Morphodynamics

Ammonoid Septa and Sutures

“…4. The Cartesian Diver Model (Seilacher and Labarbera 1995;Seilacher & Gishlick 2015) assumes that the last septum remained uncalcified until the beginning of the next chamber cycle; in that interval, the space behind the septal mantle might have been filled with gas and functioned like a swim bladder. The enclosed gas volume could be adjusted by sutural muscles spanning across the septal mantle to change buoyancy.…”

The taxonomic status of the genus Hubertoceras Spath: A new light on sexual dimorphism from the Callovian ammonites of Kutch, India

“…Sources are: Waagen, 1875;Spath, 1931;Collignon, 1958;Callomon, 1993;Dutta and Bardhan, 2016. nites who lived in shallow waters (as was the Callovian Kutch basin; see Fürsich and Oschmann, 1993) and were nectobenthos (cf. Kennedy and Cobban, 1976;Batt, 1989;Tsujita and Westermann, 1998) and sluggish to moderately mobile (also see Ward, 1981;Seilacher and Gishlick, 2015). They were also strongly dimorphic whose microconchs were evolute, lappeted and having bifid ribs like Hubertoceras.…”

“…This ammonite community faced competition for food, habitat and mates (Bardhan and Chattopadhyay, 2003;Jana et al, 2005). The functions of lappets have been variously interpreated (fabricational noise, Seilacher, 1974;Seilacher and Gishlick, 2015; change of habitat or behaviour, Westermann, 1990;Davis et al, 1996;Klug, 2001; defence against predation or intersexual cannibalism, Keupp and Riedel, 2010;sexual characters, Landman et al, 2012). Many workers, however, believed its utility to facilitate mate recognition system among other functions (Kennedy and Cobban, 1976;Jana et al, 2005;Bardhan et al, 2010;Keupp and Riedel, 2010).…”

Developmental bias, macroevolution, and the fossil record

“…However, at least 95% of the multicellular species that ever lived are extinct (Sepkoski, 1992), and among those extinct taxa are many unexpected forms. The fossil menagerie is almost endless but includes echinoids with periscope-like test extensions, tightly coiled and helical oysters, uncoiled, helical, and spiny nautiloids, sharks with coiled tooth batteries, six-fingered tetrapods (basally and with a second derivation in ichthyosaurs), crocodilians with bulbous shell-crushing teeth, and giant ground sloths including aquatic ones (for sources see Amson, Argot, McDonald, & de Muizon, 2015;Caldwell, 2002;Coates & Clack, 1990;Gale & Smith, 1982;Oyston, Hughes, Wagner, Gerber, & Wills, 2015;Seilacher & Gishlick, 2014;Tapanila, Pruitt, Wilga, & Pradel, 2018;Vermeij, 2015;Westermann, 1998). The fossil menagerie is almost endless but includes echinoids with periscope-like test extensions, tightly coiled and helical oysters, uncoiled, helical, and spiny nautiloids, sharks with coiled tooth batteries, six-fingered tetrapods (basally and with a second derivation in ichthyosaurs), crocodilians with bulbous shell-crushing teeth, and giant ground sloths including aquatic ones (for sources see Amson, Argot, McDonald, & de Muizon, 2015;Caldwell, 2002;Coates & Clack, 1990;Gale & Smith, 1982;Oyston, Hughes, Wagner, Gerber, & Wills, 2015;Seilacher & Gishlick, 2014;Tapanila, Pruitt, Wilga, & Pradel, 2018;Vermeij, 2015;Westermann, 1998).…”

“…However, the first inequivalved taxa appeared in the Early Ordovician during the initial diversification of the group (Cope & Babin, 1999), and this dissociation has occurred repeatedly in multiple lineages since then, with shifts in shell geometry not seen in extant forms (Seilacher & Gishlick, 2014). To take just one direction, the evolutionary dissociation of right and left valves to break bilateral symmetry has produced a wide array of extreme morphologies, many of which are extinct.…”