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1 2006

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Ontogeny in a macroconchiate Idoceras from Mexico: a key for interpreting suprageneric classification of genus Idoceras based on phenotype expression

Villasenor A., Oloriz F.

Volumina Jurassica

2006

Vol. 4, no. 1

245-247

Genus Idoceras was created one hundred years ago by C. Burckhardt (1906). Mainly focused on "un grand nombre d'especes appartenant aux groupes de l'Ammonites planula Hehl, et de l’Ammonites balderus Oppel" collected from Kimmeridgian beds at Sierra de Mazapil (Zacatecas), Burckhardt did not give precise hypotheses about relationships with morphologically related taxa. Aside from comparisons with Middle Jurassic taxa (e.g., Parkinsonia) made by pioneer European ammonitologists, Burckhardt recognized that the European ammonites planula, balderus and related ones had been interpreted as related to Perisphinctes or Simoceras. In addition, he stated that both ventral sculpture and sutural pattern serve for separation from typical Perisphinctes. On the assumption that ventral sculpture is of subordinate value in the case-study, the following quotations taken from Burckhardt's definition of genus Idoceras are both of relevance and introductory for its updated interpretation at the suprageneric level: Coquille discoidale, generalement aplatie, peu renflee… La plupart des cotes se subdivide en deux branches secondaires souvent mal reliees…, quelques-unes resten simples; quelquefois aussi il y a des cotes tripartites, bidichotomes ou intercalees… avec l'age… les cotes ont la tendance a de se renfler faiblement pourtour de l'ombilic et sourtout au bord externe des tours, tandis qu'elles s'affaiblissent sur les flancs jusqu'a s'effacer. According to Burckhardt, Idoceras" identification involves: occurrence of loose coiling, common simple ribs, and the sutural pattern, all these being of use for easy distinction from typical simoceratids; difficulty for separation from some, unnamed groups of both perisphinctids and "simoceratids"; and the foreseeable inclusion of mainly Tethyan "simoceratids" in his new genus Idoceras. Together with inconclusive knowledge about the origin and phylogenetical relationships of Idoceras in the original description, its suprageneric interpretation has been controversial during the past century. Idoceras has been included in the subfamilies Idoceratinae and Ataxioceratinae, mainly included within the family Perisphinctidae. The inclusion of genus Idoceras in the polyphyletic Ataxioceratinae has been made taking Subnebrodites as synonym and, therefore, embracing Late Oxfordian ammonites of the planula group and related species. Restriction of Ataxioceratinae to embrace mainly the Kimmeridgian perisphinctids is widely accepted, and the appearance of its "key-innovation" (i.e., virgatotome ribbing, and related ones such as polygyrate, fascipartite) has been proposed to identify ataxioceratids from the Upper Oxfordian onward in the Upper Jurassic. Disassembling of a well-preserved, macroconchiate Idoceras showing the greatest size known, collected ex-situ from "Idoceras beds" in Sierra de Mazapil (Fig. 1), allows the precise description of the ontogenetical course in terms of sculpture and shell-type: from quasi-platycone to discocone shell; increasing relative compression and, therefore, from suboval to high-oval whorl-section; from moderately involute to involute shell; decreasing expression of narrow, shallow, oblique constrictions; from bifurcated, isocostate ribbing to increasing more or less complete ataxioceratoid and periumbilically reinforced ribs; and increasing shell smoothing from the end of the phragmocone. Thus, the combination of shell-type and ataxioceratoid ribbing, as well as stratigraphy (upper Lower Kimmeridgian), support the interpretation of genus Idoceras as belonging to Ataxioceratinae. The use of subfamily Idoceratinae is disfavored.