Hildoglochiceras, gen. and sp. ind., 1924
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https://dx.doi.org/10.3897/zitteliana.96.73892 |
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https://treatment.plazi.org/id/627AE0F1-7A62-521A-AB1A-1E94FBF2F0AF |
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Hildoglochiceras |
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Taramelliceratinae gen. and sp. ind.
Fig. 11A-F View Figure 11
Taramelliceras sp. gr. compsum ( (Oppel)- Taramelliceras kachhense Spath or Parastreblites sp. gr. hoelderi Donze and Énay)
Material.
Two specimens, Hildoglochiceras Bed of Jara Dome (Lower Tithonian); KSKV2019Jara/17 (figured), 79.
Description.
Fragments of phragmocone of small size, moderately involute, compressed, with oval whorl section, indistinct ventro-lateral shoulders, and obtusely rounded ventral region. Ornamentation consisting of blunt, falcate, primary ribs with marked inflection slightly below the mid-flank. Swell at branching points at mid whorl height, along thin, faintly developed spiral groove. Secondary ribs crescentic, occasionally showing very subtle swellings that barely define the indistinct flank periphery and shoulders. Ventral region without identifiable ribs or tubercles. Suture lines relatively well preserved with smoothed peripheral frilling.
Remarks.
The first specimen (KSKV2019Jara/17), a small fragment of a phragmocone, ca. 50 mm in size is an internal cast with accentuated variably preserved flanks. The left side has been abraded, except for the extreme outer flank that shows remnants of blunt secondaries, while the right side shows remains of a moderately coarse ornamentation across the flank, with a shallow and discontinuous lateral groove-like depression and selective collapse areas. The ventral region is unornamented, subtly raised on the mid-line, and flanked by the external ends of ribs, some of which show incipient oblique-radial swellings barely differentiated from the ribs, which do not contribute to the distinctness of shoulders. The state of preservation precludes any clear remains of tuberculation.
It seems that the right flank exhibits glochiceratin-taramelliceratin traits: (1) peripheral and widely spaced incipient swellings ("remains of tuberculation"?), occasionally located at points where two secondary ribs connect; and (2) a lateral groove-like depression slightly above the mid-flank. In contrast, the smoothed left flank preserves suture lines in such a way that attrition has been pervasive enough to distort ribbing severely and produced the peripheral frilling of septa; hence the suture-line smoothing corresponds to an erosion level being at least equivalent to rib thickness (compared with the sharper suture-line frilling preserved on the rightside). It is unclear whether differential preservation operated on a pathologic specimen (note symmetrical thickness of the internal cast with respect to the siphuncle) showing different lateral sculpture (Fig. 11A, B View Figure 11 ), or whether the present appearance merely is a taphonomic feature (comparatively smoothed left flank). The latter option would suggest weathering or reworking and abrasion. The latter would point to a distinct break in sedimentation and later bioturbation during renewed continuous deposition. Reworking is of special interest in the context of the biostratigraphic interpretation as commented in previous sections.
Comparative analysis.
The taxonomic interpretation of incomplete oppeliids, as in the case of other Late Jurassic ammonites, is a very difficult task since diagnostic morphological features for identification at the genus and species levels only developed on middle and outer whorls, inner whorls being largely indistinct. This general pattern is taxon-dependent. This situation is accentuated when natural conditions (outcrop, deposition, preservation) and/or collecting limitations (sample size, sampling process) impede access to large samples from a given stratigraphic horizon, i.e., a particular bed representing continuous deposition during a relatively “short” time with no or only a low degree of within-habitat time-averaging. Overall, dominant depositional conditions in Kachchh during the Late Jurassic determined the rarity of records of large, “isochronous” ammonite samples enabling an analysis at the population level.
Based on the assumption that the lateral groove is real though defectively preserved, Paralingulaticeras may show a similar sculpture but it shows well-developed ventro-lateral tubercles, a much more slender, flatter shell with a lower degree of coiling, and the lateral groove is mainly developed on the body chamber. Among well-known European Paralingulaticeras species (cf. Ziegler 1958), P. nodosum Berckhemer with scarcely marked lateral groove and tuberculation, and the sparsely tuberculated P. parcevali Fontannes, are differently ribbed on the flanks, while the densely tuberculate and coarsely ribbed P. lithographicum Oppel clearly differs in shape and sculpture. These species of Paralingulaticeras occur in the latest Kimmeridgian Beckeri Zone and in the lowermost Tithonian, lower part of the Hybonotum Zone of Submediterranean ( Ziegler 1958; Berckhemer and Hölder 1959) and Mediaterranean areas of the Western Tethys ( Olóriz 1978; Caracuel and Olóriz 1999).
