Evolution of Crustaceans at the edge of the end-Permian crisis: ichnonetwork analysis of the fluvial succession of Nurra (Permian-Triassic, Sardinia, Italy).

Decapod crustaceans are among the most efficient ecosystemengineers of the Phanerozoic, but the path that led
to their engineering success is poorly known. The Permian–Triassic continental succession of Nurra (early
Cisuralian–early Middle Triassic; Sardinia, Italy) sheds light on this obscure subject, because it preserves the
oldest (Roadian) fluvial Ophiomorpha and Camborygma, ascribed to ghost shrimps (Decapoda: Axiidea, Gebiidea)
and crayfishes (Decapoda: Astacidea, Parastacidea), respectively. These crustacean trace fossils are part of awellpreserved
ichnofauna including Arenicolites, Diplocraterion, Helminthoidichnites, Palaeophycus, Planolites,
rhizoliths, Skolithos, Spongeliomorpha, Taenidium, Treptichnus, alongside an undetermined tetrapod footprint, helical
burrows and Sinusichnus-like traces.
In light of the abundance of crustacean trace fossils, the Cala Viola section (Roadian–Anisian) was selected for
ichnonetwork analysis. This novel approach, focusing on the topology of the association relationships of
ichnotaxa, revealed six ichnoassociations, i.e., Rhizohaloes, Taenidium/Ophiomorpha, Helminthoidichnites,
Palaeophycus, Skolithos and Arenicolites ichnoassociations. In addition, ichnonetwork analysis shown that
Camborygma and Ophiomorpha colonised adjacent, partially overlapping fluvial subenvironments, ranging from
active fluvial channels to periaquatic overbanks. As the Nurra occurrences are penecontemporaneous of the
astacid/thalassinid diversification (Carboniferous–Early Permian) and coeval with the appearance of crayfishes
(Middle Permian), fluvial environments played an important role in the evolution of both crayfishes and ghost
shrimps. The most parsimonious explanation of the observed scenario is that astacid–thalassinid diversification
happened in fluvial environments between the Carboniferous and Early Permian, while ghost shrimps invaded
marine environments at the Permian–Triassic boundary. In addition, ichnonetwork analysis revealed that the
ichnological system simplified across the Permian–Triassic boundary, accounting for a structured Permian ecosystem
and fragmented, stressed habitats during the Early Triassic. This scenario is explained by a progressive
drying trend that dramatically changed the hydrological features across the Permian–Triassic boundary. This supports
the crucial role of global warming in driving the end-Permian crisis and its prolonged recovery.