The Geology cover features my research!

Across the past 500 million years, deep-sea animal burrows have followed assembly rules remarkably similar to those that shape Facebook, Instagram, Twitter and other human social networks. This conclusion has been reached in a new study carried out by a multidisciplinary team of scientists led by myself and including Carlos Neto de Carvalho (Naturtejo UNESCO Global Geopark/D. Luiz Institute, Portugal), Gabriele Tosadori (University of Verona, Italy) and Alexandre Antonelli (Royal Botanic Gardens, Kew and University of Gothenburg, Sweden). The study has been published in the June issue of the peer-reviewed journal Geology (https://doi.org/10.1130/G48523.1), which has been the Web of Science’s #1 ranked “geology” journal for 12 years in a row. Geology devoted its June 2021 cover to my study!

The study combines fossil burrows (ichnofossils) from 45 fossil sites and sophisticated computer simulations to explain one of the most fundamental hypotheses of palaeontology: the puzzling divergence in pattern and complexity between shallow- and deep-sea burrows.

Since the 1950s, scientists have noted a striking pattern in the fossil record, that is, deep-sea burrows have been different from shallow-marine ones for more than 500 million years. Specifically, deep-sea burrows are geometrically patterned, whereas shallow-marine burrows display simpler, straight morphologies. No matter how far researchers looked into the past, no matter on which continent they looked, they always found this pattern. This divergent pattern was revealed to be so persistent in the rock record that the hydrocarbon industry is still using it for discovering new oil reservoirs by using characteristic associations of ichnofossils that typify the sedimentary environment properties (including depth) where they were produced.

Despite its popularity, the divergence hypothesis has never been tested, and the organizing principles that underpin it have remained unknown… until now. Are deep-sea burrows truly different from shallow-marine ones, and, if yes, why? To answer these questions, we used the mathematics of Facebook to depict – in a single web-like picture! – the past 500 million years of biological evolution. Specifically, we patiently represented each burrow as a circle (node) and connected with a line (link) those burrows that are found in the same palaeontological site. Just like the Facebook network is made up of people connected by friendship relations, the resulting ‘fossil network’ is made by fossil burrows connected by co-occurrence relationships. The network consists of more than 600 links!

When we first looked at the fossil network, we were amazed by its intricate beauty and organization. We observed two groups of ‘friends’ in the fossil network. The past researchers’ intuition has been now confirmed by statistical analysis, according to which the fossil network is optimally partitioned into two groups of friends corresponding to shallow- and deep-marine environments. Fifty years after its proposal by one of the most celebrated paleontologists in the world, Prof. Adolf Seilacher, the divergence between deep-sea and shallow-sea burrows has been demonstrated. But why it is so?

We knew that the answer was in the connection pattern. Just like the Facebook network emerges from social phenomena, the fossil network has necessarily been shaped by biological ones. The connection pattern of the fossil network was revealed to be a small world – a unique network type in which most nodes can be reached from every other node by a small number of hops. This is behind the ‘Six degrees of separation’ phenomenon, i.e., the idea that all people on Earth are few handshakes away from each other. The small-world structure of the fossil network arises from two universal ecological guilds—specialists and generalists—controlling the spatial distribution of burrows across the past 500 million years. It finds a parallel in the development of social networks, with most individuals associating with (making friends with) individuals who are geographically close (environment-specific burrows) and few moving around (generalists). Two simple rules explain 500 million years of evolution!

The divergence reported here has applied implications for scientific research and for the energy industry, which relies on fossil burrows for identifying resource-bearing deposits. By demonstrating the persisting divergence of burrow shape over evolutionary time scales, this work suggests that small-world dynamics controlled gene flow and natural variation in heritable behaviour across the past 500 million years. This indicates that small-world dynamics has been a major, but hitherto neglected, evolutionary force in the oceans.

Reference

Baucon, A., Neto de Carvalho, C., Felletti, F., Tosadori, G., Antonelli, A. (2021). Small-world dynamics drove Phanerozoic divergence of burrowing behaviors. Geology 49: 748–752. https://doi.org/10.1130/G48523.1