ECOLOGY OF LATE MESOZOIC HETEROMORPHIC AMMONITES: A CASE FOR ALGAL SYMBIOSIS?

Once touted as inadaptive products of “racial senility”, the heteromorphs curiously depart from the typical planispiral form of ammonites and occupy a radically divergent range of shell morphologies. Some resembled hairpins, others snails or even worms. Late Mesozoic heteromorphs, which include the ancylocones and hamiticones, probably arose suddenly through a single mutation and have been subject to a number of ecological interpretations. The recognition that algal symbiosis is widespread among bivalves and gastropods suggests that perhaps other molluscan stocks, for instance these aberrant ammonites, also served as hosts for photosynthetic unicellular algae. To benefit from this mutualistic relationship, such hosts obviously require tissues that are exposed to sunlight. Although ontogeny controls life position, functional morphology strongly indicates that the terminal aperture of adult ancyloconic and hamiticonic shells were oriented upward towards the ocean surface. These openings conceivably sported a radial fan of delicate filtering tentacles, which may have been adapted both for ensnaring plankton and providing the extensive surface area necessary for efficient algal photosynthesis. Several species possess structural features characteristic of an internal or semi-internal shell, allowing for increased mantle exposure. Indeed, most reconstructions of these heteromorphs argue that they were shallow-water vertical migrants living in the epipelagic zone, well within the bathymetric depths at which light penetrates. While ancylocones and hamiticones were not exclusively restricted to tropical latitudes, they were apparently more abundant and competitive in oligotrophic habitats. And though massive, robust skeletons and rapid calcification rates are usually associated with benthic hosts, such as reef-building corals, giant heteromorphs are known from the fossil record. Lastly, geochemical evidence from the Cretaceous hamiticone Polyptychoceras reveals a significant inverse correlation between δ¹⁸O and δ¹³C stable isotope values (‰ VPDB), which is consistent with the hypothesis of photosymbiosis.