Palaeospondylus as reconstructed by synchrotron radiation x-ray computed tomography. Credit: RIKEN
Evidence that the mysterious historical fish-like vertebrate Palaeospondylus was seemingly one of many earliest ancestors of four-limbed animals, together with people, has been found by the Evolutionary Morphology Laboratory led by Shigeru Kuratani on the RIKEN Cluster for Pioneering Research (CPR) in Japan, together with collaborators. Published immediately (May 25, 2022) within the scientific journal Nature, the examine unmasks this unusual animal from the deep previous and units its place on the evolutionary tree.
Palaeospondylus was a small fish-like vertebrate, about 5 cm (2 inches) lengthy, which had an eel-like physique and lived within the Devonian interval about 390 million years in the past. Although fossils are plentiful, its small dimension and the poor high quality of cranial reconstructions—by each CT scan and wax fashions—have made putting it on the evolutionary tree troublesome ever because it was found in 1890. It’s been thought to share options with each jawed and jawless fish and its physique has baffled evolutionary scientists as a thriller. Among a number of uncommon options, probably the most perplexing is the absence of tooth or dermal bones within the fossil file.
To resolve a few of these points, the researchers used the extremely highly effective RIKEN SPring-8 synchrotron to generate high-resolution micro-CT scans using synchrotron radiation X-rays. Furthermore, unlike most studies that have used excavated fossil heads, the new study used carefully selected fossils in which the heads remained completely embedded in the rock. “Choosing the best specimens for the micro-CT scans and carefully trimming away the rock surrounding the fossilized skull allowed us to improve the resolution of the scans,” says lead author Tatsuya Hirasawa. “Although not quite cutting-edge technology, these preparations were certainly keys to our achievement.”
The high-resolution scans revealed multiple important features. First, researchers discovered three semi-circular canals, clearly indicating an inner-ear morphology of jawed vertebrates. This resolved an issue because previous studies suggested that Palaeospondylus was evolutionarily closer to primitive jawless vertebrates. Next, they found key cranial features that place Palaeospondylus into the tetrapodomorph category, which is made from tetrapods—four-limbed animals—and their closest ancient relatives. Several analyses revealed that Palaeospondylus was more closely related to limbed tetrapods than to many other known tetrapodomorphs that still retained fins.
However, unlike tetrapodomorphs in general, teeth, dermal bones, and paired appendages have never been associated with Palaeospondylus fossils, although these features are readily found in fossils of other animals that lived around the same time and in the same place in the Achanarras fish bed in Scotland. The lack of these features can be explained by the splitting of a set of developmental features, resulting in a larval-like body. “Whether these features were evolutionarily lost or whether normal development froze halfway in fossils might never be known,” says Hirasawa. “Nevertheless, this heterochronic evolution might have facilitated the development of new features like limbs.”
Kuratani and his research group do not limit their study of early vertebrate evolution to the fossil record. They also use molecular biology and genetics to study developing embryos of key modern vertebrates. “The strange morphology of Palaeospondylus, which is comparable to that of tetrapod larvae, is very interesting from a developmental genetic point of view,” says Hirasawa. “Taking this into consideration, we will continue to study the developmental genetics that brought about this and other morphological changes that occurred at the water-to-land transition in vertebrate history.”
Reference: “Morphology of Palaeospondylus shows affinity to tetrapod ancestors” by Hirasawa T, Hu Y, Uesugi K, Hoshino M, Manabe M, Kuratani S, 25 May 2022, Nature.
DOI: 10.1038/s41586-022-04781-3
