Evaluating paedomorphic heterochrony in trilobites: the case of the diminutive trilobite Flexicalymene retrorsa minuens from the Cincinnatian Series (Upper Ordovician), Cincinnati region

Abstract

Flexicalymene retrorsa minuens from the uppermost 3 m of the Waynesville Formation of the Cincinnatian Series (Upper Ordovician) of North America lived approximately 445 Ma and exhibited marked reduction in maximum size relative to its stratigraphically subjacent sister subspecies, Flexicalymene retrorsa retrorsa. Phylogenetic analysis is consistent with the notion that F. retrorsa retrorsa was the ancestor of F. retrorsa minuens. F. retrorsa minuens has been claimed to differ from F. retrorsa retrorsa"in size alone," and thus presents a plausible example of global paedomorphic evolution in trilobites. Despite strong similarity in the overall form of the two subspecies, F. retrorsa minuens is neither a dwarf nor a simple progenetic descendant of F. retrorsa retrorsa. More complex patterns of global heterochronic paedomorphosis, such as a neotonic decrease in the rate of progress along a common ontogenetic trajectory with respect to size, coupled with growth cessation at a small size, "sequential" progenesis, or non-uniform changes in the rate of progress along a shared ontogenetic trajectory with respect to size, can also be rejected. Rather, differences between these subspecies are more consistent with localized changes in rates of character development than with a global heterochronic modification of the ancestral ontogeny. The evolution of F. retrorsa minuens from F. retrorsa retrorsa was largely dominated by modifications of the development of characters already evident in the ancestral ontogeny, not by the origin of novel structures. Factors promoting size reduction in F. retrorsa minuens appear to have been specific to this subspecies, because other co-occurring taxa, including other trilobite species, do not show marked differences in mean size.

Reply to Comment on: Álvaro J.J., Esteve, J. & Zamora, S. 2019. Morphological assessment of the earliest paradoxidid trilobites (Cambrian Series 3) from Morocco and Spain [Geological Magazine] by Geyer G, Nowicki J, Żylińska A & Landing E

“…Numerous studies cover small morphological structures using a low number of landmarks and their results seem to be satisfactory, when an adequate set of landmarks is available over only half of the anatomical element shape (e.g. Hughes & Chapman, 1995;Kim et al 2002Kim et al , 2009Hunda & Hughes, 2007;Webster, 2007).…”

Heterochronic phenotypic plasticity with lack of genetic differentiation in the southeastern Pacific squat lobster Pleuroncodes monodon

“…Phenotypic plasticity in the developmental pathway is an important mode of evolution for many marine organisms (e.g., Hunda and Hughes 2007;Nielsen 2008). Heterochronic phenotypic variation has been reported for several species.…”

Contribution of Morphometrics to the Systematics of the Ordovician GenusNeseuretus(Calymenidae, Trilobita) from the Armorican Massif, France

“…Recent studies have shown the relevance of geometric morphometrics to assess morphological diversity of trilobites. In calymenid trilobites in particular, Hunda and Hughes (2007) studied the species Flexicalymene retrorsa (Foerste, 1910) and Chestnut (2008) worked on Silurian calymenids. Finally, Delabroye and Cronier (2008) recently studied the ontogeny of the Armorican trinucleid trilobite Marrolithus bureaui (Oehlert, 1895) using a landmark-based approach.…”