Comments are provided on a published paper on Middle Jurassic Laevaptychus from central Mexico [C.EsquivelMacías, P.Zell, J.A.Moreno-Bedmar and K.Flores-Castro, Giant Middle Jurassic Bathonian) cf. Laevaptychus sp. of the Aztl´ an section, Hidalgo State, central Mexico, Journal of South American Earth Sciences, 110, 103302]. This article describes an interesting finding of large-sized ammonite lower jaws (aptychi referred to Laevaptychus paragenus), claimed as the largest Jurassic aptychi ever known. However, the age of these specimens was erroneously defined due to misidentification of an associated ammonite specimen as Bathonian Procerites. Although poorly preserved, this ammonite shows typical features of the Kimmeridgian genus Idoceras. The Kimmeridgian age of these occurrences is in agreement with findings of Laevaptychus, as this is one of few aptychi formal genera, which belongs to a single ammonite family (Aspidoceratidae). Aspidoceratids appeared in the late Callovian and during the evolution of this lineage maximum sizes of adult specimens and the relative whorl height gradually increased up to Kimmeridgian - Tithonian; only prior to their extinction in early Berriasian, aspidoceratids became uncommon and smaller in size. Laevaptychi are thick-valved aptychi, which have high preservation potential while compared with other aptychi of Jurassic ammonites and their host shells. Giant laevaptychi reported in previous publications (the largest of which reaches 35 cm in length) are briefly reviewed. In adult aspidoceratids the maximum length of aptychi is slightly less than the maximum whorl height. Thus, taking into account the size of the largest aspidoceratid ammonites (up to 85 cm in diameter), the estimated length of the largest laevaptychi can be expected to be ~35–40 cm, which is close to their known record.
This publication represents comments on an article previously published by Mexican paleontologists. They published interesting findings of large aptychi belonging to the formal genus Laevaptychus. These aptychi had a considerably large size: they were 18 mm long, and our colleagues erroneously assumed that these were the largest known aptychi of this genus. However, we have shown that earlier even larger aptychi have been described, the maximum size of which reached 35 cm. In addition, in their article, the authors made a mistake with the age of the findings due to the poor preservation of the ammonites found together with the aptychi. They suggested that these aptychi came from the Bathonian layers, and this assumption created a paradox: the hosts of the Laevaptychus, ammonites of the family Aspidoceratidae, appeared much later, at the end of the Callovian. However, in fact, the Mexican Laevaptichi are younger than the authors suggested: they were found in the Kimmeridgian layers, which were formed when Aspidoceratids flourished.
Aptychi, Ammonites, Evolution, Aptychophora