The oldest known relative of all vertebrates on Earth swam across the oceans 518 million years ago, a new study reveals.
Researchers in China have analyzed fossils of yunnanozous, an extinct soft-bodied organism that lived during the Cambrian period of our planet’s history.
The fossils, found in Yunnan Province, China, show that the creature is the oldest known “stem vertebrate” on Earth, a vertebrate that has become extinct but closely related to living vertebrates.
Yunnanozoans were very simple fish-like organisms that lived underwater, but had “basket-like” skeletons similar to modern vertebrates.
They are also believed to have been deuterostomes, meaning their anus formed before the mouth during embryonic development.
Artistic reconstruction of the underwater yunnanozoan shows “basket-like” skeletons similar to modern vertebrates
Vertebrates, such as fish, amphibians, reptiles, birds, mammals, and humans, share unique characteristics, such as a spine and a skull, while invertebrates are animals without a spine.
WHAT WERE THE YUNNANOZOANS?
The Yunnanozoans are an extinct fish-like soft-bodied organism that lived 518 million years ago.
Yunnanozous are the oldest known “stem vertebrates”: extinct vertebrates, but closely related to living vertebrates.
They are considered deuterostomes: their anus formed before the mouth during embryonic development.
Yunnanozoan fossils have been discovered in China’s Yunnan Province
The new study was conducted by experts from the Nanjing Institute of Geology and Paleontology and Nanjing University in Jiangsu Province of China.
“Yunnanozoans are Cambrian animals with a taxonomic position that has been debated for a long time,” the team says in its article.
“Our phylogenetic analysis provides more support than yunnanozoa are stem vertebrates.”
Scientists have long been baffled about the gap in the fossil record that would explain the evolution of invertebrates to vertebrates.
The evolutionary process that caused invertebrates to develop the spine, and how the first vertebrates were, has been a mystery for centuries.
As scientists have studied how vertebrates evolved, a key focus has been pharyngeal arches: paired structures that grow on either side of the future head and neck of the developing embryo and fuse in the middle.
Mammalian embryos have five pairs of these pharyngeal arches. As a mammalian embryo grows, the pharyngeal arches produce parts of the face and neck, such as muscles, bones, and connective tissue.
The authors say pharyngeal arches are a “key innovation” that probably contributed to the evolution of vertebrate jaws and brainstem.
The pharyngeal arch is thought to have evolved from an unarticulated cartilage “wand” in vertebrate ancestors, such as the amphioxus cordate, a small “fish-like” organism and close invertebrate relative of vertebrates.
In an effort to better understand the role of the pharyngeal arc in ancient vertebrates, the research team studied the fossils of soft-bodied yunnanozoans found in Yunnan Province, China.
But it is not known for sure whether this anatomy really existed in the ancient ancestors.
In an effort to better understand the role of the pharyngeal arc in ancient vertebrates, the research team studied the fossils of 127 yunnanozous specimens.
The specimens have well-preserved carbon residues that allowed the team to perform detailed analyzes, using microscopy, spectrometry, and other methods.
The results confirmed that yunnanozoans have cellular cartilage in the pharynx, a feature considered specific to vertebrates, suggesting that they are stem vertebrates.
During their study, the team also observed that all seven pharyngeal arches of yunnanozoan fossils are similar to each other.
All arches have segments and filaments similar to bamboo. The neighboring arches are all connected by horizontal bars and the top and bottom, forming a basket.
A basket-like pharyngeal skeleton is a feature found today in live jawless fish, such as lampreys and mixina fish.
Pharyngeal arches are paired structures that grow on either side of the future head and neck of the developing embryo and fuse in the middle. Here is a rat embryo with pharyngeal arches
“Two types of pharyngeal skeletons, the basket types and the isolated ones, are found in the Cambrian and living vertebrates,” study author Tian Qingyi said.
“This implies that the shape of the pharyngeal skeletons has a more complex early evolutionary history than previously thought.”
Researchers describe yunnanozous as “controversial,” because their classification has been debated for about three decades.
But new anatomical observations support the evolutionary placement of yunnanozoans at the bottom of the vertebrate life tree.
His new study has been published in the journal Science.
DIFFERENCES BETWEEN AN INVERTEBRATE AND A VERTEBRATE
A vertebrate is an animal with a spinal cord covered in the back or cartilage. The term comes from the word vertebrae, the bones that make up the spine.
Animals that do not have a spine or spinal cord covered with cartilage are called invertebrates. Vertebrates include birds, fish, amphibians, reptiles, and mammals.
About 97 percent of the animals on our planet are invertebrates, or animals without a spine, although when we think of “animals” we tend to think of vertebrates.
Invertebrates are sometimes mistakenly thought to be primitive due to their lack of developed organs.
Their simple internal systems include respiratory systems such as the gills or trachea and often use an open circulatory system to pump blood.
Because invertebrates do not have an internal skeletal structure, they sometimes have an external skeleton that protects their soft bodies called an “exoskeleton.”
In general, the success of invertebrates often comes from their ability to reproduce very quickly, unlike many vertebrates that take years to grow fully.
Vertebrate animals, on the other hand, have a spine that develops from a notochord they possess as an embryo.
They also have internal systems defined as complex respiratory structures, a closed circulatory system, and sensory organs that build the nervous system.
Vertebrates are usually larger than invertebrates, thanks to their spine, which allows their bodies to grow and move faster than many invertebrates.
Source: Ellen Eisenbeis / Butterfly Pavilion