The oldest shipworms (Bivalvia, Pholadoidea, Teredinidae) preserved with soft parts (western France): insights into the fossil record and evolution of Pholadoidea

Abstract

Teredinidae are obligate xylophagous bivalves that colonize drift wood. They display a highly derived anatomy with a reduced shell; most of their body consists of soft tissues. Consequently, fossil teredinids mostly correspond to burrows, shells or small terminal aragonite structures called ‘pallets’. We report, from mid‐Cretaceous logs of the Envigne Valley, France, exceptionally preserved wood‐boring bivalves with silicified soft parts. After characterizing the wood, we report both the molluscs’ anatomy and their distribution inside the wood (using computed tomography). The 3D‐reconstructions reveal rarely preserved soft tissues (mantle, respiratory siphons, visceral pouch) but surprisingly no mineralized pallets. Envigne shipworms display neat dorsal condyles and a vermiform body plan, making them the oldest known Teredinidae. To document the evolutionary or taphonomic origin of this lack of pallet, the mineralization of these bivalves was investigated, and it is concluded that pallets could have dissolved prior to other carbonate components. From a survey of the fossil wood‐boring bivalves and their characters, we provide a time scaled origin of the main pholadoid clades supported in recent phylogenies. Since we found no correlation between the presence of pallets in Pholadoidea and their occurrence in the stratigraphic record, we suggest that their absence during the Mesozoic could be the result of a taphonomic bias related to the ‘calcite vs aragonite seas’ paradigm. The Cenomanian Envigne Valley was an estuarine to intertidal environment in which the Teredinidae appear to have selected their wood habitat on the basis of size but not type.

New fossil assemblage with amber, plants and vertebrates from the lower Cenomanian near Châtellerault (Vienne, western France)

“…1). The series continues up into green sandstones, already described by Mathieu (1960Mathieu ( , 1968, and progressively reaches up into 30 m in thickness in Colombiers; they contain rich fossil wood and oyster shells (Exogyra colomba Lamarck, 1819), which are dated as middle Cenomanian in age (Robin et al, 2018;Boura et al, 2019). They are overlaid by upper Cenomanian marlstones, locally called "Chalk tufa", which bear Calycoceras Hyatt, 1900, by lower Turonian beds with Mammites nodosoides Schlüter, 1871 and Lewesiceras peramplum Mantell 1822, and by flint breccia deposits containing a few plant megafossil inclusions (Gomez et al, 2018).…”

An early record of a vesselless angiosperm from the middle Cenomanian of the Envigne valley (Vienne, Western France)

“…In the last years, thousands of silicified wood fragments have been collected from the middle Cenomanian of two localities in the Envigne valley (Vienne department, western France) (Robin et al 2018). Only trunks and branches of various lengths and diameters have been preserved.…”

“…Both fossil assemblages are exceptional in their high plant diversity and the exquisite tissue preservation, as well as in their association with animal fossils. For instance, some of the petrified woods have allowed for the preservation of the oldest shipworms described with soft parts (Robin et al 2018). Shipworms, bivalve mollusks of the family Teredinidae, are notorious for boring into wood that is immersed in sea water.…”

Tamilokus mabinia, a new, anatomically divergent genus and species of wood-boring bivalve from the Philippines

“…The extension of the crystalline style, from a position ventral to and near the anterior end of the foot to a position well posterior to the shell valves, is unique to Tamilokus (Turner, 1966). In other bivalves, cilia-mediated rotation of the crystalline style is thought to aid digestion by mixing enzymes and reducing food particle size through mechanical action (Robin et al, 2018; Nelson, 1918; Edmondson, 1920; Yonge, 1923; Lavine, 1946; Morton, 1952; Horiuchi & Lane, 1982; Alyakrinskaya, 2001; Sakamoto et al, 2008; Mackenzie & Marshall, 2014). However, the functional significance of the exceptionally large style of Tamilokus remains to be explained.…”

“…Previously, Micro-CT has been used to determine quantitative measurements of wood degradation and boring rates of Teredinidae (Amon et al, 2015). Additionally, Micro-CT has been used to reveal exceptionally well-preserved fossilized wood-boring bivalves in mid-Cretaceous wood, with silicified soft parts and overall body-plan characteristic of the Teredinidae (Robin et al., 2018). Herein, we further demonstrate the power of this technique to reveal the relationships among soft tissue features in three dimensions, and in the evaluation of likely mode of nutrition and life history strategy, while minimizing distortion that can result from manual dissection.…”