Terreneuvian Orthothecid (Hyolitha) Digestive Tracts from Northern Montagne Noire, France; Taphonomic, Ontogenetic and Phylogenetic Implications

Abstract

More than 285 specimens of Conotheca subcurvata with three-dimensionally preserved digestive tracts were recovered from the Terreneuvian (early Cambrian) Heraultia Limestone of the northern Montagne Noire, southern France. They represent one of the oldest occurrences of such preserved guts. The newly discovered operculum of some complete specimens provides additional data allowing emendation of the species diagnosis. Infestation of the U-shaped digestive tracts by smooth uniseriate, branching to anastomosing filaments along with isolated botryoidal coccoids attests to their early, microbially mediated phosphatisation. Apart from taphonomic deformation, C. subcurvata exhibits three different configurations of the digestive tract: (1) anal tube and gut parallel, straight to slightly undulating; (2) anal tube straight and loosely folded gut; and (3) anal tube straight and gut straight with local zigzag folds. The arrangement of the digestive tracts and its correlation with the mean apertural diameter of the specimens are interpreted as ontogenetically dependent. The simple U-shaped gut, usually considered as characteristic of the Hyolithida, developed in earlier stages of C. subcurvata, whereas the more complex orthothecid type-3 only appears in largest specimens. This growth pattern suggests a distinct phylogenetic relationship between these two hyolith orders through heterochronic processes.

An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark

“…Thus, the Bornholm specimen with the preserved digestive tract is of the same age as the specimens with digestive tracts described from the Atdabanian of Siberia (Meškova & Sysoev 1981). The Bornholm specimen is younger than hyoliths with preserved digestive tracts from the Terranuevian of Montagne Noire (Devaere et al 2014). …”

“…Hyolithids with soft parts preserved are generally rare but Moysiuk et al (2017) Devaere et al 2014 for references).…”

“…Preserved soft parts in hyoliths are rare and only about a dozen reports of are known with material from the Cambrian, Ordovician and Devonian (see Devaere et al 2014 andMoysiuk et al 2017 for references). The soft part morphology is important to distinguish between the order Hyolithida and the order Orthothecida.…”

“…The orthothecid digestive tracts are the best documented and is complex and generally sinuously folded, while it is simple and U-shaped in the hyolithids, which may reflect different feeding habits. However, Devaere et al (2014) showed in an ontogenetic series of the orthothecid Conotheca subcurvata from Originally, the lower Cambrian strata were regarded as one uniform unfossiliferous quartzite and very little attention was paid to the later called Green Shales (Pingel 1828;Forchhammer 1835). Eventually, J.F.…”

A new type of entombment of Peronopsis (Agnostida) in a hyolithid conch

“…Errant polychaetes, see BERTLING, 1992, p. 182). The tunnel by burrowing organism fed on decating soft tissue of the hyolithid; the general arrangement of these tunnels obviously copies the disposition of the gut inside a hyolithid conch (see DEVAERE et al, 2014).…”