Surprising Clues from 390-Million-Year-Old Vertebrate Fossils

Tatsuya Hirasawa

(Associate Professor, Department of Earth and Planetary Science)

Earth is home to a vast array of life forms, but how this diversity has come about raises intriguing questions. In ancient times, myths provided the answers. However, since biology became a full-fledged scientific discipline, generations of researchers have been trying to solve this central mystery. Our laboratory is also tackling this issue by investigating the mechanisms of biological formation and the shapes of prehistoric organisms.

The developmental process from the fertilized egg to the formation of the body involves the regulation of gene expression and many cell-cell interactions. By systematically comparing the development of different organisms, we can understand the evolutionary change of developmental processes and the corresponding body parts of different groups of organisms. This is evolutionary developmental biology, a field that focuses on clarifying evolutionary mechanisms by comparing developmental processes. In recent years, various organisms have been the subject of such rigorous comparative research. Our laboratory, too, uses vertebrate embryos to observe and conduct manipulative experiments on their developmental processes.

In contrast to living organisms, studying geologically preserved fossil species produce fragmented and much less informative data. However, just as we can gain insight into the structure and the history of the universe by observing faraway astronomical objects in deep space, some of the mysteries of evolution can only be solved by "observing" fragmented pieces of information, the fossils left behind from deep time.

Even though the evolutionary placement, where some fossil species belong on the tree of life, remains unknown, such morphological “mystery organisms” could provide keys to understanding the evolution of diversity. For example, the five-centimeter-long Palaeospondylus, a lake dweller in the Middle Devonian period (about 390 million years ago), has remained such a mystery organism despite the efforts of several prominent researchers who have tried to reveal its identity since its first description in 1890. In a recent endeavor, our lab selected the best Palaeospondylus fossils suitable for detailed observations and, using synchrotron radiation X-ray micro-computed tomography (CT) powered by a large accelerator, eventually succeeded in making detailed observations of the microstructure of the skeleton and the three-dimensional morphology of the entire skull for the first time. We discovered a slightly odd proportion, the lower jaw being shorter than the upper jaw, but we managed to compare the results with the known vertebrate skull patterns. To our surprise, the phylogenetic analyses based on the morphological data obtained from our detailed observations of Palaeospondylus suggested that it fit into the transitional grade between fish and land vertebrates. The ancestral lineage of land vertebrates seems to include strangely shaped members, such as this mystery organism.

These odd features of Palaeospondylus resemble larval morphological patterns, which is a surprising finding for evolutionary developmental biology. Since some organs are still underdeveloped in larval bodies, the interactions between developing organs and tissues usually seen in embryonic bodies may have become decoupled in animals with a larval stage. It may be worthwhile to investigate if such changes in developmental interactions in larval bodies could have brought about the evolution of limbs and other traits in land vertebrates.

References
T. Hirasawa, et al. "Morphology of Palaeospondylus shows affinity to tetrapod ancestors," Nature 606, 109-112. 2022.

Figure: Detailed observation of the skull of Palaeospondylus (resolution 1.46 µm) using synchrotron-radiation X-ray micro-CT. Based on phylogenetic analyses, Palaeospondylus was inferred to be between the fish grade with fins (e.g., Eusthenopteron) and vertebrates with evolved limbs that crawled on land for the first time (e.g., Acanthostega). However, Palaeospondylus has some curious features, lacking the dermal bone on the surface of the skull, teeth, and pectoral and pelvic fins. These features are consistent with those found in larvae.

― This article is from the "Mysteries in Science" series in The Rigakubu News ―