# Pathological caudal skeleton of an ichthyodectiform fish from the Upper Cretaceous Niobrara Formation of western Kansas, USA

## TL;DR

A fossil fish from Kansas shows unusual bone growth in its tail, likely caused by infection, which would have impaired its movement and survival.

## Contribution

Describes a unique case of combined bacterial and fungal infection-induced pathology in an ichthyodectiform fish skeleton.

## Key findings

- The fish's caudal vertebrae show extensive pathological bone overgrowth and coossification.
- Pathology started posteriorly and progressed anteriorly, affecting preural vertebrae most severely.
- Preserved calcified soft tissues suggest infection impacted neural and haemal canals.

## Abstract

A series of 12 contiguous caudal vertebrae of an ichthyodectiform fish from the Smoky Hill Chalk Member of the Niobrara Formation is described. The vertebral centra exhibit extensive overgrowth of pathological bone and there is additional pathological bone within the centra and intervertebral spaces, which together resulted in the coossification of most centra. The extent of the pathology is greatest on preural vertebrae 1-3 and decreases anteriorly, which suggests that the pathology began posteriorly and progressed anteriorly. In addition to the pathological overgrowth on bones, the specimen preserves features interpreted as calcified and/or ossified soft tissues associated with the neural and haemal canals. The pathologies are unlike previously described examples of bony pathologies in fish, and it is suggested that they resulted from combined bacterial and fungal infections. As the pathologies developed, they would have adversely impacted the fish’s swimming and feeding abilities, and presumably eventually led to the fish’s death.