Fossilization processes of graptolites: insights from the experimental decay of Rhabdopleura sp. (Pterobranchia)

Abstract

Laboratory experiments documenting the decomposition pattern of extant organisms are used to reconstruct the anatomy and taphonomy of fossil taxa. The subclass Graptolithina (Hemichordata: Pterobranchia) is a significant fossil taxon of the Palaeozoic era, represented by just one modern genus, Rhabdopleura. The rich graptolite fossil record is characterized by an almost total absence of fossil zooids. Here we investigated the temporal decay pattern of Rhabdopleura sp. tubes, stolons and single zooids removed from the tubarium. Tubes showed decay after four days, when fuselli began to separate from the tube walls. This rapid loss may explain the absence of fuselli from some graptolite fossils. The black stolon did not show decay until day 155. One day after their removal, zooids quickly decomposed in the following temporal sequence: (1) tentacles; (2) ectoderm; (3) arms; (4) gut; (5) cephalic shield, leading to complete disappearance of recognizable body parts in the majority of experimental zooids within 64-104 h. The most resistant zooid features to decay (61 days) were black-pigmented granules. These results indicate that tubes and the black stolon would persist for weeks across death, transport and burial, whereas a complete decay of zooid features occurs in few days, providing an explanation for the overall poor record of fossil graptolite zooids and suggesting that recorded silhouettes of fossil zooids may be attributed to fossil decay-resistant pigments.

Comparative experimental taphonomy of eight marine arthropods indicates distinct differences in preservation potential

“…; Beli et al . ). Of the ten taphonomic characters scored (Table ), four may be used to evaluate stemward slippage: carapace completeness, claw completeness, abdomen completeness and telson completeness.…”

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

“…[51] The same decay experiments allowed the chevron-shaped structures in Conopiscius, a Carboniferous chordate, to be interpreted as myomeres rather than external scales, and also indicated that a decay-resistant cuticle was not necessarily present in Pikaia from the Burgess Shale. [51,55] Decay in seawater has now been monitored in a range of taxa in laboratory experiments (see Table S1, Supporting Information): anthozoans, [56] annelids, [48] chaetognaths, [57] priapulids, [18] onychophorans, [17] pterobranchs, [58] enteropneusts, [59] nonvertebrate chordates, [20] and cyclostomes. [60] Thus the sequence of character loss has been determined for taxa representing most clades of eumetazoans.…”

Malongitubus: a possible pterobranch hemichordate from the early Cambrian of South China

“…While the chitinous perisarc and hydrothecae of colonial hydrozoans may have some fossilization potential, it is rather unlikely that the coenosarc would be preserved in the fossil record. By contrast, decay experiments on modern pterobranchs have demonstrated that the sclerotized stolon system is one of the most durable structures of pterobranch colonies and may prevail until most of the zooids and tubarium have decayed (Beli et al, 2017). In Malongitubus , and notably also in the closely related Dalyia racemata from the Cambrian Stage 5 Burgess Shale, the putative stolon system is the most resistant structure of the fossil remains.…”

“…Taphonomic experiments on modern pterobranchs (Briggs et al, 1995; Beli et al, 2017) show different rates of decay between zooids, tubaria, and the stolon system. The zooids decay rapidly and become unrecognizable after a few days, whereas the tubes and the interior stolon system can last for several months.…”