René Hoffmann; PNAS December 21, 2021 118 (51) e2118894118

Paleontologists always seek the most complete reconstruction of their beloved study objects, e.g., dinosaurs, mammals, or extinct invertebrates like the ammonites. The latter are representatives of the cephalopods that populated the oceans for more than 340 My. Among modern cephalopods only Nautilus shares an external shell with ammonites, while the majority of modern cephalopods evolved an internal and often reduced shell. All shell-forming mollusks—such as cephalopods and gastropods, among others—share the same mechanism of shell formation, continuously adding material to the growth front by a specialized tissue called the mantle (1). This means that their complete ontogeny can be recorded in the shell. Unfortunately, the fossil record of mollusks is mostly limited to shells and much is unknown about their soft-tissue organization. Chirat et al. (1) present a physical model to look at mollusk soft-body organization and growth as manifested in shell form, which also informs us about the soft-body symmetry of the extinct ammonites.

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Finally, a few questions remain in light of this mechanical model. Why did pathological heteromorphs (Aegocrioceras, Pictetia, Proaustraliceras, and Ptychoceras) with nontouching whorls (figure 5 of ref. 20) not develop meandering or helicospiral shells, assuming a slight mismatch between growth rates was induced by traumatic events? Why does Spirula have a slightly helicospiral internal shell mismatch between the two growth rates resulting in compression of the soft body. A completely straight body would create very high stresses according to the model, and the energy-minimizing shell forms would be quite unrealistic. It is concluded by Chirat et al. (1) that these shells are unlikely to exist, but they do: juvenile Didymoceras, Baculites, Sciponoceras, Bochianites, Bactrites, many Paleozoic nautiloids, and scaphopods (Fig. 1B). If the occurrence of helicospiral shells is largely related to a mismatch between two growth rates, why does it occur only in 1% of all externally shelled cephalopods and within these forms so consistently? This model is an important step that will stimulate many interesting questions regarding the occurrences of asymmetry in mollusk and asymmetry in general, providing a new innovative computer-based approach.

See: https://www.pnas.org/content/118/51/e2118894118

(Fig. 1C)? The internal position of the shell should exclude a Figure: Helicospiral cephalopod shells. (A) Turonian Hyphantoceras reussianum from Halle (Northrhine-Westfalia, Germany). Both specimens are about 6.5 cm high; the upper specimen sinistral, the lower specimen dextral. (Scale bar, 1 cm.) (B) Virtual model of Campanian Didymoceras stevensoni from the Western Interior Seaway with a planispiral shell of the hatchling (not shown), straight to slightly bent juvenile shell parts, turning into a helicospiral shell that transitions into a meandering and finally planispiral shell part (both green). (C) Internal helicospiral shell of the mesopelagic coleoid cephalopod Spirula spirula. (Scale bar, 2 mm.)