A Size-Independent Revision of the Fractal Step Method for Ammonite Sutures

The novel coronavirus has presented specimen-access challenges to geoscientific researchers, including paleobiologists interested in fossil ammonoids. Ammonoid sutures are geometric patterns formed by the intersection of the septa and the shell wall, and have long been a diagnostic tool for ammonite researchers for such applications as species identification, taxonomic relationships, ontogenetic change, functional and evolutionary morphology, and other aspects of ammonoid paleobiology. Without access to specimens in museum and institutional collections, researchers must rely on previously published illustrations and photographs of ammonoid sutures. However, many of these illustrations were published decades ago without an index of scale. Suture tracings lacking a scale bar are not usable by researchers interested in applying the quantitativeness of fractal geometry to the interpretation of septal complexity. Additonally, distortion of the marginal elements of suture geometry occurs due to shell curvature near the venter and umbilicus. The revised approach described here eliminates the problem of missing scale information in fractal analysis of ammonite sutures, making use of just the lateral lobe and adjacent saddle. Our revised method’s non-requirement of a full hemisuture also facilitates comparisons among sutures within an ontogenetic sequence, or sutures from multiple ammonite taxa.

Ontogenetic change in the diet of Semaprochilodus insignis (Characiformes: Prochilodontidae) during migration between two limnologically distinct environments in the Amazon Basin

Semaprochilodus insignis is an Amazonian migratory fish species, moving in large shoals between white- and black-water rivers. It has long been classified as a detritivorous fish. However, it is possible that the trophic plasticity of S. insignis could be higher than previously assumed. The objective of this study was to investigate the relative contributions of autotrophic energy sources to the diet of S. insignis in the Negro and Solimões rivers and to determine if the species undergoes an ontogenetic change in the diet. We found variations between the δ13C and δ15N values of S. insignis between the rivers. In the Negro River, periphyton (84%) was the principal energy source for adults, while juveniles in the Solimões River foraged predominantly on terrestrial plants/C3 macrophytes (50%) and phytoplankton (42%). These variations in isotopic signatures are likely associated with migratory movements of S. insignis at different life stages and hydrological periods. Instead of the previously assumed dietary classification, we suggest that S. insignis varies its diet ontogenetically, with adults acting as illiophagous in black-water while the young are detritivorous in white-water rivers. The results show that this species creates complex links between food chains, thus emphasizing the importance of conserving flooded areas.

Taxonomy and functional morphology of the Urasterellidae (Paleozoic Asteroidea, Echinodermata)

Family-level taxonomic concepts for Paleozoic Asteroidea are problematic; data availability for the family Urasterellidae, however, is judged sufficient for familial reappraisal. The survey cannot be fully comprehensive because certain available names are founded on very limited material, with specimens widely scattered among collections. Recognized urasterellid genera include Urasterella Schuchert, 1914; Anthroosasterias Blake, 2017; Meturaster Blake, Guensburg, and Lefebvre, 2016; Salteraster Stürtz, 1893; Staurasterias Blake, 2017; Ulrichaster Spencer, 1950; and Pegoasterella pompom n. gen. n. sp. Other available generic names are held in abeyance. The name Urasterella has approached form-genus status, an outcome exacerbated by incomplete understanding of the type species. Urasterella huxleyi (Billings, 1865) is reassigned to Anthroosasterias. Relatively large specimen suites for Anthroosasterias huxleyi (Billings, 1865) and Salteraster grandis (Meek, 1872) enable some interpretation of urasterellid intrataxon variation, ontogenetic change, and life mode. Morphologic changes accompanying ontogeny of A. huxleyi are argued as favoring a shift from substrate surface feeding to feeding through extension of arms into the water column in a manner comparable to that seen among extant Forcipulatida and ophiurans.UUID: http://zoobank.org/82d2eca7-55e0-42ed-a164-c666652e3972