Ammonoidea, Zittel, 1884

Webster, Nicole B & Vermeij, Geerat J, 2017, The varix: evolution, distribution, and phylogenetic clumping of a repeated gastropod innovation, Zoological Journal of the Linnean Society 180 (4), pp. 732-754 : 744-745

Ammonoidea

Many ammonoids have periodic constrictions – grooves in the shell that appear to cut through other ribs and are sometimes associated with distinct shell thickenings ( Arkell, Kummel & Wright, 1957). Shell thickenings are also found as internal ridges or pseudo-constrictions, without external structures ( Westermann, 1990; Bucher et al., 1996). These periodic shell thickenings are thought to indicate episodic growth and have occasionally been called varices, but are generally called constrictions ( Seeley, 1865; Moore, Lalicker & Fischer, 1952; Bucher et al., 1996). Many constrictions clearly share ontogenetic similarity with gastropod varices and could be termed varices. All ammonoid constrictions are regularly spaced – although not necessarily synchronized between whorls – with a set number of interribs. This is almost certainly due to the septa and sutures present in ammonoids, but not in gastropods. Another major form of ammonoid shell sculpture similar to varices is megastriae, distinct radial elements that include a discontinuity in shell secretion ( Bucher & Guex, 1990; Bucher et al., 1996). The most intriguing type of megastriae are the parabolae of Phylloceratoidea, Lytoceratoidea, Perisphinctoidea, and the simpler flares of some Lytoceratoidea ( Radtke, Hoffmann & Keupp, 2016). Although in most cases these structures are worn smooth, they were often large, sometimes undulating or spiny extensions of the shell. Cross sections of the shell reveal a discontinuity, a presumed hiatus in growth, and share many features in common with varices, especially those of muricids ( Radtke et al., 2016). Interestingly, it appears that these apertural flares were sometimes resorbed altogether, with the outer most edge falling off as an intact ring of shell, as seen in Lytoceras ( Seilacher & Gunji, 1993; Radtke et al., 2016). Megastriae are not generally synchronous, but may relate to periods of rapid growth ( Bucher & Guex, 1990). Just like many muricids, juvenile megastriae are not as complex and may not clearly fit the definition, as they show an ontogenetic progression.

In at least some cases, both ammonoid constrictions and megastriae share sufficient similarities with our definition to be considered varices, although an exhaustive search, and perhaps a much better understanding of ammonoid shell growth, would be necessary to firmly establish where the boundaries lie.

Some clear varix examples include most members of Ancyloceratidae (E. Cretaceous) showing periodically thickened ‘major’ ribs interspersed with smaller ribs ( Arkell et al., 1957). Acantholytoceras longispinus (Uhlig, 1883) (E. Cretaceous, Barremian; Lytoceratidae ) has some striking spines on each constriction, although tubercles are found on the constrictions in a few other genera, and in Hyphantoceras (L. Cretaceous, Turonian-Santonian; Nostoceratidae ), the constrictions are thin and flaring ( Arkell et al., 1957).

Several families of Ammonitina with periodic ‘major’ ribs fit the varix pattern, such as Puzosiinae (Hauterivian-Maastrichtian), Holcodiscidae (E. Hauterivian-L. Barremian), and Cheloniceratinae (Barremian-L. Aptian). Several of these ammonite groups show loose or irregular coiling in contrast to gastropods, where no varices are known among groups with loosely coiled shells.