In new research, paleontologists used X-ray microtomography to characterize the feeding apparatus of an exceptionally well-preserved specimen of the Early Devonian jawless fish Rhinopteraspis dunensis.
Feeding behaviors are commonly used by scientists to help piece together early evolution of vertebrates, and different jaw shapes and constructions can suggest a broad range of feeding strategies.
In the absence of jaws, many competing theories have been developed ranging from biting and slicing, to filtering food from sediment or water.
In a new study, University of Birmingham paleontologist Ivan Sansom and his colleagues were able to visualize the mouth parts of one of these jawless fish, Rhinopteraspis dunensis, in detail.
The images revealed the structure and arrangement of finger-like bones that project from the lower ‘lip’ of the animal’s mouth, which the scientists believe acted to control the mouth’s size and shape as it captured food particles from surrounding water.
“The application of CT scanning techniques to the study of fossil fish is revealing so much new information about these ancient vertebrates and giving us the opportunity to study precious and unique specimens without destructive investigation,” Dr. Sansom said.
“In this case, these methods have allowed us to fit all of the small bones of this animal’s mouth together, and try and understand how it fed from this integrated system rather than by using isolated bones,” said Dr. Richard Dearden, a researcher at the University of Birmingham and the Naturalis Biodiversity Center.
“Instead of a steady trend towards ‘active food acquisition’ — scavenging or hunting — we see a real diversity and range of feeding behaviors among our earliest vertebrate relatives.”
The reconstruction produced by the researchers shows that the bony plates around the mouth would have had limited movement, making it unlikely that the animals were hunters capable of ‘biting.’
In combination with an elongated snout, they would also have found it difficult to scoop and filter sediment directly from the bottom of the sea.
However these plates would have allowed it to control opening of the mouth, and perhaps strain food from water in a way also used by animals such as flamingos or oysters.
The findings offer a new perspective on theories of vertebrate evolution, since current hypotheses argue that long term evolutionary trends move from passive food consumption to increasingly predatory behavior.
In contrast, the study suggests that in fact, early vertebrates had a broad range of different feeding behaviors long before jawed animals started to appear.
“The functional morphology of the apparatus in Rhinopteraspis dunensis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape,” the authors concluded.
“This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterized by a directional trend towards increasingly active food acquisition.”
The study was published in the Proceedings of the Royal Society B.
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Richard P. Dearden et al. 2024. The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes. Proc. R. Soc. B 291 (2019): 20232258; doi: 10.1098/rspb.2023.2258