Among ornamented glochiceratins from eastern Gondwana, the Madagascan Paraglochiceras from the Hildoglochiceras kobelli Zone (interpreted as Early Tithonian in Madagascar) commonly have more globose shells with unsculptured inner flanks. P. hirtzi Collignon (1960: pl. 146, fig. 576) and P. aff. propinquum (Waagen) are relatively close in shape but clearly differ in sculpture. Hildoglochiceras shows ventrolateral ribs that rarely bifurcate in the more finely ribbed species and does not have any dorsolateral ribs. Hildoglochiceras colei Spath ( Collignon 1960: pl. 144, fig. 561) shows slightly more flattened and convergent flanks, and different external ribs. H. parceumbilicatum parceumbilicatum Collignon (1960: pl. 144, fig. 563) curiously resembles the smoothed left flank of specimen described herein. Hildoglochiceras tenuicostulatum Collignon (1960: pl. 145, fig. 568) shows a more crowded and delicate ribbing. Aside from the usual schemes of correlation, the precise biostratigraphic range of the genus Hildoglochiceras is not conclusively known, as has been discussed above.
If alternatively, the lateral groove is a secondary, preservational feature, there are two interpretations of the inner cast described:
(1) Parastreblites Donze and Énay 1961, which does not possess a lateral groove, shows a streblitoid whorl section in the outer whorls, but its inner whorls could be similar in sculpture to the Taramelliceras compsum group, since the mid-flank tuberculation of Streblites does not occur ( Olóriz 1978). In fact, this tuberculation reveals Taramelliceras inheritance s. str., excluding the group of Taramelliceras trachinotum Oppel ( Donze and Énay 1961) due to its clearly older age. Parastreblites has been recorded in the Lower Tithonian (three-fold division) Darwini Zone and in the upper Hybonotum-Albertinum (=Darwini) Zone in the western Tethys (southern Spain; Énay and Geyssant 1975 and Olóriz 1978, respectively), but potential records from latest Kimmeridgian to earliest Tithonian horizons cannot be excluded, if Oppelia gaetanoi Fontannes is considered. Inner whorls of some species of Parastreblites show morphologic affinity to the group of Taramelliceras compsum (Oppel) (e.g., Parastreblites hoelderi in Donze and Énay 1961). In fact, Parastreblites has been reported from distant areas outside Europe ( Imlay 1942; Donze and Énay 1961; Leanza 1980; Parent et al. 2015), but there it is a rare, inconclusively known taxon showing a distinct morphology and wider biostratigraphic range with respect to European specimens. Even allusions to the Kimmeridgian genus Metahaploceras have been made for those Lower Tithonian ammonites.
(2) Taramelliceras is the alternative option for interpreting the incomplete inner cast described. First revised by Hölder (1955), Taramelliceras exhibits a notable intra-species variability and is represented by three main species groups in the Upper Jurassic, with the T. compsum group as the source for Mid-to-Late Kimmeridgian forms and their youngest offsprings. According to Hölder (1955), the T. compsum group was widely distributed, from Mexico to India, and Taramelliceras (Metahaploceras) occurred throughout East Africa. All these data reveal a Tethyan influence in the surrounding epicontinental areas on both sides of the Trans-Erythraean Gulf or incipient seaway, during rising sea-levels throughout late Middle to Late Kimmeridgian and Early Tithonian times (three-fold divisions). Berckhemer and Hölder (1959) revised Malm ε to ζ 3 taramelliceratins from southern Germany and split the Taramelliceras compsum group in subspecies, with diversified descendants during latest Kimmeridgian and earliest Tithonian times up to probably ζ 3 e.g., Taramelliceras franciscanum Fontannes, which was preliminarily included by Hölder (1955) in Taramelliceras compsum . Olóriz (1978) confirmed the pronounced morphological variability in Tethyan specimens of Taramelliceras compsum (Oppel) from the Betic Cordillera in southern Spain and highlighted the relevance of this species and related forms during the Late Kimmeridgian (two-fold division) and the earliest Tithonian. Working on better preserved material from epicontinental deposits with a continuous sedimentary record, Baudouin et al. ( 2011) were able to demonstrate a high within-species variability in a large sample of Taramelliceras compsum , collected from two successive beds in fine-grained siliciclastic-carbonate rhythmites of the Mount Crussol type-section, southern France. From Hölder (1955) to Baudouin et al. ( 2011), Taramelliceras compsum has been interpreted to be very variable with respect to ribbing and tuberculation, including its inner whorls. Commonly, two lateral ribs connect to a single ventro-lateral tubercle at a relatively small shell size; other specimens with broader and slightly domed vs. raised ventral regions have a virtually smooth to clearly tuberculate midventer, respectively. Taramelliceras kachchense Spath has been interpreted to be a derived form from across Trans-Erythraean Through areas, where references to the group of Taramelliceras compsum (Oppel) have been common from Uhlig (1903) onwards.
