Infaunal molting in Viaphacops orurensis (Bonarelli 1921) and its evolutionary implications

RÚSTAN, J.J.; BALSEIRO, D.; IWASAKI, Y; FOGLIA, R.D.

Praga

Congreso; The 5th Conference on Trilobites and their relatives; 2012

Infaunal molting behavior in trilobites has been only recently recognized. The hypothesis has been supported by direct taphonomic indicators based on a Late Silurian-Early Devonian phacopid lineage, involving three species of Paciphacops Maksimova 1972, from western Argentina. The infaunal molt pattern is characterized by the thoracopygidium being perfectly articulated, frequently exhibiting a gentle dorsal bending; the cephalon in front of the first thoracic segment being separated and nearly perpendicularly oriented; and the hypostome, when preserved, being barely displaced. Because no other taphonomic agents can be invoked for explaining this distinct three-dimensional arrangement, it has been interpreted as biologically produced during the molt, during which the trilobite was entirely buried in soft sediment. Since it is exclusively related to the most vulnerable period of the trilobite adult life (exuviation), this trait is understood as an evolutionary acquisition of a predator-deterrent behavior. In addition, an evolutionary trend toward the thickening of the carapace, development of spinosity, and increase in size have been documented in this Paciphacops lineage. These trends have been interpreted as escalated, in concert with the evolutionary-ecological context of the so-called Middle Paleozoic Marine Revolution. This bioevent is related to the sudden appearance of several groups of durophagous predators that greatly intensified predation pressure in Silurian-Devonian marine ecosystems, triggering coeval defensive trends in a number of prey groups. Here we report new evidence based on several specimens of Viaphacops orurensis (Bonarelli 1921) coming from the Middle Devonian (Givetian?) of the Sica Sica Formation at Patacamaya, La Paz Department, Bolivian Altiplano. They are housed in the AMNH, New York, USA (Brani?a collection), and consist of molt assemblages that conspicuously exhibit the very same described infaunal pattern. Although previous evidence suggested that the infaunal molt behavior was restricted to a monophyletic Paciphacops group of species, endemic to a single basin (Argentine Precordillera), the specimens of V. orurensis show that this behavior also existed in other genera and different basins, thus questioning the phylogenetic and paleobiogeographic context of the original proposal. In addition, since V. orurensis is much younger that the species of Paciphacops in which infaunal molting was reported, and also lacks pronounced spinosity, it would suggest a complex mosaic evolutionary pattern of these traits, implying that infaunalism, spinosity, and increase in size may have evolved more than once (i.e. is polyphyletic). A phylogenetic revision focused on genera closely allied to Paciphacops and Viaphacops, such as Ananaspis, Kainops, Echidnops, etc., is therefore necessary. Because this would be the first attempt to perform a phylogenetic study including behavioral characters in Trilobita, it would help to elucidate the phylogenetic patterns of responses previously supposed as escalated in nature. Moreover, taking into account that we now have data from a larger stratigraphic and geographic range and, eventually, a more significant stock of taxa to include in a phylogenetic test, such a revision would also help to solve the systematics and relationships of the complex group of phacopid trilobites usually grouped within the tribe Ananaspini of Haas, 1998.