Decay of Branchiostoma : implications for soft‐tissue preservation in conodonts and other primitive chordates

Abstract

Decay experiments on the cephalochordate Branchiostoma lanceolatum (‘amphioxus’) demonstrate that the most decay resistant structures are the notochord sheath and the cartilaginous rods which support the gill bars. However, even more labile soft parts, such as the muscles and skin may survive for at least 124 days under totally anoxic conditions. As the chevron‐shaped muscles of the myomeres shrink and collapse, those on opposite sides of the trunk maybe displaced, resulting in pronounced offsetting. Only 1.42% of the initial dry weight of Branchiostoma is resistant to alkali and acid hydrolysis, compared to 46% in the polychaete Nereis virens. Branchiostoma is only likely to be fossilized as a result of decay inhibition and replication by early diagenetic minerals. The results of these experiments cast light on the interpretation of a number of primitive fossil chordates. There is no reason to infer extracellular decay‐resistant cuticle in the Burgess Shale Pikaia. The axial lies preserved in the conodont animal specimens from the Carboniferous of Edinburgh, Scotland, represent the notochord. The displacement of the elements to one side of the head reflects the true position of the apparatus ‐ the surrounding tissue has been lost through decay. The chevron‐shaped structures in the Carboniferous chordate Conopiscius are the muscles of the myomeres, not external scales. The lines delineating the segments in the Silurian Jamoytius most likely represent the myosepta. There is some doubt about the nature of the only specimen interpreted as a fossil cephalochordate, Palaeobranchiostoma hamatotergum from the Permian of South Africa. □Taphonomy, decay, softparts, Cephalochordata, Branchiostoma, lancelet, Chordata, Pikaia, conodont, Conopiscius, Jamoytius, Palaeobranchiostoma.

Decay of vertebrate characters in hagfish and lamprey (Cyclostomata) and the implications for the vertebrate fossil record

“…Some previous studies of decay have used asphyxia to kill animals, but MS222 treatment does not adversely affect bacterial flora [33] and our previous results with Branchiostoma demonstrate that using MS222 has no effect on the patterns or rate of decay [19,23].…”

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

“…[46,54] Observations of decay of the lancelet Branchiostoma lanceolatum, for example, were used to argue that the axial lines preserved along the trunk of conodonts represent the notochord, and that the apparent offset position of the conodont elements below the head reflects the decay of the supporting tissue. [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.…”

“…[46][47][48][49][50][51][52] Annelids and arthropods decaying under different conditions of oxygen and temperature, for example, showed consistent patterns of morphological decay, reflecting the nature of their tissues. [49][50][51]53] Interpretations of soft-bodied fossils were informed by which features were more likely to survive decay versus those that degraded rapidly. [46,54] Observations of decay of the lancelet Branchiostoma lanceolatum, for example, were used to argue that the axial lines preserved along the trunk of conodonts represent the notochord, and that the apparent offset position of the conodont elements below the head reflects the decay of the supporting tissue.…”

Sediment Effects on the Preservation of Burgess Shale-Type Compression Fossils

“…By contrast, secondarily tanned structures tend to be much more uniformly expressed and can be exceptionally thick and/or reflective, as in the case of Eldonia guts (Butterfield 1996). A similar, highly reflective style of carbonaceous preservation occurs in the stemgroup chordate Pikaia (Butterfield 1990(Butterfield , 2003Briggs and Kear 1994b;Conway Morris and Caron 2012), which might also be ascribed to the taphonomic tanning of a collagen-based integument.…”