Deep Sea Fossil Spines Provide Evidence of the First Forms of Life on Earth

It is believed that the first very simple life forms on Earth emerged at the bottom of the sea a long time ago. In our modern times, the deep sea is known for its bizarre fauna. Scientists have been conducting intensive research into the changes demonstrated by several species living on the seafloor. There have been theories claiming that the ecosystems of the deep sea have emerged repeatedly after multiple mass extinctions and oceanic disturbances.

In the history of our planet, the deep sea is considered a relatively young ecosystem. However, there is increasing evidence which suggests that this part of the Earth is much older than previously thought.

Early Occurrence of Irregular Sea Urchins

At the University of Göttingen, a team of researchers have provided the first fossil evidence for a stable colonization of the deep sea floor by a higher form of invertebrates. They discovered fossil spines of irregular echinoids (sea urchins), which suggest the long-standing existence of these animals since the Cretaceous period, as well as their evolution under the impacts of fluctuating environmental conditions.

More than 1,400 sediment samples from the boreholes in the Southern, Atlantic, and Pacific Oceans were gathered by the researchers, representing former water depths of 200 to 4,700 meters. They found over 40,000 spine fragments assigned to a group of animals called irregular echinoids based on their structure and shape.

The scientists recorded the morphological characteristics of the spines, such as shape and length. At the same time, the thickness of around 170 spines from each of the two time periods was also determined for comparison. They also chose the amount of spiny material present in the sediments as an indicator of the total mass of the sea urchins in the habitat.

Their analysis revealed that irregular echinoids have continuously populated the deep sea since the early Cretaceous period about 104 million years ago. They also provide insights into the destructive meteorite impact at the end of the period, which resulted in a worldwide mass extinction of dinosaurs and caused disturbances in the deep sea.

This evidence was shown by the morphological changes in the spines, since they appeared to be thinner and less diverse in shape after the event than before. This phenomenon, interpreted by scientists as the "Lilliput Effect," suggests that smaller pieces demonstrated a greater survival advantage after a mass extinction, leading to a smaller body size of a species. It was assumed to be due to the lack of food available at the bottom of the deep sea.

What are Irregular Echinoids?

Echinoids refer to a group of marine animals that belong to the phylum Echinodermata and the class Echinodea. Their name is derived from the Greek word 'echinos,' which means 'spiny' and includes heart urchins, sand dollars, and heart urchins. Echinoids take two general forms: regular and irregular.

Irregular echinoids are named that way because of the evolution in the morphological innovations that depart them from the regular and pentaradial symmetry. Most of these innovations convergently evolved in various groups and are mostly adapted for their infaunal lifestyle.

Scientists believe irregular echinoids evolved during the Mesozoic Marine Revolution, with morphological changes being a small part of their evolutionary history. They have also been the subject of archeological studies due to their excellent fossil record, with very contrasting evolutionary histories displayed by major groups.