Nautilus —a poor model for the function and behavior of ammonoids?

Abstract

Inferences drawn from the biology, function, and behavior of closely related living forms facilitate interpretation of the mode of life of groups known only from the fossil record. The choice of phylogenetically relevant modern ‘model organisms’ can have critical bearing on the resulting interpretations. The biology and behavior of fossil ammonoids are often interpreted in the light of evidence derived from the study of modern Nautilus. However, examination of the fossil record and cladistic analyses both indicate that coleoids are much more closely related to ammonoids than is Nautilus. Coleoid biology and behavior differ dramatically from the biology and behavior of Nautilus. Thus, the inclusion of coleoids as examples, rather than reliance on Nautilus alone, produces a strikingly different vision of ammonoid biology and suggests that inferences of ammonoid biology and behavior that rely exclusively on Nautilus should be reviewed. Two features related to swimming ability in Nautilus, static stability and large retractor muscles, are much reduced in many ammonoids, leading to the interpretation that ammonoids were poorer swimmers than Nautilus. However, reexamination of the evidence indicates that static stability should not play a role in the swimming of ammonoids with long body chambers. In addition, functional arguments suggest that a coleoid‐like swimming mechanism should have evolved prior to the loss of the body chamber in coleoids. Thus, a coleoid‐like swimming mechanism is likely to have evolved prior to the separation of ammonoid and coleoid lineages. A mechanism is proposed by which a coleoid swimming mechanism, independent of retractor muscle size, could function in ammonoids with long body chambers.□Ammonoids, ammonites, evolution, functional morphology, Nautilus, phylogeny.

Ammonite habitat revealed via isotopic composition and comparisons with co-occurring benthic and planktonic organisms

“…However, these studies have had limited success for both biological and geological reasons. Ammonites are extinct, and their closest living relatives, the octopods, squids, cuttlefish, and Nautilus, are all in different orders/subclasses (14). Even among living cephalopods, a variety of behaviors are observed, including vertical and lateral migrations (15).…”

Wrinkle layer and supracephalic attachment area: implications for ammonoid paleobiology

“…To solve this problem, some authors have suggested that ammonites could have had a split hyponome with one opening on each side of the rostrum (Schmidt 1930, Trueman 1941, Westermann 2013. Other authors even speculated about the possible presence of fins in ammonoids (Jacobs & Chamberlain 1996, Andrew et al 2011). However, double or twin-nozzled hyponomes have never been found in any modern or extinct cephalopods.…”

“…In fact, not all researchers were confident of the correctness of the usage of existing calculations with upward oriented apertures for reconstructions of the ammonoid mode of life and its swimming position, although the majority agrees on that. For example, Jacobs & Landman (1993) stated that the assumption of the entire body of the ammonoid being contained within the body chamber permanently was unwarranted. They assumed that many ammonoids were capable of mantle extension and could have controlled the orientation of the shell to some degree.…”

“…Mutvei & Reyment (1973) suggested the presence of a mantle cavity with long gills in the ammonoid soft body. Jacobs & Chamberlain (1996) noted that if ammonoids had large mantle cavities, comparable in size with coleoid mantle cavities, their body chambers might not have been completely filled with soft tissues. When active, the mantle cavity must have been filled with water for ammonite respiration (Fig.…”

“…Thus, the ancestors of coleoids likely had no hood and ammonites inherited ten arms from their ancestors but did not inherit a nautilid-like hood. It is unlikely that ammonoids would have independently and completely repeated the evolutionary path of the Nautilida and developed such a complex organ with regard to their differences from nautilids (Jacobs & Landman 1993).…”