The strongly ribbed left side of the analysed phragmocone excludes comparison with smooth forms such as Taramelliceras nivale (Stolizcka), an insufficiently known species reported from Himalayan and Madagascan areas.
Biostratigraphy.
All this information supports the tentative interpretation of the incomplete phragmocone described as belonging to Taramelliceras sp. of the T. compsum (Oppel) - T. kachhense Spath groups. These two nominal species are most probably evidence of a Tethyan source and a local, derived taxon, respectively, the latter being a local phenotype expression related to colonization of shelves bordering the Trans-Erythraean Through. Thus, the biostratigraphic range of Taramelliceras could extend from Middle Kimmeridgian horizons to the lower part of the Lower Tithonian (three-fold-division), if the total range of the former species in west-Tethyan areas applies. Southwards, at the Indian-Malagasy palaeomargin, Collignon (1960) documented what he interpreted as Lower Tithonian Taramelliceras in Madagascar, but a more accurate biostratigraphy is needed before a definite conclusion can be reached, also with respect to its actual co-occurrence with Hildoglochiceras there, i.e., without reworking.
The stratigraphic range assumed for the Oppel species in west-Tethyan areas, with the youngest Taramelliceratinae occurring in the Albertinum/Darwini Zone, and the limited evidence of reworking in the stratigraphic interval sampled (a single specimen; see description above) points to the possibility that Taramelliceras occurs from levels with a minimum age of Early (earliest?) Tithonian (three-fold division). The assumed co-occurrence with Hildoglochiceras in Kachchh and Madagascar should be consistent with an age of the oldest Hildoglochiceras older than usually interpreted. The inconclusive evidence of lowermost Tithonian horizons in these areas could be the result of unfavourable conditions for ammonites and/or of stratigraphic gaps in connection with the change from coarse-grained siliciclastics to calcareous sediments. Apparently, regional tectonic forcing during earliest Tithonian times occurred close to the Jurassic eustatic maximum. Local variation in the time span involved in the stratigraphic gap cannot be dismissed in accordance with lateral facies changes of deposits containing Hildoglochiceras in the area (e.g., Pandey et al. 2016).
The alternative interpretation of the described phragmocone as Parastreblites sp. gr. hoelderi Donze and Énay (1961) is compatible with the biostratigraphic interpretation just proposed, based on the assumption that the biostratigraphic range of European Parastreblites (Parastreblites) s. str. is the reference for correlation, which might include the Ulmensis horizon (=uppermost Kimmeridgian or lowermost Tithonian according Donze and Énay (1961). However, even though being difficult to evaluate (e.g., Parent et al. 2011), the rare records of Parastreblites from the Zitteli Zone in Argentina ( Leanza 1980; Parent et al. 2011, 2015; Vennari 2013) could be taken into account, but the illustration by Parent et al. (2015) of material collected from horizons above their Zitteli and Proximus zones rather excludes the assignment to Parastreblites s. str.
In the absence of age-diagnostic Tethyan and Indian ammonites in the reported ammonite assemblage, the Hildoglochiceras described here represent a local record but, being a sample of population size, it is the most relevant record of this genus that is available. The favoured biostratigraphic interpretation points to indeterminate upper Lower Tithonian horizons (three-fold division), correlated with a lower part of the Tethyan Albertinum/Darwini Zone, but slightly older horizons also might apply. This interpretation is based on: (1) underlying ammonite-poor, sandy deposits without evidence of relevant erosion at the top; (2) lacking records of Hybonoticeras , which are mainly interpreted to represent the uppermost Kimmeridgian across epicontinental deposits in the Trans-Erythraean Trough, and rarely lowermost Tithonian horizons; (3) the occurrence of transient forms between Neochetoceras and early Semiformiceras in neighbouring areas (Nepal), interpreted as probable evidence of morphological evolution towards early forms of Semiformiceras rather than a case of local, diachronous homeomorphism; (4) occurrence elsewhere in the Trans-Erythraean Trough of ammonites morphologically close to those belonging to Lower-to-lowermost Tithonian in West-Tethyan areas, and (5) the interpretation of the Hildoglochiceras horizon as recording a local maximum flooding zone. This interpretation agrees with the occurrence of ammonite remains morphologically close to virgatosphinctins, reported and illustrated with precise stratigraphic control ("Couches à Virgatosphinctes et Aulacosphinctoides " at Nupra, Thakkhola, central Nepal, by Énay (2009); and comments with precise citations in previous sections). The second alternative pointing to some horizons within the upper Albertinum/Darwini to lower Semiforme-Verruciferum zones is potentially correlatable with the early Middle Tithonian transgressive pulse in different areas worldwide but could involve a wider stratigraphic gap (Fig. 2 View Figure 2 ).
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