New paper on tetrapod-like fish from Queensland

Hello! I’m taking a brief break from my maternity leave to tell you about my latest paper, published last week in the Journal, PeerJ; A fresh look at Cladarosymblema narrienense, a tetrapodomorph fish (Sarcopterygii: Megalichthyidae) from the Carboniferous of Australia, illuminated via X-ray tomography.

Four panels: a photo of the specimen, a radiograph of the specimen, a tomogram (slice) of the specimen, and a 3D model of the specimen. Image credit: Alice Clement.

In my paper we use microCT and synchrotron technology to image some spectacular 3D fossils of a fish known as Cladarosymblema from about 330 million years ago in what is today known as Queensland, Australia.

Cladarosyblema was a type of tetrapod-like fish known as a ‘megalichthyid‘. These fish grew to large sizes, lived in freshwater environments, and would have been fearsome predators. They were one of the few tetrapodomorph groups that survived the end Devonian extinctions, and persisted up until the Permian Period (299-252 mya). Cladarosymblema is the only megalichthyid known from Australia, and one of just two known from the ancient southern supercontinent, Gondwana.

Cladarosymblema was originally described in 1995 from several specimens, but using scanning technology we were able to uncover much of its internal anatomy that had until now remained hidden. In particular we could describe the gill arch skeleton, parts of the shoulder girdle, vertebrae and upper roof of the mouth bones (palate).

Additionally, we were also able to isolate the cranial endocast from Cladarosymblema, which gives insights into the size and shape of the brain of this animal. The area for the pituitary gland (so-called the master gland) is relatively large, suggesting a significant role in regulating various important endocrine glands. The overall shape of the endocast is more similar to that of early terrestrial vertebrates (tetrapods) than to most of the fish left living in the water. Was it some of these adaptations that enabled Cladarosymblema’s relatives to colonise land?

Furthermore, the membership of the ‘megalichthyids’ has been controversial, with several recent studies finding conflicting results. We ran a phylogenetic analysis (analysis of relationships) and found that the megalichthyids form a natural clade (are monophyletic).

Section of the evolutionary family tree showing that megalichthyid fishes form a natural clade.

I want to thank the Queensland Museum for allowing us access to the beautiful specimens, as well as all of my co-authors, reviewers, and the editor who handled the paper at PeerJ. All of the scan data and 3D models are available at MorphoSource, and the phylogenetic matrix can be accessed on MorphoBank.

Cladarosymblema narrienense head restoration in right lateral view. Image: John Long.

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