More than 500 million years ago, as myriad new lifeforms were emerging and diversifying in the world’s oceans, a small, wormlike fish called the yunnanozoan (Yunnanozoan livitidum) flourished around what is now southwestern China.
Based on the numerous fossils that have been found, this sea creature seemingly had little more to it than eyes, a rudimentary brain, and a basketlike skeleton of cartilage supporting its digestive tract. Yet as with many other organisms that emerged during this time, paleontologists have long known that yunnanozoans represent important developments in evolutionary history, especially regarding the emergence of the skeleton and, eventually, the spine. Despite this, yunnanozoans’ classification has been controversial among researchers for years, explains Nanjing University paleontologist Baoyu Jiang, as experts have interpreted various parts of its anatomy as evidence that it belonged to different ancient clades. Now, research by Jiang and his colleagues published today (July 7) in Science concludes that yunnanozoans may be among the earliest vertebrates.
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Vertebrates are thought to have emerged during the Cambrian explosion around 520 million years ago, but the point in time at which these animals branched away from the phylum of chordata, which lack the rigid structures that protect vertebrates’ nervous system, is difficult to pin down. Over the years, numerous hypotheses for how vertebrates first appeared and developed have emerged, Jiang tells The Scientist. “Any progress about the [yunnanozoan’s] phylogenetic position is key to understanding the origin of vertebrates,” he says, so the researchers decided to apply newer imaging techniques to 127 recently collected yunnanozoan fossils in order to determine how best to classify the animals and how to place them within evolutionary history.
In the past, various researchers have placed yunnanozoans into multiple clades, including cephalochordates (a clade of rudimentary invertebrate marine animals), hemichordates (considered to be a link between invertebrates and vertebrates), and even in a far more ancient group called the stem deuterostomes. These conflicting designations tend to be based on observations of the same fossils, with researchers disagreeing on how to interpret various structures.
In the new study, the team examined the fossils with X-ray computed microtomography (micro-CT) and scanning electron microscopy—two techniques that Mount Holyoke College paleontologist Mark McMenamin, who didn’t work on the study, says have recently become the standard for the field but haven’t yet been applied to many types of fossils. The researchers focused primarily on seven pairs of symmetrical arches that are thought to be a precursor to modern skeletons, and especially of the head and jaws of vertebrates.
Jiang and his colleagues found that those arches contain a pattern of filaments, interconnected by bamboo-like bars of cellular cartilage, that closely resemble the gill arches of modern fish and align with earlier predictions for what an early vertebrate ancestor would look like. These structures, overlooked by prior analyses, are “typical of a vertebrate,” Jiang tells The Scientist, and are missing from fossils of early chordates. The findings, he says, support the hypothesis that vertebrates’ heads and jaws did indeed develop from the first two arches found in yunnanozoans and similar organisms.
The arches’ structure, the authors say, also resembles the modern vertebrates’ pharyngeal skeletons—which develops from three embryonic germ layers and neural crest cells—and provide important details regarding when in evolutionary history these skeletons emerged. Collectively, they write, the arches’ features suggest that yunnanozoans may be stem (early) vertebrates.
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“What makes this exciting is not so much the actual old age of the fossils, but [the] placement of the yunnanozoans on the phylogenetic tree,” David Jandzik, an evolutionary biologist at Comenius University in Slovakia and the University of Colorado Boulder who didn’t work on the study, tells The Scientist over email. “Yunnanozoans had been (controversially) placed among stem vertebrates before, and it is not surprising that the new details on fossil morphology can lead to dramatic changes in the topology of a phylogenetic tree—which is basically a hypothesis on who is more closely related to whom.”
The finding that yunnanozoan pharyngeal arches are homologous to those of vertebrates suggests that yunnanozoans belong among vertebrates rather than chordates, Jandzik explains, and the paper “shows that [an] elaborate pharyngeal skeleton composed of cellular cartilage might have existed earlier [than previously assumed] and that its evolution was probably even more rapid than we had thought before.”
Jandzik notes that reclassifications of this sort happen “all the time due to the incompleteness and imperfect preservation of the fossil material,” but says that the authors’ conclusion that yunnanozoans are vertebrates may be difficult to topple. “The new material presented in this study is relatively robust,” he says, “so it might require even more compelling new evidence.”
McMenamin says he found the paper’s claim that the yunnanozoan fossils presently contain the earliest evidence of a pharyngeal skeleton—which served as the foundation of the skeletons of the vertebrates still alive today and gave rise to structures such as the head and jaw—particularly compelling. “If their interpretation that yunnanozoans are vertebrates are correct, that pushes the origins of vertebrates into the early Cambrian,” he says, as yunnanozoan fossils predate the time vertebrates were thought to have first evolved, “and it will expand our notion of the diversity of form very early in the history of these creatures.”
However, McMenamin says he’s not completely convinced that interpretation is correct. He points out that the fossils seem to be missing a notochord—a rod typically found behind the neural tube in chordates, which provides structural support and assists in the patterning of cells into tissues during embryonic development. This structure is flexible in chordates but stiffer in vertebrates, and McMenamin says he would have expected it to have been preserved in fossils across hundreds of millions of years better than the arches identified in the paper, had it been present in yunnanozoans. The absence of a notochord, he says, is one factor that has allowed for the many interpretations of yunnanozoan phylogeny, and therefore he says it was “a little bit of a surprise” to see the fish definitively classified as a vertebrate, rather than a chordate, without one.
“I think [the authors] made good advances in the imaging, and their interpretation of these structures is reasonable, but I don’t think they’ve knocked down the controversy of the affinity of the yunnanozoans,” McMenamin says.
Jiang says that there was some evidence of a physical groove in the fossils where a notochord would be expected, potentially indicating its presence, but that he and his colleagues didn’t mention it in the paper because “in this study, we focused on the arches.”
Despite his reservations about the notochord, “I would just say that this is a very welcome techniques paper that is adding new data to the mix,” McMenamin tells The Scientist, adding that “it is going to really help us understand early chordate—and possibly as well early vertebrate—evolution.”
