3D imaged head of ancient jawless fish hints at evolution of vertebrate skull

(CNN) — Hundreds of millions of years ago, jawless fish swam the Earth’s oceans, their brains protected by armored skin on the outside and cartilaginous plates on the inside. Scientists still don’t know how the skulls of modern vertebrates evolved from these ancient ancestors of fish, the first animals with vertebrae. Now, a recent analysis of a fascinating fossil fills in some of the gaps.

This specimen, a 455-million-year-old articulated skull, belongs to a jawless fish. Eryptychius americanus. It was discovered in the Harding Sandstone Formation in Colorado and is the oldest 3D fossil evidence of early vertebrate cranial anatomy, according to a study published this Wednesday. Nature magazine.

Modern vertebrates descended from jawless fish form two groups: the jawed vertebrates and the jawless hagfish and lampreys. Structure of the skull E. americanus It differs from that found in living vertebrates or extinct relatives of fish, with unfused cartilaginous regions—some symmetrical, some not—surrounding the front of the head and the mouth, olfactory organs, and eyes.

“You don’t see this strange paired and unfused cartilage anywhere else,” says Dr. Richard Dearden, lead author of the study and a postdoctoral researcher at the Center for Natural Biodiversity in Leiden, the Netherlands. “So it’s very exciting.”

Extraction of details

Fossil cartilage of the head was recovered in 1949 Described in 1967 By Robert Dennison, Curator of Fossil Fishes at the Field Museum of Natural History, Chicago. Denison divided the fossiliferous rock into two parts; In one, he dissolved the rock matrix with acid and suspended the fossil in epoxy, Dearden explained.

Dennison’s analysis revealed distinctively shaped armor scales and cartilage-like structures. But at that point you can’t go deep without cutting and destroying the fossil, Dearden explained.

“The detail you can extract from the surface of a fossil is not enough to actually do anything with them,” he explained. For this reason, this model was scientifically pushed for decades, considered interesting “but fundamentally unusable.”

Using CT scans, Dearden and his co-authors were able to identify and visualize the cartilage. The team reconstructed a digital model of the fish’s skull in 3D, Dearden told CNN. (He did this research while at the University of Birmingham in the United Kingdom.)

“The reason it took so long for someone to scan this is because very few people actually work with this Ordovician fish, which is the type of fossil that you have to be an expert to recognize its potential,” he said. In an email.

Fish without armor and jaws

Jawless fish Ordovician period — between 488.3 and 443.7 million years ago — are called ostracoderms because of their armored skin, and most of them are known from fossils that preserve only their outer armor, Dearden explained.

“Everything we know about the inside of his head is basically based on that shield,” Dearden explained. “We see eye holes in this outer shell, and we assume that’s where the eyes are in the skull. But we don’t know what’s actually there.”

The study authors identified 10 pieces of cartilage in the sample: six in epoxy and four in rock matrix. Cartilage is surrounded by scales and canals that can hold sensory or vascular structures.

There are questions about the evolution of the skull, such as the purpose of the canals found in the fossil and why all the cartilage is concentrated at the front of the fish’s skull. According to Dearden, there was more cartilage at the back of the head, which is not preserved in this specimen.

It is also unclear when jaws appeared in fish.

However, “this fossil fills a gap in our knowledge of vertebrate head evolution,” said paleobiologist Lauren Sullen, an associate professor in the Division of Macroevolution at Okinawa Graduate University of Science and Technology (Japan).

“This gap is partly because the Ordovician ancestors of jawed fishes were relatively rare and mostly restricted to very shallow marine waters. After their deaths, including the remains of these early fishes. ErypticiusThey were destroyed by the waves, and we mainly found fragments,” explains Sallan, who was not involved in the study, researching the origins of marine biodiversity.

“As a result, we have limited fossil material and almost no complete heads,” Sallan told CNN. “Preserving the internal material from these fish is a big discovery and a big step forward.”