Infaunal molting in Mid Paleozoic trilobites: new insights based on data from South America

RUSTÁN, J.J.; VACCARI, N.E.; BALSEIRO, D.

Conferencia; IGCP 596 - SDS Symposium (September 20-22, 2015, Brussels) Climate change and Biodiversity patterns in the Mid-Palaeozoic; 2015

Infaunalism for molting in trilobites has been compellingly supported by a striking three-dimensional moltpattern, which was firstly reported from Late Silurian?Early Devonian phacopids of western Argentina(Rustán et al., 2011). The infaunal molt pattern (Fig. 1) is characterized by the thoracopygidium beingperfectly articulated, frequently exhibiting dorsal bending; the cephalon separated and located in front ordirectly above the first thoracic segments, trending to be overturned or perpendicularly oriented, and thehypostome dettached and displaced. Because no other taphonomic agent can be invoked for explaining thisdistinct spatial arrangement, it has been interpreted as biologically produced during molting, the trilobite beingentirely buried in soft sediment.Since it is exclusively related to the most vulnerable period of the trilobite adult life (exuviation), this traitis understood as an evolutionary acquisition of a predator-deterrent behavior. Preliminar interpretations(Rustán et al., 2011) indicated that this behavior was only present in a putative lineage involving three speciesof Paciphacops from the Argentine Precordillera basin. In addition, an evolutionary trend toward thethickening of the carapace, development of spinosity, and size increase, was preliminary proposed for thisPaciphacops lineage. Later, however, a thorough taxonomic revision indicated that one of the Argentinianspecies should be considered as Echidnops (Fig 1.1), refusing the idea that the infaunal molt behavior wasrestricted to a monophyletic Paciphacops lineage, endemic from Argentina. A revision of the Brani?acollection of trilobites, housed at the AMNH, USA (Rustán et al., 2012 a, b), allowed the identification of thesame molt pattern in several species of the phacopid Viaphacops and also in the calmoniid Pennaia verneuili,coming mainly from the Middle Devonian of Bolivia. Moreover, a recent revisit of the poorly known MiddleDevonian Chigua Formation (Baldis & Longobucco, 1977; Amenábar, 2009), in western ArgentinePrecordillera, has provided additional specimens of ?Phacops? chavelai along with the calmoniid Punillaspisargentina, both of which also exhibit unquestionable infaunal molt patterns (Fig. 1.2 and 4).Regarding the phylogenetical context, these new evidences definitively confirm that infaunal molting, asan ethological defensive character, appeared more than once in different trilobite families (namely phacopidsand calmoniids), thus being clearly polyphyletic. In addition, since Middle Devonian phacopids andcalmoniids exhibiting infaunal behavior, lack spinosity and are usually smaller than Lower Devonianrepresentatives, a complex mosaic evolutionary pattern of these traits is suggested, implying that spinosity andsize increase would have independently evolved in several different lineages. In accordance with initialinterpretations, evidences as a whole reinforce the idea that these trends are in concert with the ecologicalevolutionary context of the so-called Middle Paleozoic Marine Revolution (Signor & Brett, 1984). This majorbioevent is related to the sudden appearance of several groups of durophagous predators which greatlyintensified predation pressure in Silurian?Devonian marine ecosystems, triggering defensive evolutionarytrends in a number of prey groups. Furthermore, in light of the current larger stock of taxa, embracing a widerstratigraphic and geographic range, causal connections between the burst diversification of predators andevolution of active defensive responses in trilobites are now evident, since they share a common pattern in thefossil record. Mirroring the postulated trend in predation pressure (Nagel-Myers et al., 2009), defensiveinfaunal molting initiated in the Silurian and rapidly became frequent during the Early Devonian, reaching amaximum during the Middle Devonian, in taxonomic diversity at species, genus and family levels (Fig. 2).