Gastrochaenolites is a clavate boring produced primarily by endolithic bivalves in lithic substrates. Here, we documented this trace fossil from multiple stratigraphic intervals in Iran: Upper Cretaceous (Tarbur Formation), Oligocene (Qom Formation), Miocene (Makran Basin), and recent specimens (Persian Gulf coast). Integration of ichnological, paleontological, and bioerosional evidence indicates uniform paleoenvironmental conditions throughout the studied intervals, characterized by moderate to high hydrodynamic energy, oligotrophic to slightly mesotrophic nutrient regimes, and shallow euphotic depths from intertidal to a few meters below the low-tide level. The borings are confined exclusively to carbonate substrates, occurring mainly within coral-bearing assemblages and beach-related rock fragments. This distribution corroborates the clear-water, low-sedimentation preferences of their bivalve producers. The combination of in-situ articulated bivalves and thin calcareous linings constitutes unequivocal evidence for the identity of the boring organisms and active chemical bioerosion. The progressive erosional truncation of the borings documents increasing surface abrasion, providing a valuable tool for reconstructing taphonomic histories. Beyond confirming Gastrochaenolites as a reliable proxy in a previously under-represented region, this study offers two global contributions: (1) the first complete three-stage erosional sequence (clavate -> U-shaped -> bowl-shaped), providing a quantifiable taphonomic tool; and (2) rare preservation of in-situ articulated bivalves with calcareous linings and siphonal canals, yielding direct evidence of tracemaker identity and active chemical bioerosion. These findings highlight the value of integrating trace fossils with body fossils for paleoenvironmental and taphonomic reconstructions.
Faecal pellet stuffed trace fossils from the Middle Triassic (Ladinian) Liard Formation at Williston Lake in northeastern British Columbia are described. The traces occur primarily within hummocky cross-stratified sandstone beds within a mixed silicilclastic-carbonate succession interpreted to represent deposition in offshore transition and lower shoreface depositional settings. The traces consist of slender strings of faecal pellets (Tomaculum problematicum) that occur horizontally, vertically and oblique to bedding and are consistent with the original type material of Alcyonidiopsis Massalongo. Although solitary strings have been observed, many specimens occur as composite forms that emanate outwards from a single central point. These traces comprise a new ichnospecies herein named Alcyonidiopsis liardensis. The traces occur in abundance solely in beds that contain both abundant isolated faecal pellets as well as ophiuroid body fossils. In identical lithologies that are devoid of ophiuroid fossils, Alcyonidiopsis is absent as well. Available evidence supports the hypothesis that these traces were produced by ophiuroids gathering and feeding on the faecal pellets of other marine invertebrates.
Spongeliomorpha is a common component in Mesozoic and younger strata. However, Paleozoic records of Spongeliomorpha are rare. Here, we report widespread distribution of Spongeliomorpha from the upper Silurian shallow marine siliciclastic succession of South China, which is by far the oldest record of this ichnogenus. The trace fossil displays consistent characteristics along a depositional stretch of more than 900 km. Specimens show Y- or T-shaped bifurcations and bundles of fan-shaped scratches converging to form a plaited pattern, allowing assignment to S. aff. sudolica. Comparative study regarding neoichnological observations and functional morphology analysis indicates that early crustaceans, or other arthropods with rigid appendages were plausible candidates. These burrows most likely functioned as dwelling structures of a detritus feeder that obtained food from the sediment surface. The occurrence of Spongeliomorpha in the late Silurian demonstrates that complex side-burrowing behavior, previously associated primarily with Mesozoic and Cenozoic decapod crustaceans, was already well established in Paleozoic arthropods. This finding highlights behavioral convergence across distantly related arthropod lineages and cautions against the routine assignment of Spongeliomorpha to a single producer such as decapods.
