Why did an organism produce Chondrites?

The iconic fossil Chondrites is broadly seaweed-shaped, and is one of the most common—and historically important—fossil burrows. What animal produced such a complex burrow, and why? I tried to answer this question by assembling a team of 16 researchers from different disciplines. The results have just been published on Earth-Science Reviews (Baucon et al., 2020).

The publisher kindly provided 50 days’ free access to our Chondrites paper: click on this link before April 17, 2020 to freely read and download our paper!

We re-evaluated the behaviour of Chondrites based not only on review of existing literature but also analysis of novel data. These include macroscopic, thin section and ESEM-EDX observations; CT-scans and resin peels of modern analogues; computer-controlled serial grinding; morphometric analysis and theoretical morphology. Different types of tracemakers built Chondrites to obtain food: (1) worm-like animals produced Chondrites for searching for food in the seafloor sediment; (2) bivalves built Chondrites for cultivating and directly ingesting bacteria; (3) bivalves produced Chondrites to provide symbionts with chemical agents.

The Chondrites team quantified the shape of the burrow: (1) the angle of formed by a pair of adjacent branches is typically between 30° and 56°; (2) branching order is between 1 and 9; (3) branches are, on average, nine times longer than wide. In the third dimension, downward branching is dominant but bundled shafts and upward branching may be present. Chondrites was modified through the life of the tracemaker or it represented a part of the producer’s lifespan. The size of Chondrites increased markedly from the Late Jurassic to the Late Cretaceous, suggesting that the tracemakers became larger and larger.

Observations at the petrographic microscope shown that Chondrites was actively filled by the tracemaker; alternatively, currents or clay percolation filled empty tunnels. Chondrites — and its modern tracemaker(s) — is associated with a range of marine settings, including well-oxygenated, dysoxic and space-limited environments.

References

Baucon, A., Bednarz, M., Dufour, S., Felletti, F., Malgesini, G., Neto de Carvalho, C., Niklas, K.J., Wehrmann, A., Batstone, R., Bernardini, F., Briguglio, A., Cabella, R., Cavalazzi, B., Ferretti, A., Zanzerl, H., McIlroy, D. (2020). Ethology of the trace fossil Chondrites: form, function and environment. Earth-Science Reviews 202