadfall communities were not assembled the same way modern whalefalls are.

Vesicomyid bivalves that define the sulfophilic stage today are absent from these sites, but also from most methane seep and vent deposits from the mid Cretaceous; at this point there were only transitional forms in shallower waters. Lucinid bivalves filled their place. Fossils that provide this kind of insight are very lacking as most deep sea marine sites come from anoxic areas. 

With that said, there are a handful of other individual specimens that preserve traces of deadfall communities, likely from individual instances where the conditions were just right for supporting life. Ornithomimosaur remains (Brownstein, 2018) and Cretoxyrhina specimens (Amalfitano et al., 2019) preserve traces of invertebrates, while a specimen of “Prognathodon” waiparensis was found covered in barnacles post mortem (Buckeridge, 2011), and multiple Mosasaurus vertebrae from the Maastricht formation have been found with marks from urchins on them (Jagt et al., 2020): one specimen of Mosasaurus from Alabama, called the Braggs Mosasaur, had several urchins in its skull (Bryan, 1992).

Even with these monumental specimens, there are many, many unknowns in terms of Mesozoic deadfalls. How did they start, and why? How did they link to the evolution of Mesozoic chemosynthetic ecosystems? How truly diverse were they, and which of those animals survived the KPG? It’s possible that we’ll never know those answers, but equally as likely that important specimens will be uncovered that illuminate these fascinating ecosystems.

Osedax image from –
https://www.montereybayaquarium.org/animals/animals-a-to-z/whale-worm

Papers can be read here -
https://drive.google.com/drive/u/3/folders/1k93ashotZkFWgOfssen_79C2UV18YAbj