Life-cycles of some Devonian ammonoids
Article first published online: 2 JAN 2007
Volume 34, Issue 3, pages 215–233, September 2001
How to Cite
Klug, C. (2001), Life-cycles of some Devonian ammonoids. Lethaia, 34: 215–233. doi: 10.1111/j.1502-3931.2001.tb00051.x
- Issue published online: 2 JAN 2007
- Article first published online: 2 JAN 2007
- 15th September 2000, revised 2nd March 2001.
Whorl expansion rates of six representative ammonoid genera from late Emsian and Eifelian strata of Morocco were calculated for each whorl. The corresponding body chamber lengths and the orientations of the apertures were computed based on these values. The resulting body chamber length and orientation of the aperture graphs were compared with other conch features, ecology of Recent cephalopods, and sedimentological data of the host rocks in the Tafilalt (eastern Anti-Atlas, Morocco). A subdivision of the ontogeny of these ammonoids was achieved comprising the early and late embryonic periods, the juvenile period, the preadult, and the adult growth period. All growth periods are defined by specific changes in growth, conch morphology, and mode of life. According to this reconstruction, hatchlings were probably already capable of active movements. Differentiation in two main modes of life of the examined taxa occurred in the late juvenile or early preadult period. As preadult animals, most of the Mimagoniatitoidea and Agoniatitoidea became active swimmers (Nektonic), whereas the representatives of the Anarcestoidea were capable of slow movements only (Planktonic). As adults, most representatives of the three superfamilies had an approximately horizontally oriented aperture, allowing active swimming and possibly active choice of spawning sites. Additionally, the new ammonoid taxon Rherisites tuba gen. nov., sp. nov. from the late Emsian is introduced.