Abstract:Acanthodians may represent a paraphyletic assemblage of stem chondrichthyans, stem osteichthyans, stem gnathostomes, or some combination of the three. One of the difficulties in determining the phylogenetic affinities of this group of mostly small, spiny fishes is that several subgroups of acanthodians are represented by relatively little information in the fossil record. It is becoming increasingly apparent that to understand the evolution of gnathostomes, we must understand more about acanthodians. This stud… Show more
“…As for acanthodians, a combination of habitat availability (potential ecological biases) and lack of available rock volume (sampling biases) might explain at least some of the observed relationships. Information on acanthodian ecology and habitat preference is scarce and limited to the general environment of the deposits they were recovered from such as freshwater, marine, deltas, tidal flats, estuaries and lagoons (Denison 1979; Blais 2017). However, as our results indicate, a significant proportion of more complete acanthodian specimens are obtained from benthic assemblage zones in shallow waters which suggests, tentatively, a habit preference.…”
Section: Discussionmentioning
confidence: 99%
“…Notably, possession of a pre‐anal‐fin spine remains the only putative synapomorphy of a monophyletic Acanthodii (Brazeau & Friedman 2014). Acanthodians show little diversity in range of body shape, inferred function and ecomorphology compared to other major groups of fishes (Denison 1979; Blais 2017).…”
Acanthodians are a poorly understood paraphyletic grade of extinct Palaeozoic fishes. They play an increasingly prominent role in our understanding of vertebrate evolution as part of the chondrichthyan stem-group even though their evolutionary history is scarce. The limited preservation of their mostly cartilaginous skeleton largely results in a bias towards isolated remains such as fin spines and scales. Here, we quantify the quality of the acanthodian fossil record by using a variation of the Skeletal Completeness Metric (SCM), an approach that calculates how complete the skeletons of individuals are compared to their theoretical complete skeleton. A novel Soft Tissue Completeness Metric (STCM) is introduced to estimate the percentage of soft body tissue preserved as an alternate measurement of completeness. Completeness scores for >1600 specimens comprising >300 taxa obtained from museum collection visits and literature surveys were assembled into a database. Acanthodian completeness peaks in
“…As for acanthodians, a combination of habitat availability (potential ecological biases) and lack of available rock volume (sampling biases) might explain at least some of the observed relationships. Information on acanthodian ecology and habitat preference is scarce and limited to the general environment of the deposits they were recovered from such as freshwater, marine, deltas, tidal flats, estuaries and lagoons (Denison 1979; Blais 2017). However, as our results indicate, a significant proportion of more complete acanthodian specimens are obtained from benthic assemblage zones in shallow waters which suggests, tentatively, a habit preference.…”
Section: Discussionmentioning
confidence: 99%
“…Notably, possession of a pre‐anal‐fin spine remains the only putative synapomorphy of a monophyletic Acanthodii (Brazeau & Friedman 2014). Acanthodians show little diversity in range of body shape, inferred function and ecomorphology compared to other major groups of fishes (Denison 1979; Blais 2017).…”
Acanthodians are a poorly understood paraphyletic grade of extinct Palaeozoic fishes. They play an increasingly prominent role in our understanding of vertebrate evolution as part of the chondrichthyan stem-group even though their evolutionary history is scarce. The limited preservation of their mostly cartilaginous skeleton largely results in a bias towards isolated remains such as fin spines and scales. Here, we quantify the quality of the acanthodian fossil record by using a variation of the Skeletal Completeness Metric (SCM), an approach that calculates how complete the skeletons of individuals are compared to their theoretical complete skeleton. A novel Soft Tissue Completeness Metric (STCM) is introduced to estimate the percentage of soft body tissue preserved as an alternate measurement of completeness. Completeness scores for >1600 specimens comprising >300 taxa obtained from museum collection visits and literature surveys were assembled into a database. Acanthodian completeness peaks in
“…Taxa included in this study were typically categorized by species, with the exception of indeterminate specimens (as Taxon sp. #); Cladoselache (specimens identified only to genus except for Cladoselache fyleri AMNH FF 240); and Ischnacanthus gracilis , because all full-body fossils of Ischnacanthus are assigned to I. gracilis , even though multiple species are likely present (Blais 2017). Figure 1.Diagrams showing measurements described in Supplementary Table T1 and taken from a wide range of Silurian and Devonian vertebrate fossils.…”
The rise of jawed vertebrates (gnathostomes) and extinction of nearly all jawless vertebrates (agnathans) is one of the most important transitions in vertebrate evolution, but the causes are poorly understood. Competition between agnathans and gnathostomes during the Devonian period is the most commonly hypothesized cause; however, no formal attempts to test this hypothesis have been made. Generally, competition between species increases as morphological similarity increases; therefore, this study uses the largest to date morphometric comparison of Silurian and Devonian agnathan and gnathostome groups to determine which groups were most and least likely to have competed. Five agnathan groups (Anaspida, Heterostraci, Osteostraci, Thelodonti, and Furcacaudiformes) were compared with five gnathostome groups (Acanthodii, Actinopterygii, Chondrichthyes, Placodermi, and Sarcopterygii) including taxa from most major orders. Morphological dissimilarity was measured by Gower's dissimilarity coefficient, and the differences between agnathan and gnathostome body forms across early vertebrate morphospace were compared using principal coordinate analysis. Our results indicate competition between some agnathans and gnathostomes is plausible, but not all agnathan groups were similar to gnathostomes. Furcacaudiformes (fork-tailed thelodonts) are distinct from other early vertebrate groups and the least likely to have competed with other groups.
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