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Welcome to the website of Richard Dearden

The early evolution of cartilaginous fishes

About Me

I am a postdoctoral researcher at the Muséum national d'histoire naturelle, Paris, based in the Centre de recherche en paléontologie (CR2P), and working with Alan Pradel. My research focuses on the early evolution of the cartilaginous fishes: the group of animals which today comprises living sharks, rays, and chimaeras. In particular I am interested in the very earliest evolution of this group in the Palaeozoic, between about 440-270 million years ago. More broadly I have a general interest in early vertebrates as well as tomographic imaging, phylogenetic methods, and macroevolutionary processes. Outside fossil fishes my pursuits of choice include cooking (bread/pizza specialist), reading (particularly history/sci-fi), and gaming (mainly grand strategy).

Research

Today’s cartilaginous fishes (or chondrichthyans) include the familiar sharks and rays, as well as the less familiar chimaeras. They are ubiquitous in modern oceans (and occasionally freshwater) and occupy a great range of ecological roles as pelagic predators, bottom dwelling mollusc-munchers, enormous filter feeders, deep sea lurkers, and more. Despite this they are eclipsed in diversity by their cousins, the bony fishes (ray-finned and lobe-finned fishes, including tetrapods) and are often unfairly maligned as terrifying, antediluvian monstrosities. Even in science, the anatomical differences that separate them from their bony cousins (ever-replacing teeth, a cartilaginous skeleton, and a shagreen of small scales) are often characterised as primitive and used as a stand-in for the anatomy of their common ancestor with bony fishes.

Extant cartilaginous fishes: elasmobranchs (sharks and rays) and holocephalans (chimaeras)

It is true that recognisably modern chondrichthyans evolved a long time ago, with the earliest fossils coming from the early Mesozoic (at least 174 million years ago). However, this is only half the story. In fact, the chondrichthyan lineage stretches over twice this far back in time, to their split from bony fishes at least 420 million years ago in the Palaeozoic. The early chondrichthyans alive during the Palaeozoic were very different to modern species, exhibiting a wide array of sometimes bizarre forms, and sometimes filling ecological roles untouched by modern chondrichthyans. These fossils, as well as those of other early fishes, also show us that modern chondrichthyan anatomy isn’t all that primitive after all, and that in fact many aspects (including their teeth, cartilage, and shagreen) are evolutionary innovations.

Three acanthodians (Palaeozoic chondrichthyans) from the Early Devonian of the UK: clockwise from top left Mesacanthus, Parexus and Ischnacanthus (not to scale, images from NHM, London)

Acanthodians

In my research I am interested in these Palaeozoic chondrichthyans and what they can tell us about how chondrichthyans, and jawed vertebrates, evolved. Chondrichthyans’ cartilaginous skeletons mean that their rare body fossils are often somewhat ropey; I use imaging methods such as computed tomography (CT) and synchrotron tomography to try and extract information on their anatomy in 3D. Using this, as well as data on the anatomy of living chondrichthyans and phylogenetic methods, I aim to piece together the early evolution of the cartilaginous fishes.

A tomogram (single tomographic slice) through an acanthodian's head: arrows indicate parts of the fish's skeleton

At the moment I am a postdoctoral researcher at the MNHN, Paris working on early Permian specimens of the acanthodian-grade stem-chondrichthyan Acanthodes. Acanthodes has by far the most completely preserved internal skeleton of any acanthodian, but almost all specimens were prepared with acid in the 19th and early 20th centuries leaving sometimes unclear moulds of the skeleton. A handful of fossils in the MNHN collections were however never prepared, and using modern 3D imaging techniques we can visualise the skeletons within the rock in 3D, giving us an unprecedentedly detailed look at Acanthodes. We aim to compare this information with data on the skeleton and muscles of living chondrichthyans to reconstruct the muscle anatomy and functional morphology of Acanthodes. Before this I did my PhD at Imperial College London with Martin Brazeau where I worked on Devonian acanthodians, using 3D scanning to try and better understand their anatomy.

Video of the CT scanned head skeleton of the stem-chondrichthyan Acanthodes from the MNHN, Paris

TL:DR?

My research written in Up-Goer 5

Humans, other land animals with four legs, and lots of things that live in the water come from animals that lived in the water a long, long time ago. The bodies of these animals can be found in rocks. I use computers to look inside these rocks to understand what these animals looked like. From this we can understand the changes that led to them turning into living animals over time. The old animals I look at are usually like the living group that has big teeth, lives in the water, and which was in the movie ''Jaws''.

PhD Thesis

You can download my full PhD thesis at the link here: Download PhD Thesis (270.1 MB).