Asteroid and Ophiuroid Trace Fossils from the Lower Cretaceous of Chile

Abstract

ABSTRACT. Abundant asterozoan trace fossils in the Lower Cretaceous Apeleg Formation of southern Chile were produced by the in®lling of traces made by asteroids and ophiuroids on a muddy surface. Most are preserved as hypichnial ridges formed as casts on the bottom of ®ne-grained sandstone laminae. Star-shaped Asteriacites lumbricalis are interpreted as the in®llings of shallow excavations made by asteroids. Hook-shaped and sinuous ridges of the newly de®ned ichnogenus and ichnospecies Ophioichnus aysenensis are interpreted as the casts of imprints made by the walking action of the arms of ophiuroids. Deposition probably took place in an offshore tideswept marine shelf environment. In life the animals formed an assemblage equivalent to echinoderm aggregations on the modern sea¯oor.

BIOTURBATION BY THE COMMON ANTARCTIC SCALLOP (ADAMUSSIUM COLBECKI) AND OPHIUROID (OPHIONOTUS VICTORIAE) UNDER MULTI-YEAR SEA ICE: ECOLOGIC AND STRATIGRAPHIC IMPLICATIONS

“…Ophionotus victoriae can overturn a square meter in as little as week through its various feeding activities despite alternating between periods of activity and quiescence; periodic activity is consistent with in situ studies of their fluctuating metabolism (Obermüller et al 2010). Though rowing motion of ophiuroids can be preserved in the record (Bell 2004), O. victoriae's arm sweeps and self-burial can quickly Despite the different feeding modes and movement of A. colbecki and O. victoriae, both animals appear important to Explorers Cove mixing processes. Large deposit feeders such as O. victoriae typically dominate sediment mixing processes over suspension feeders, including A. colbecki (Wheatcroft et al 1990).…”

Cretaceous slab segmentation in southwestern Gondwana

“…Easterly-derived deltaic deposits have been recognized further east (Cerro Bayo: González-Bonorino & Suárez 1995). To the east of Lago La Plata, in Argentina, only continental facies have been described (Ploszkiewicz, 1987), however, the presence of trace fossils comparable to those in the marine beds of this formation exposed to the west (Bell, 2004), including Gyrochorte sp. in some of these rocks, indicate marine deposition for those beds.…”

Late Miocene Asterozoans (Echinodermata) in the James Ross Island Volcanic Group

“…Asteroids have a flattened flexible body and their arms are typically two to three times the diameter of the central area, with which they merge (Bell 2004, p. 55). Ophiuroids typically have a central disc and long slender arms with pinnate lateral spines (Bell 2004). The asterozoan skeleton consists of thousands of discrete ossicles linked in a dermal layer at the surface of the body.…”

“…After death, most asterozoans suffer rapid destruction (see Blake 2000, Salamon & Zaton 2004, Twitchett et al 2005, hence their sparse fossil record as complete specimens. Asterozoans are also known as trace fossils, typically preserved on the underside of sandstone beds (Bell 2004 and references therein) which cast excavations made in underlying mudstones. There are two possible interpretations for the nature of the asterozoan fossils in the JRIVG (Fig.…”