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Ammonoids of the Cretaceous superfamily Desmoceratoidea are geographically and stratigraphically widespread. However, the group is understudied, with just one prominent Japanese worker, T. Matsumoto, who worked on desmoceratoids extensively in the 1970s-1990s. Since then, few researchers have studied desmoceratoid taxonomy, phylogeny, or ecology. Based on their geographic distributions and typical geologic contexts, desmoceratoids are traditionally interpreted to have been open ocean dwellers. However, details of their paleoecology and mode of life are lacking, limiting our ability to interpret desmoceratoid diversity dynamics through the Cretaceous. The Westermann morphospace method is a way to predict the life mode of coiled (non-heteromorph) ammonoids based on morphological measurements of shell shape. The morphospace has three end-member morphotypes, spherocone, oxycone, and serpenticone, with predicted modes of life for different morphospace regions. Shell shape measurements were made on 112 desmoceratoid specimens from collections at the U.S. National Museum of Natural History and the Natural History Museum in London (UK); these specimens are from every continent except Australia and Antarctica. Westermann parameters were computed and specimens were plotted in Westermann morphospace so that patterns of morphospace occupation could be assessed. Desmoceratoids generally fall towards the oxycone corner of the morphospace, with genera falling into two groups, one more platyconic with predicted good swimming efficiency and one more discoconic with more limited swimming efficiency. Lewesiceras and Desmoceras have specimens in both groups, while Puzosia uniquely occupies a more central position in the morphospace. Temporally and regionally, there were no significant variations in morphology. While often interpreted as uniformly effective open ocean swimmers, these results indicate that desmoceratoid ammonoids had varying modes of life.
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The concept of ‘heteromorph ammonoids’
‘Heteromorph ammonoids’ encompass all ammonoid species whose shapes do not conform to a closely coiled planispiral shell. The term is useful as a broad description for such ammonoids. However, as a concept, ‘heteromorph ammonoids’ no longer has any scientific value or explanatory power. Although such ammonoids have traditionally been considered aberrant forms, they represent instead an integral part of the evolutionary history of the Ammonoidea. ‘Heteromorph ammonoids’, as a whole, are a poly- phyletic group, consisting of a heterogeneous mixture of taxa without any phylogenetic, morphological or ecological coherence. Their treatment as a single entity risks conflating convergences and phylogenetic affinities. It also vastly oversimplifies the stunning array of morphologies and ecological niches occupied by these animals. Investigation into the uncoiling (and recoiling) of ammonoids is a legitimate and worthwhile enterprise, especially in view of the realization that this phenomenon occurred several times in the history of the Ammonoidea. However, few insights can be gained by treating ‘heteromorph ammonoids’ as a single entity. Studies of such ammonoids should focus on monophyletic groups within a well‐constrained phylogenetic and stratigraphical framework to yield meaningful results.
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- Award ID(s):
- 2010822
- PAR ID:
- 10353207
- Date Published:
- Journal Name:
- Lethaia
- Volume:
- 54
- ISSN:
- 0024-1164
- Page Range / eLocation ID:
- 595-602
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1111/let.12443
