How starfish grew arms has always puzzled scientists. A chance discovery in the desserts of Morocco may finally answer the great mystery.
Starfish are among the most recognizable animals on Earth. Most people associate sea stars with trips to the beach or rocky pools of water. Despite appearing as simple creatures, their distinctive biological evolution has been unknown until recently.
What is Starfish?
Asteroidea or commonly known as starfish or sea stars, these carnivorous invertebrates can live up to 35-year-old. Despite the namesake, starfish are part of the animal group echinoderms, which is spiny skinned animals that include sea cucumbers, sea lilies, and sea urchins.
There are roughly 2,000 species of sea stars living in the Earth's oceans. The five-arm species is the most common, which explains the namesake. However, some species can have 10 to 40 arms.
Mysteries of the Sea Star
Sea stars are known not to have heads, brains, or even circulatory system. Instead, these invertebrates have a water vascular system that uses seawater as a replacement for blood. Also, sea stars can regenerate more than 75% of their body mass.
The evolution of the sea star has been baffling scientists and researchers for generations. For over 480 million years, starfish have not changed their physical appearance and have lived through 5 mass extinction events.
A recent study published in the journal Biology Letters explains how the starfish developed their unique shape.
A chance discovery in the Moroccan deserts was found in the Fezuoata formations made of sedimentary rock depots dating to the early Ordovician period that ended roughly 460 million years ago.
Paleontologists believe that life is rapidly diversified during this time, which is now referred to as the "Great Ordovician Biodiversification," which is when animals that we recognize today might have first appeared.
Despite their appearances, sea stars are made of interconnected, hard parts attached by soft tissues and ligaments that quickly degrade upon death.
The Cantabrigiaster is the oldest Early Ordovician ancestor found. It was discovered in 2003 but has taken roughly 17 years to figure out its significance in the evolutionary chain.
What makes it unique is its complete lack of characteristics commonly found in starfish and brittle stars today.
The breakthrough came when paleontologists compared the arms of modern sea lilies--filter feeders with feathery arms attached to the seafloor-- to the Cantabrigiaster.
The striking resemblance between modern filter feeders and ancient sea stars led the University of Cambridge and Harvard University teams to formulate the new analysis. Applying biological model features of current Asterozoa fossils.
The results showed that the most primitive Asterozoa is the Cantabrigiaster that most probably evolved from animal crinoids who lived before the dinosaurs 250 million years ago.
The five arms of today's starfish are remnants of ancestors' past. In the case of Cantabrigiasters, it evolved by flipping upside-down, so its arms fed on sediments.
Although researchers were only able to sample a small number of ancestors, the chance discovery provides an idea of how they could relate to each other. Today, paleontologists continue studying echinoderms with varying characteristics that make it hard to tell which was the first species to evolve.
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