DOI: 10.15468/w5vv9y
title: Large dendrinids meet giant clam: the bioerosion trace fossil Neodendrina carnelia igen. et isp. n. in a Tridacna shell from Pleistocene-Holocene coral reef deposits, Red Sea, Egypt

description: This dataset contains the digitized treatments in Plazi based on the original journal article Wisshak, Max, Neumann, Christian (2018): Large dendrinids meet giant clam: the bioerosion trace fossil Neodendrina carnelia igen. et isp. n. in a Tridacna shell from Pleistocene-Holocene coral reef deposits, Red Sea, Egypt. Fossil Record 21 (1): 1-9, DOI: 10.5194/fr-21-1-2018, URL: http://dx.doi.org/10.5194/fr-21-1-2018Abstract. The rosette-shaped microboring trace fossil Neodendrina carnelia igen. et isp. n. – a large representative of the ichnofamily Dendrinidae – is identified on the inner side of the giant clam Tridacna maxima from Pleistocene to Holocene coral reef deposits of the El Quseir district at the Egyptian Red Sea coast. The new dendritic bioerosion trace fossil is diagnosed as a radial rosette comprised of a prostrate system of roofless canals that ramify in a strictly dichotomous fashion forming intermittent branches of uniform width and rounded terminations. The trace appears to be rare, although in the type material it occurs in a cluster of more than a hundred specimens. The location of traces on the interior surface of the shell suggests that boring occurred post-mortem to the host. Its record is presently restricted to shallow marine, euphotic, tropical coral reef settings in the Western Indo-Pacific (Red Sea and Madagascar). The biological identity of the trace maker cannot be resolved yet, but several lines of reasoning allow speculations directed towards a complex attachment scar, perhaps produced by a benthic foraminiferan or a macrophyte.1 IntroductionThe Dendrinidae are a diverse ichnofamily of dendritic and rosette-shaped marine microboring trace fossils (Bromley et al., 2007) whose unknown trace makers appear to have flourished particularly during the Devonian and the Late Cretaceous (Wisshak, 2017). While they had been considered largely extinct since the end-Cretaceous mass extinction event, a recent revision of the group revealed that several members of the group still occur in modern oceans, preferentially in temperate to polar waters (Wisshak, 2017). In this study, we describe a new conspicuous dendrinid from a tropic coral reef setting from the Pleistocene–Holocene of the north-eastern Red Sea coast, El Quseir district, Egypt. In a single valve of the giant clam Tridacna maxima (Röding, 1798), a large pelecypod common in these settings, hundreds of specimens of this bioerosion trace fossil have been recognized. Their unique habitus and branching pattern merits establishment of a new ichnogenus, addressed as Neodendrina, in reference to the type ichnogenus Dendrina Quenstedt, 1849 from the Upper Cretaceous, which on first sight exhibits a remarkable similarity but after closer examination reveals a quite distinct architecture.Geological contextPleistocene raised coral reefs are a common feature along the Red Sea coast of Egypt, forming terraces at different elevations above the present-day sea level. Between El Quseir and Marsa Alam, three reefal units forming six morphological terraces can be recognized (El-Sorogy, 1997). The reefs form discontinuous strips which run parallel to the shore line, separated by conglomerates, quartz sands and gravels. At the study area, the lithology of the lowermost terrace represents a transition of mixed carbonate–siliciclastic rocks with scleractinians as primary frame builders (Fig. 1a). These Pleistocene reefal limestones are usually referred to as the Samadai Formation (Philobbos et al., 1989) and are conformably overlain by Pleistocene raised beaches and coral reefs (Kora et al., 2013, 2014). Their age and diagenesis have been discussed by several workers during the last decades (see Dullo, 1990; El Sorogy, 2002, for further references) and have been a classical study area for palaeontological studies (see Kora et al., 2014).Figure 1. The Pleistocene raised coral reef limestones exposed at the type locality of Neodendrina carnelia igen. et isp. n. just south of the Carnelia Beach Resort, located between El Quseir and Marsa Alam, exhibiting scleractinian corals as primary reef builders (a) and giant clams Tridacna spp. weathering from the carbonate–siliciclastic rocks (b) that mix with Holocene and modern Tridacna valves, forming a highly time-averaged assemblage (c).Among other fossils (Fig. 1a), the rocks contain numerous large molluscs with giant clams (Tridacna spp.) and conchs (Lambis and Strombus spp.) being the most conspicuous. As the poorly lithified sediments erode, exposed Tridacna valves accumulate in large numbers on the surface (Fig. 1b). Here, they mix with subfossil (Holocene) and modern Tridacna valves, forming a highly time-averaged assemblage (Fig. 1c). This process was (and still is) promoted by active fishery of giant clams by the native Bedouin population and their ancestors over the course of the last&gt; 125 000 years, producing huge piles of discarded Tridacna shells along Red Sea beaches (e.g. Ashworth et al., 2004; Benzoni et al., 2006; Richter et al., 2008).