Growth and variation in the Silurian proetide trilobiteAulacopleura koninckiand its implications for trilobite palaeobiology

Abstract

Morphometric analysis of growth in Aulacopleura konincki reveals several important features: (1) morphological variability is approximately constant throughout growth, increasing slightly in later ontogeny; (2) shape is more tightly constrained than the numbers of postcephalic segments, which can be quite variable; and (3) a major ontogenetic transition occurs at glabellar lengths of about 1.7 mm. This transition divides the ontogeny into two distinct growth phases, is smooth rather than abrupt, and is expressed as changes in growth trajectories, especially in the pygidium. The transition is not strictly correlated with the number of thoracic or pygidial segments. These results suggest a re‐evaluation of the concepts of meraspid and holaspid growth stages in A. konincki, using growth trajectories rather than thoracic segment number to define the stages. Developmental flexibility in holaspid segment numbers in this phylogenetically advanced trilobite suggests that environmental factors, rather than phylogenetic constraint, may explain variations in segment production patterns within some advanced Trilobita.Morphometrics, trilobites, ontogeny, variability, segmentation, Silurian, Czech Republic.

Trilobite body patterning and the evolution of arthropod tagmosis

“…1C, 2A, 5). This appears to have been the basal condition of the clade, and species that varied in the numbers of adult thoracic segments (16,24) were of this kind. In such cases, allocation of segments to the thorax or pygidium may have had limited functional significance.…”

“…The overall tempo and mode of arachnate ontogenetic change is apparently gradual and linear compared to the intermittent and markedly non-linear changes that characterize many derived mandibulates (of which insect metamorphosis is a prime example). (16) Thus, the ability to modify limb morphology markedly during ontogeny, with associated changes in Hox gene expression, seems to have been limited in arachnates. The extent to which this inability to modularize cephalic appendages contributed to the demise of marine arachnates, and extinction of the Trilobita, is unknown.…”

The depositional environment and taphonomy of the Homerian “Aulacopleura shales” fossil assemblage near Loděnice, Czech Republic (Prague Basin, Perunican microcontinent)

“…The variable elongation of crinoid ossicles may partially reflect small-scale spatial variation in the degree of compaction. This might explain the rather varied degrees of compaction witnessed among conspecific specimens from these beds (Hughes & Chapman 1995). Given that the exoskeleton of trilobites such as Aulacopleura koninckii from these beds was much thinner than columnal stereom, but much more robust than the fecal pellets, we would anticipate the degree of compaction in these animals to be on the order of 25% to 50%.…”

“…Several recent studies of trilobite growth have also been based on material from the site (Fusco et al 2004(Fusco et al , 2014Hughes & Chapman 1995) with a particular focus on the development of Aulacopleura koninckii (Barrande, 1846). To date, documentation of the palaeoecology and palaeoenvironment of the Na Černid-lech Hill site has largely been limited to lists of faunal occurrence (e.g.…”

A nektaspid arthropod from the Early Cambrian Sirius Passet fauna, with a description of retrodeformation based on functional morphology

“…An implicit claim in Fortey and Theron's model is that the total number of segments in the adult naraoiids under question is a constant, with only their distribution between thorax and pygidium being affected by heterochrony. There is, however, no particular reason to think that this is true, and the number of segments released into the thorax may not be a reliable guide to heterochronic processes (Hughes and Chapman (1995), in a study of the proetide Aulacopleura konincki (Barrande, 1846) went further and suggested that the ontogenetic dynamics of trilobites may have been decoupled even from the traditional staging of growth based on segment release). Although a 'giant protaspid' from the Chengjiang fauna has been described and attributed to Naraoia, too little is known about the ontogeny to be able to ascertain the sequence of segment addition, which in other trilobites is known to be highly variable, with some trilobites apparently having released their full complement of thoracic segments from the transitory pygidium all at once, whereas others both released segments to the thorax more or less sequentially whilst at the same time adding segments to the transitory pygidium.…”