Ontogenetic analysis of Anisian (Middle Triassic) ptychitid ammonoids from Nevada, USA

Abstract

Ptychites is among the most widely distributed ammonoid genera of the Triassic and is namesake of a family and superfamily. However, representatives of the genus mostly show low-level phenotypic disparity. Furthermore, a large number of taxa are based on only a few poorly preserved specimens, creating challenges to determine ptychitid taxonomy. Consequently, a novel approach is needed to improve ptychitid diversity studies. Here, we investigate Ptychites spp. from the middle and late Anisian of Nevada. The species recorded include Ptychites embreei n. sp., which is distinguished by an average conch diameter that is much smaller and shows a more evolute coiling than most of its relatives. The new species ranges from the Gymnotoceras mimetus to the Gymnotoceras rotelliformis zones, which makes it the longest-ranging species of the genus. For the first time, the ontogenetic development of Ptychites was obtained from cross sections where possible. Cross-sectioning highlights unique ontogenetic trajectories in ptychitids. This demonstrates that, despite showing little phenotypic disparity, Ptychites was highly ontogenetically differentiated, and thus the high taxonomic diversity at the species level is justified for the species investigated. UUID: http://zoobank.org/5abe2487-8a00-4b48-adc7-ec7db7a097f7

Ontogeny of highly variable ceratitid ammonoids from the Anisian (Middle Triassic)

“…The process of lengthening and shortening of the trajectories (i.e., related to changes in rate and timing of the development) account for the ontogenetic differentiation of the species in focus. This contrasts a previous traditional morphometric analysis by Bischof & Lehmann (2020) of ptychitids, which revealed that the spherocone-cadicone morphospace is much more distinct. The highly ontogenetically differentiated genus Ptychites directly differed through characteristic ontogenetic trajectories.…”

“…The results obtained here do justice to the general opinion that ontogenetic trajectories can be a powerful tool to describe (e.g., Korn & Klug, 2007) and discriminate ammonoid species (e.g., Rieber, 1962;Bischof & Lehmann, 2020): The newly introduced methods succeeded in statistically discriminating the ontogenetic pathways of the pre-defined ceratitid species. Based on this analysis, the high diversity of the Anisian ammonoid assemblages of Nevada appears not to be artificially inflated and the alpha taxonomy is regarded to be adequate.…”

“…To visualize the ontogenetic development of ammonoids, there are different types of morphospaces. Ontogenetic trajectory spaces (originally called ontogenetic morphospaces), as defined by Bischof & Lehmann (2020) , p.2), illustrate the differences in total ontogenetic development of individuals. They show the data in an artificial state of combined morphologies of different ontogenetic stages.…”

“…Thereafter, the scan images were digitized. CT scan images of Anisian ammonoids from Nevada do not provide sufficient contrast of the internal structures for a reliable analysis (Bischof & Lehmann, 2020). Based on the digitized cross-sections, we performed a 2D landmark-based geometric morphometrics analysis.…”

“…Previous studies have proven that—particularly due to their accretionary planispiral conch growth with conservation of previous growth stages—ammonoids offer a high-resolution data set for ontogenetic, developmental and also taxonomic studies. While the study of conch ontogeny has a long history in the study of Paleozoic ammonoids (e.g., Korn & Klug, 2007 ; Korn, 2010 ; Monnet, De Baets & Klug, 2011 ; Naglik et al, 2015 ), it was only rarely examined on Mesozoic ammonoids (e.g., Rieber, 1962 ; Tajika et al, 2015 ; Bischof & Lehmann, 2020 ). So far, the morphology and ontogeny of ammonoids was mainly assessed using descriptive, comparative or traditional morphometric methods (linear measurements).…”