Ammonites, Zittel, 1884

Pandey, Dhirendra Kumar, Fuersich, Franz T., Alberti, Matthias, Das, Ranajit & Saez, Federico Oloriz, 2022, First population-level study of the ammonite genus Hildoglochiceras Spath, and the Lower Tithonian record of the Hildoglochiceras Horizon in the Kachchh Basin, India, Zitteliana 96, pp. 1-49 : 1

Family Haploceratidae Zittel, 1884

Remarks.

The taxa described under this family are Haploceras Zittel, 1870 and Hildoglochiceras Spath, 1924. The shell of Haploceras shows fine growth lines when epigenized shell is preserved, while inner casts are smooth, with a small but variable umbilicus that initially opens gradually until the last ontogenic stage when it slightly unfolds. Macroconchiate Haploceras show a slightly flexuous peristome, while assumed microconchs are variable in size, incorporate wide and short lappets rather than narrow and pedunculated ones, but can resemble glochiceratins in the absence of peristomal structures. Incomplete specimens of Haploceras make species-level interpretations difficult.

Reports of the genus Haploceras have been variably interpreted before the mid-twentieth century when this genus was commonly applied to Kimmeridgian and Lower Tithonian glochiceratins elsewhere in the world (e.g., Ziegler 1958), while the present knowledge about microconchiates is inconclusive. Across the Trans-Erythraean Trough, Haploceras has been more frequently reported from the northern and eastern margins than from western ones, although it is a rare genus across the Himalayas (from Uhlig 1903 to Énay 2009). Southwards across eastern margins, Haploceras has been reported from southern Pakistan to Madagascar throughout the twentieth century (e.g., Lemoine 1910; Collignon 1960; Fatmi and Zeiss 1994). In contrast, across western margins, Haploceras was only registered from southern Yemen ( Howarth and Morris 1998) and Tanzania (from Zwierczyki 1914 to Bussert et al. 2009), being absent or very rare in between.

Haploceras elimatum (Oppel, 1965) and allies represent the morphological group more widespread and have been commonly reported throughout epicontinental areas in the Trans-Erythraean Trough. Haploceras elimatum (Oppel, 1965) and Haploceras staszycii (Zeuschner, 1846) have commonly been recognized as close species, difficult to separate, for instance by Rod (1937) and by ( Hölder and Ziegler 1959), even as representing a single species ( Rod 1946). They represent a dimorphic pair ( Barthel 1962). The macroconch embraces all but one Tithonian haploceratin species with polymorphic microconchs ( Énay and Cecca 1986). According to the information available, the latter is a rather speculative proposal. Zeiss (2001) highlighted the existing options for interpreting dimorphism in haploceratins, and treated the morphologically close species Haploceras elimatum and Haploceras staszycii as separate units only for convenience. Based on all the above mentioned, these two morphospecies are here interpreted as conspecific and referred to as Haploceras staszycii by priority, with reference to the closest nominal morphological species reported from the Trans-Erythraean Trough.

Hildoglochiceras is commonly more evolute, characterized by a median lateral groove and, consequently, with an acute-oval to oval whorl-section, venter of variable width and height, a lateral sulcus above the lower one-third of the flanks, a variable shell-size for the beginning of ribbing, and more or less ornamented outer whorls. In the present collection, we recognise two groups within Hildoglochiceras ; one interpreted as microconch and other as the corresponding macroconch. These two morphs have been separated on the basis of shell size and the diameter of the umbilicus. The macroconch shows a subrectangular to oval whorl section. See previous chapters for a revision of reports of Hildoglochiceras across the Trans-Erythraean Trough.

Another comparable genus to inner whorls of Haploceras and Hildoglochiceras is Glochiceras Hyatt, 1900, the shell of which is smaller and shows a variable whorl section, sculpture and peristomal structure. The umbilicus of Glochiceras opens suddenly at the end of the growth. In addition, some species of Glochiceras are also characterised by a median lateral groove like in Hildoglochiceras . But small size, a rather discoid shell with narrow venter, the type of peristome, and the biostratigraphic range of typical Glochiceras , i.e. from Oxfordian to Kimmeridgian, with scarce records from the Lower Tithonian (e.g., Ziegler 1958; Collignon 1960; Barthel 1962; Grigore 2019), may be diagnostic. These morphological features persist in the scarce Glochiceras reported from the lowermost Tithonian of Europe. Separation of glochiceratins and microconchite haploceratins can be difficult with incomplete specimens lacking the peristome.

Among evolute haploceratins with a comparatively wide ventral region, Lingulaticeras Ziegler, 1958 and Paralingulaticeras Ziegler, 1958 are relatively close in shell-type to Hildoglochiceras . Supposed lowermost Tithonian forms of the former are more involute and show a sculptured venter of variable width, while those of the latter develop a latero-ventral tuberculation.