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More than 500 million years (Ma), The oldest known vertebrates Resembling small, jawless larvae that filter nutrient particles from water. In time, they resembled our present-day fishes, but that took about 100 Ma The first jaw vertebrae appear. The latter (gnathostomes) have flourished to this day. In contrast, jawless vertebrates (agnathans), except for lampreys, disappeared after a phase of decline by about 370 Ma. This extinction was explained by competition with gnathostomes, which succeeded thanks to their morphological innovations. But how this rivalry between these two groups would have unfolded remains unclear today. In this context, Bradley R. from the University of Illinois at Urbana-Champaign. Scott and Phillip S.L. Andersen published a study on September 16, 2022 in the journal Paleobiology They try to use statistical analysis to determine whether competition was the determining factor in the disappearance of most of the Agnathans.
Morphology is a valuable clue that provides information about past lifestyles
Competition is a biological relationship that the study authors define as “Use of the same resources (food or habitat) by more than one taxon (group of organisms that share common characteristics, author note) Same place, same time”. But unlike today, direct observation of biological interactions in fossils is usually impossible. So it is important to focus on the only thing that fossil organisms give us to see with the naked eye: their morphology. In fact, some traits have uses, functions in living things. For example fins allow you to swim, but still go further. The shape of the fins, or the overall shape of the body, are highly variable traits according to the ecology of an organism. They combine to meet a need in the living environment, allowing researchers to draw typical examples. from this,”Fish living in open environments have a hydrodynamic body, large antero-dorsal fins (…). Also benthic vertebrates (living on the sea floor, editor’s note) have large pelvic fins […] or laterally compressed body, especially in rocky environments or with vegetation“, explain the authors of the study.
There may be competition between certain groups…
These different anatomical characteristics were pooled for different major groups of agnathans and gnathostomes to carry out a statistical analysis.
More than 500 million years (Ma), The oldest known vertebrates Resembling small, jawless larvae that filter nutrient particles from water. In time, they resembled our present-day fishes, but that took about 100 Ma The first jaw vertebrae appear. The latter (gnathostomes) have flourished to this day. In contrast, jawless vertebrates (agnathans), except for lampreys, disappeared after a phase of decline by about 370 Ma. This extinction was explained by competition with gnathostomes, which succeeded thanks to their morphological innovations. But how this rivalry between these two groups would have unfolded remains unclear today. In this context, Bradley R. from the University of Illinois Urbana-Champaign. Scott and Phillip S.L. Andersen published a study on September 16, 2022 in the journal Paleobiology They try to use statistical analysis to determine whether competition was the determining factor in the disappearance of most of the Agnathans.
Morphology is a valuable clue that provides information about past lifestyles
Competition is a biological relationship that the study authors define as “Use of the same resources (food or habitat) by more than one taxon (group of organisms that share common characteristics, author note) Same place, same time”. But unlike today, direct observation of biological interactions in fossils is usually impossible. So it is important to focus on the only thing that fossil organisms give us to see with the naked eye: their morphology. In fact, some traits have uses, functions in living things. For example fins allow you to swim, but still go further. The shape of the fins, or the overall shape of the body, are highly variable traits according to the ecology of an organism. They combine to meet a need in the living environment, allowing researchers to draw typical examples. from this,”Fish living in open environments have a hydrodynamic body, large antero-dorsal fins (…). Also benthic vertebrates (living on the sea floor, editor’s note) have large pelvic fins […] or laterally compressed body, especially in rocky environments or with vegetation“, explain the authors of the study.
There may be competition between certain groups…
These different anatomical characteristics were pooled for different major groups of agnathans and gnathostomes to carry out a statistical analysis. Given the fact that it is difficult to infer the exact ecology of ancient species based on their morphology alone, this analysis mainly helps to know which groups are most similar or different in morphology. Based on this, major trends can be determined. Indeed, morphological similarity between species suggests that they may exploit the same resources and thus enter into competition. On the contrary, the case of different groups would be indicative of a move in the direction of lack of competition.
The results of the analysis show that some groups are more likely to compete than others. The study’s authors report possible instances of competition between agnathans and gnathostomes, particularly between heterostracons (agnathans with bony plates) and phyllolipids (slightly cuirassed gnathostomes).
Photo of Akhenaten, a heterostragon from the collection of the Natural History Museum in Berlin. Its anatomical proximity to phyllolipids suggests that heterostracans entered into competition with the latter, according to the study. Credit: Bradley R. Scott.
A phyllolipid, a gnathostum photograph from the collection of the Natural History Museum in Berlin. The similarity of this group to the heterostragons suggests that they may have competed. Credit: Bradley R. Scott.
… but certainly not everywhere
But the most important element of these results is the large morphological difference between some groups, such as the rhenids (ray-like fish), which are very different from the agnathans.
Photograph of a Rhinelander from the collection of the Natural History Museum in Berlin. It is unlikely that this group competed with the Akhnaths due to its specific morphology. Credit: Bradley R. Scott.
The same is true for a group of agnathans named Fargacadiformes, vis-a-vis the gnathostomes. This group is revealing, because morphologically distant from jawed fishes, its extinction is unlikely to have been due to competition with them. The sediment in which they were found and their appearance similar to that of some living fish suggests that they lived in a rocky environment. gold,” The environment (temperature, sea level, oxygen and other species) changed a lot during the Silurian (444-419 Ma) and especially the Devonian (419-359 Ma). ” says Bradley R. Scott insists Science and the futureleading to the modification or disappearance of their preferred habitat, probably leading to their extinction.
Photo by Agnathan, a Fargacadiform from the collection of the Natural History Museum, Berlin. This group probably disappeared due to environmental changes, not competition with gnathostomes. Credit: Bradley R. Scott.
But could there be other possible causes? Bradley R. For Scott” […] Poaching is another possibility.” or their endemism (organisms that live in specific places and nowhere else). This may have made them more sensitive to environmental changes. But for this last possibility, he defers to caution Science and the future : “We still do not know whether gnathostome species are less endemic and more widely distributed compared to Agnathan “.
This study shows that competition between some groups of agnathans and gnathostomes may have contributed to the disappearance of the former, as had long been assumed. Nevertheless, the hypothesis that agnathans have been replaced by generalized competition from gnathostomes is not sustained here. Other phenomena such as changes or disappearances in habitats, predation or possible endemism of certain groups must be taken into account. These first results, Bradley R. It shows which group is very different from the other group that Scott believed Science and the future : “ Our future work will focus on whether morphological differences between agnathans and gnathostomes are functional […] “. It is calculated for each group of vertebrae.” […] Performance differences based on hydrodynamic computer models “. To be continued !
