The oldest species with a spine has been identified – more than 500 million years old.
A new study has revealed that yunnanozoa are the oldest known vertebrate ancestral animals.
Scientists have long puzzled over the gap in the fossil record that would explain the evolution from invertebrates to vertebrates.
Vertebrates, including fish, amphibians, reptiles, birds, mammals, and humans, share unique features, such as a spine and skull. Invertebrates are animals without backbones.
The process that led invertebrates to become vertebrates — and what those earliest vertebrates looked like — has been a mystery to scientists for centuries.
Now a Chinese research team has been investigating yunnanozoa, extinct creatures from the early Cambrian period 518 million years ago.
The researchers discovered evidence that yunnanozoa are the oldest known stem vertebrae. The term stem vertebrate refers to those vertebrates that are extinct but very closely related to living vertebrates.
Over the years, as scientists have studied how vertebrates evolved, a major focus of research has been the pharyngeal arches, which are structures that produce parts of the face and neck, such as the muscles, bones and connective tissue.
Researchers have hypothesized that the pharyngeal arch evolved from an unconnected cartilaginous rod in vertebrate ancestors, such as the chordate amphioxus, a close relative of the vertebrates.
But whether such an anatomy actually existed in the ancient ancestors is not known with certainty.
In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the Chinese team studied the fossils of the gentle yunnanozoans found in Yunnan province.
For years, researchers have studied the Yunnanozoa, coming to different conclusions about how to interpret the creature’s anatomy.
The Chinese team set out to find newly collected fossil specimens of Yunnanozoa in previously unexplored ways, and conducted a high-resolution anatomical and ultrastructural study.
The 127 specimens they studied contained well-preserved carbonaceous residues that enabled the team to conduct ultrastructural observations and detailed geochemical analyses.
The team applied X-ray microtomography, scanning electron microscopy, transmission electron microscopy, Raman spectrometry, Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy to the fossil specimens.
Their findings, published in the journal Science, variously confirmed that yunnanozoa have cellular cartilage in the pharynx, a feature considered specific to vertebrates.
The discovery supports the theory that Yunnanozoa are tribal vertebrates. The results of the study show that the Yunnanozoa are the earliest and also the most primitive relatives of vertebrates.
During their study, the team noted that all seven pharyngeal arches in the yunnanozoa fossils resemble each other. All arcs have bamboo-like segments and filaments.
Neighboring arches are all connected by dorsal and ventral horizontal bars, forming a basket. A basket-like pharyngeal skeleton is a feature found today in live jawless fish, such as lampreys and hagfish.
Study lead author Tian Qingyi, of Nanjing University and Nanjing Institute of Geology and Paleontology, said: ‘Two types of pharyngeal skeletons — the basket-like and isolated types — occur in the Cambrian and living vertebrates.
“This implies that the shape of pharyngeal skeletons has a more complex early evolutionary history than previously thought.”
The new anatomical observations made by the Chinese team in their study support the evolutionary placement of yunnanozoans on the very basal part of the vertebrate tree of life.
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