Researchers Trace Evolutionary Trajectory of Starfish's Unique Symmetry Through a 500 Million-Year-Old Fossil

The star shape of the starfish (five arms) is intriguing to many marine aficionados. But how it came to be is a question not many can answer. It is due to a gap in the fossil record of echinoderms, according to IFL Science. Researchers were unable to trace how starfish went from having ancestors with no specialized features of this sort to exhibiting pentaradial symmetry. Many believed some species depicted this evolution, but they were missing from the database. The same is the case for other echinoderms, displaying radial symmetry. One of them has emerged in Morocco recently. This specimen sheds light on how the stunning feature possibly evolved in the invertebrate. Findings regarding these fossil specimens were published in the journal Current Biology. 

Discovery of "Atlascystis acantha"

The specimen was unearthed from the Anti-Atlas Mountains of Morocco by scientists from the Natural History Museum and named 'Atlascystis acantha.' Further examinations revealed that the remains were around 500 million years old. If this assertion is true, then the specimen is the oldest known echinoderm displaying a bilateral body plan. Echinoderms are a group of invertebrates that includes creatures like starfish, sea urchins, and sea cucumbers. Apart from starfish, no other creature from the group exhibits pentaradial symmetry. 'Atlascystis acantha,' being the oldest echinoderm specimen exhibiting bilateral symmetry, allows researchers a chance to peer into a creature that possibly started it all. 

 Connection with modern-day echinoderms

Researchers described 'Atlascystis acantha' as being flat with spines all over its body, according to Phys.org. Experts noted a pair of specialized sets of skeletal plates in the specimen. More examinations revealed the pair to be ambulacra, a body part used by modern-day living echinoderms like starfish to move and feed. This part was not identified in other early living forms displaying bilateral symmetry. The common ambulacra implies that the evolution of pentaradial symmetry possibly contained a precursory phase of bilateral symmetry.

Scientists discovered a series of individuals from 'Atlascystis acantha' in the fossil specimens, according to the Natural History Museum. They had varying sizes and provided the researchers with a wide perspective. The impeccable preservation of the remains gave researchers ample avenues to formulate how echinoderms advanced from bilateral to other forms of symmetry. "What's interesting about these fossils is that they're telling us something about how that change first occurred," Dr Imran Rahman, principal researcher at the museum and co-lead author, said. "We don't have the larval stages for Atlascystis, but we can infer based on the fact that they have this bilateral symmetry as adults that they probably underwent a much less marked metamorphosis than living echinoderms."

Reconstruction of Evolutionary Trajectory 

Experts used methodologies like 3-D imaging, phylogenetic methods, and growth analyses to formulate the evolutionary trajectory from bilateral symmetry to pentaradial symmetry in the case of starfish. The examinations indicated that creatures like 'Atlascystis acantha,' showing bilateral symmetry, were possibly at the base of the echinoderm evolutionary tree. Later, pentaradial symmetry developed in the starfish through the duplication of ambulacra, which was triggered due to the loss of a well-defined trunk region in these invertebrates. Dr. Frances Dunn, senior researcher at the Oxford University of Natural History, believes that the fossil sheds light on the "first steps of the evolution of one of the most recognizable body plans we find in animals today: the starfish."

The assertion also challenges past speculations that early bilateral echinoderms were offshoots of more complex forms, according to the Natural History Museum. Examinations showed that bilateral symmetry in echinoderms progressed from bilateral to other complex forms of symmetry. The bilateral symmetry was part of the origin and not a developed feature. Researchers are still unsure about why starfish stopped at five hands. No reason has been identified to date that justifies why only five limbs in starfish fulfill the space in the trunk region and not more. "As I said already, there's no other animals alive today that have that kind of symmetry, so we don't really have a good answer to that question," Rahman concludes.