Introduction

Phycosiphon-like trace fossils are perhaps the most common group of trace fossils identified in vertically slabbed cores of mud-rich sedimentary rocks in petroleum fields worldwide (e.g., Bockelie 1991; Goldring et al.1991; Wetzel and Bromley 1994; Bromley 1996; Pemberton and Gingras 2005). We herein use the term "phycosiphoniform" to encompass all burrows that, when seen in cross-sectional view, have a Phycosiphon-like central core of clay-grade material surrounded by a bioturbated zone of clay-poor silt or very fine-grained sand that is inferred to have been produced during deposit feeding. The ichnofabric generated is commonly termed frogspawn texture (Figure 1). Phycosiphoniform trace fossils are found in a range of marine depositional environments from marginal- to deep-marine settings in rocks ranging in age from the Palaeozoic to the recent (e.g., Goldring et al. 1991; Fu 1991; Wetzel and Bromley 1994; McIlroy 2004b). The trace maker(s) of phycosiphoniform burrows are unknown small, probably vermiform, deposit feeding organisms, which are common in clay-rich siltstones (Kern 1978; Wetzel and Bromley 1994; Bromley 1996).

While phycosiphoniform burrows are common in the rock record, there is little consistency in the literature regarding the ichnogeneric assignment of such burrows. A number of taxa with Phycosiphoniform cross section have been recognized from the core including: Phycosiphon incertum (Wetzel and Bromley 1994; McIlroy 2004b, 2007); Helminthopsis (Dafoe and Pemberton 2007; forms lacking a halo); Helminthoidichnites isp. (MacEachern et al. 2007c); Anconichnus (Kern 1978; latterly synonymized with Phycosiphon by Wetzel and Bromley 1994); Nereites isp. (Wetzel 2002); Cosmorhaphe isp. (e.g., MacEachern et al. 2007c). Most Palaeozoic occurrences of burrows in vertical cross section with a mudstone core and silty halo have been assigned to Nereites.

Since the behaviour of all of these phycosiphoniform trace fossils is conventionally interpreted to be systematic, selective deposit feeding, precise ichnogeneric identification is perhaps not necessary for palaeoenvironmental analysis. In ichnofacies studies, which rely partly upon assessment of ichnogeneric diversity, a full appreciation of ichnodiversity can be integral (MacEachern et al. 2007c; McIlroy 2008), and thus in need of careful consideration. The three- dimensional geometry and full range of potential vertical cross sections of most phycosiphoniform taxa are imperfectly known. This work focuses on reviewing existing data on the most commonly recognised phycosiphoniform burrow Phycosiphon incertum Fisher-Ooster 1858 for comparison with our three-dimensional reconstruction of well-preserved phycosiphoniform burrows from the late Cretaceous of Mexico.

The phycosiphoniform trace fossil reconstructed herein was studied from a hand specimen containing many phycosiphoniform trace fossils from a succession of well-exposed slide blocks in a slope channel complex from coastal exposures of the Upper Cretaceous Rosario Formation in the coastal outcrop at Pelican Point near Cajiloa, close to the town of El Rosario, Mexico (Figure 2). The ichnofabric is distinctive in containing anomalously large, slightly atypical, phycosiphoniform burrows. The host-sediment is a laminated turbidite siltstone. The burrow cores were subject to differential compaction relative to the host sediment, with the plane of flattening being parallel to bedding (Figure 1).