REMARKS

Umiaites belongs to the subfamily Spiticeratinae, which also includes other genera including Spiticeras and Proniceras. Spiticeratinae are found in Indo-Madagascar, Caribbean provinces, and in south and central Europe (Wright et al. 1996). Umiaites is so far known only from Kutch and is considered to be endemic. We have already mentioned the close correspondence between adult Proniceras and intermediate-sized Umiaites. Proniceras is a small genus and distributed in all major faunal provinces except the Andean (Arkell et al. 1957). Imlay (1939, pl. 18, figs. 5 and 6) reported one species from Mexico, P. scorpionum which bears an unmistakable lappet. As the microconch affinity is established, the possible macroconch requires investigation.

Dimorphism is not well known in Spiticeratinae, although in the descendant subfamily Olcostephanitinae it has been firmly established (Wright et al. 1996). Both in Olcostephanus Neumayr, 1875 and Jeannoticeras Thieuloy, 1965 microconchs are smaller, lappeted and strongly ornamented, with notably strong primary ribs. Secondaries in Olcostephanus microconchs are three to four in number, whereas they are numerous in macroconchs.

In Spiticeratinae, genera like Negreliceras Djanelide, 1922 and Kilianiceras Djanelide, 1922 are large, tuberculate and have also strong primaries and numerous fine secondaries. They appear to be macroconchs. Many species of Spiticeras, however, have a lappeted peristome (see Arkell et al. 1957), smaller adult size and are strongly tuberculate up to the end, resembling inner whorls of larger macroconchiate forms, a typical feature of ammonite dimorphism (see Callomon 1963; 1981b). Moreover, bituberculate stage in Spiticeras is suppressed compared to Kilianiceras. However, definite dimorphic pair recognition at the genus or species level is still elusive. Nature of dimorphism in both subfamilies appears to be more or less similar; microconchs being strongly ornamented and lappeted, and ornamentation continuing up to the end. They resemble inner whorls of macroconchs. Dimorphism has now become a very reliable tool in tracing phylogenies, "for it makes it possible to predict what a hitherto unrecognized or undiscovered dimorph might look like." (Callomon 1981b, p. 260). The same nature of dimorphism found in olcostephaniid phylogeny can also be observed between Umiaites and Proniceras.

Proniceras resembles intermediate-sized Umiaites, and both have common synapomorphic characters. For example, they share strong primary ribs, non-tuberculate shell and truncation of secondaries at the posterior end of the constriction, and similar septal sutural pattern. Proniceras, as with other microconchs of the family, is characterized by relatively stronger and fewer secondaries. Secondaries in Umiaites are generally finer and multifurcate in the adult phragmocone, but the ribbing pattern in inner whorls includes three secondaries. Morphometrically, the two genera also bear strong resemblance (Figure 4.1-4.2).

However, one point, which goes against the establishment of dimorphism between Umiaites and Proniceras, is the non-overlaping palaeobiogeographic distribution. Umiaites is endemic to Kutch while Proniceras is quasicosmopolitan. The objections raised against the disparity of distribution of two different sexes are well known in ammonite dimorphism. The sex ratio of ammonite macroconch and microconch varies in considerably; it may be 1:100 toward either sex (see for details in Callomon 1981b). But unlike Callomon's observation microconchs dominate Spiticeratinae assemblages. Proniceras and Spiticeras are not only widely distributed but are abundant locally or provincially (see for example Collignon 1960; Enay and Cariou 1997). Callomon (1981b) emphasized that it is very difficult to refute dimorphism unless the sample size comprises hundreds of adult specimens. The highly variable sex ratio is constrained by many factors, including biased collection, differential preservation and most importantly, migratory differences between the sexes and sexual segregation, which predominate in the living cephalopod community (see also Westermann 1990). Either variant can therefore occur to the total exclusion of others (Projeta and Gordon 1987). The Late Tithonian ammonite assemblage of Kutch was previously known by sparse genera (Spath 1931). Many new genera have been recently described (Shome et al. 2004, 2005; Shome and Roy 2006). Therefore, the discovery of Proniceras from Kutch may be a matter of time.

A literature search, however, reveals that Umiaites and Proniceras are not geographically mutually exclusive. Imlay (1939, pl. 18, figs. 1-3) described one Proniceras species, P. jimulcense from Mexico. The species is larger than the preserved diameter of 77 mm (a nearly complete whorl is missing). The ornamentation strikingly resembles the variocostate nature of Umiaites i.e., strong primaries and finer, denser secondaries, which include numerous intercalatory ribs. Moreover, its inner whorl corresponds with the adult Proniceras scorpionum described by Imlay (1939, pl. 18, figs. 5-6). Interestingly, they come from the same horizon and locality. Enay and Cariou (1997) recently mentioned large fragmentary specimens of the macroconch of Proniceras from Nepal, where the latter genus is particularly abundant in a single horizon. Photographs of Proniceras macroconch (Enay personal commun. 2005) strongly resemble the holotype of Umiaites rajnathi (Figure 4.1). This holotype is septate and bears both primary and secondary ribs until to its preserved end as with the Proniceras macroconch. Undescribed specimens of Proniceras macroconch has been reported from the Early Microcanthum Zone of southern Spain (Enay and Geyssant 1975). They appear to be adult with peristome preserved (Enay personal commun. 2005). Unfortunately, they can not be compared with the Kutch specimens, because body whorls are missing in the latter. However, they resemble adult Proniceras microconch.

One of the criteria for dimorphism is the parallel evolution of the two sexual variants (see Callomon 1963; 1981b). We also note that extinction is another attribute, which should show parallelism between the two variants. Remarkably, Proniceras and Umiaites evolved during the Late Tithonian and became extinct at the Jurassic-Cretaceous boundary, which is arguably a period of mass extinction (Bardhan et al. 2007).  Le Hegarat, 1973 reported Proniceras from the Berriasian, but for Tithonian termination of the genus see Wright et al. 1996.  Other members of the subfamily Spiticeratinae such as Spiticeras, Negreliceras, etc. occur above the Jurassic-Cretaceous boundary.