Paleontologists from Curtin University and elsewhere have studied a three-dimensionally mineralized heart (the oldest ever found), stomach, intestine and liver from Devonian arthrodire placoderms, an extinct class of armored fishes that flourished from 420 to 359 million years ago.
The origin and early diversification of jawed vertebrates involved major changes to skeletal and soft tissue anatomy.
Because skeletons are readily preserved in the fossil record, skeletal transformations in stem gnathostomes (early jawed vertebrates) can be directly examined. However, preservation of their soft tissues is exceedingly rare.
In a new study, Curtin University vertebrate paleontologist Kate Trinajstic and colleagues examined the three-dimensionally preserved soft-tissue organs — a heart, thick-walled stomach, and bilobed liver — of Late Devonian arthrodire placoderms, some of the earliest known jawed vertebrates.
The fossils came from the Gogo Formation in the Kimberley region of Western Australia.
“As a paleontologist who has studied fossils for more than 20 years, I was truly amazed to find a 3D and beautifully preserved heart in a 380-million-year-old ancestor,” Professor Trinajstic said.
“Evolution is often thought of as a series of small steps, but these ancient fossils suggest there was a larger leap between jawless and jawed vertebrates.”
“These fish literally have their hearts in their mouths and under their gills — just like sharks today.”
Professor Trinajstic and her co-authors used neutron beams and synchrotron X-rays to scan the specimens, still embedded in the limestone concretions, and constructed three-dimensional images of the soft tissues inside them based on the different densities of minerals deposited by the bacteria and the surrounding rock matrix.
They found evidence of an arthrodire’s flat s-shaped heart well separated from the liver and other abdominal organs, which is associated with the evolution of the jaws and neck.
Their findings also suggest the absence of lungs in these ancient fish, refuting a controversial hypothesis that the presence of lungs is ancestral in jawed vertebrates.
“These features were advanced in such early vertebrates, offering a unique window into how the head and neck region began to change to accommodate jaws, a critical stage in the evolution of our own bodies,” Professor Trinajstic said.
“For the first time, we can see all the organs together in a primitive jawed fish, and we were especially surprised to learn that they were not so different from us.”
“However, there was one critical difference — the liver was large and enabled the fish to remain buoyant, just like sharks today.”
“Some of today’s bony fish such as lungfish and birchers have lungs that evolved from swim bladders but it was significant that we found no evidence of lungs in any of the extinct armored fishes we examined, which suggests that they evolved independently in the bony fishes at a later date.”
“These new discoveries of soft organs in these ancient fishes are truly the stuff of paleontologists’ dreams, for without doubt these fossils are the best preserved in the world for this age,” said Flinders University’s Professor John Long.
“They show the value of the Gogo fossils for understanding the big steps in our distant evolution.”
“What’s really exceptional about the Gogo fishes is that their soft tissues are preserved in three dimensions,” added Uppsala University’s Professor Per Ahlberg.
“Most cases of soft-tissue preservation are found in flattened fossils, where the soft anatomy is little more than a stain on the rock.”
“We are also very fortunate in that modern scanning techniques allow us to study these fragile soft tissues without destroying them. A couple of decades ago, the project would have been impossible.”
The findings were published this week in the journal Science.
Kate Trinajstic et al. 2022. Exceptional preservation of organs in Devonian placoderms from the Gogo lagerstätte. Science 377 (6612): 1311-1314; doi: 10.1126/science.abf3289
Source link: https://www.sci.news/paleontology/devonian-arthrodire-placoderms-11207.html