New research is redefining the morphological and functional existence of anatomical "neck" regions to better include previously exempted vertebrates such as fishes and amphibians, changing the status quo of what it means to have a neck.
In mammals, the neck is typically defined as the region of the vertebral column located between the skull and the shoulders, enabling three-dimensional movement of the head independent of the body. However, for vertebrates like fish and amphibians that possess very different anatomy, this morphology-based definition doesn't always fit very well, but recent studies in fish have shown some functional regionalisation of the vertebral column that could be analogous to a neck.
"These findings suggest that the traditional assumption of an absent neck is not universally valid and may overlook functionally distinct anatomical regions," says Dr Roxana Taszuz, a postdoctoral research associate at the University of Liverpool, UK. "Our interest in fishes and salamanders came directly from this conceptual challenge and its broader evolutionary implications."
This project, presented at the Society for Experimental Biology conference in Florence, Italy, aims to build new definitions of the neck that factor in both morphology and functionality that applies more widely across all major vertebrate groups.
The project is funded by a Leverhulme Trust Research Project Grant and is being carried out by a team including Dr Taszus, Dr Ariel Camp and PhD student Emily Aitken at the University of Liverpool, UK, and Dr Daniel Schwarz from the Natural History Museum in Stuttgart, Germany.
To examine the morphology and functionality of the vertebral column in different species in different environments, Dr Taszus and her team captured X-ray videos of feeding behaviours and reconstructed 3D models using computer animation software. They then identified which sections of the vertebral column contributed to head and neck motion and determined where these sections start and terminate.
The team are just starting to look the neck motions of salamanders, having just gathered videos to look at how they move their spines when feeding on land and underwater. While it is accepted that salamanders do have some form of neck, there is no clear distinction on which vertebrae are involved and how they are utilised during feeding.
"During the initial literature research stages of the project, we repeatedly found inconsistencies in how the neck was defined across different vertebrate taxa," says Dr Taszus. "This led a broader investigation into historical and contemporary interpretations of the neck and an evaluation of whether a more inclusive definition could be developed that is applicable across all vertebrates."
Dr Taszus and her team wish to highlight the importance of explicitly defining the neck in a way that can be applied across different vertebrate groups, citing the historical absence of a clear definition leading to ambiguity and difficulty in comparing role and evolution of the neck across different groups.
This research is important for furthering our understanding of evolution of the first vertebrates to leave the oceans and live on land. "Salamanders represent the closest extant model animal for investigating the functional morphology of early tetrapods, the first vertebrates to adopt at least a partially terrestrial lifestyle," says Dr Taszus. "Examining how salamanders feed and coordinate movements of the head and vertebral column provides valuable insights into the evolution of these functions during the transition from aquatic to terrestrial environments."
In fish, the team are now much closer to clearly defining the difference between the morphological neck (vertebrae near the head with a distinct shape) and functional neck (vertebrae responsible for moving the head independently of the body).
"Studies going back to at least the 1940s have noted the vertebrae behind the head in fish can form a distinct anatomical region," says Dr Taszuz. "However, only recently with tools like X-ray video have we been able to show the functional neck of fish and demonstrate their ability to move the head independently from the body."
"The really challenging and exciting thing is that there is huge variation across fish species," says Dr Taszuz. "Unlike mammals with our consistent 7 cervical vertebrae, the anatomical neck of fish can look like a complete absence of vertebrae behind the head, fusion of the cranial vertebrae, or highly specialized shapes."
