An article published in the journal “Science” describes a research on the origins of the first vertebrates. A team led by paleobiologist Lauren Sallan of the University of Pennsylvania collected a series of evidence that fish, which were the first vertebrates, originated in shallow waters near the shores of primordial seas some 480 million years ago.
The origin of vertebrates has remained uncertain due to a typical problem in the field of paleontology, which is the scarcity of fossils. We know that 420 million years ago there were already many fish species, but traces of their evolution over the previous tens of millions of years are scarce and often fossils are broken into fragments, causing further problems in their study.
To try to reconstruct the first part of vertebrate history, first of all Lauren Sallan’s team put together a database in which the information available on 2,827 fossils were collected. Those are fossil fish with jaws and jawless with an age between 480 and 360 million years. These are already diversified fish ranging from Phlebolepis elegans to Pituriaspis doylei (images ©Nobu Tamura).
The researchers added to those information others concerning the environments in which those fish species lived such as the type of water: shallow near the shores, fresh water or deep ocean. This kind of information is based on the geological characteristics of the rocks in which the fossils were discovered and the invertebrate fossils discovered in the same rocks.
At that point, the researchers applied mathematical models to fill the gaps in the vertebrate family tree. In particular, based on the existing data, they calculated in which type of environment the first vertebrates were more likely to live. Coral reefs were one of the most considered possibilities for fish because today they’re very important for their biodiversity. Instead, the analysis of the collected data indicates that the various groups of vertebrates, from sharks to other groups of fish, developed in shallow water, close to the coasts.
According to the reconstruction carried out by the researchers, fish started diversifying into shallow waters. Over the course of about one hundred million years, various species developed characteristics that subsequently allowed them to expand into other environments. For example, some species became thinner and were able to swim in deeper water, others developed a thicker or even armored skin and moved into lakes and rivers.
This is a surprising result, but it also explains the scarcity and fragmentation of fossils of primitive fish as waves in shallow waters can easily break their remains into pieces. It explains the adaptations of fish species to lakes and rivers and later also to the mainland more easily than to an origin in coral reefs because it required that first them to get closer to coasts.
In more recent times, sticklebacks, family Gasterosteidae, diversified into species that mostly live in freshwater reaching such a variety that their taxonomy is under review, also after genetic research. In recent times, the environments close to the coast offered plenty of food, was there a similar situation 480 million years ago?
This research might not solve the issue of the origin of vertebrates because the accuracy of the results also depends on the quality of the data available, not always high. New discoveries could change those results and the models could be refined but this could be a big step forward in this field.