If you’ve ever wondered what a “typical” animal looks like, here it is:
Beetles are the largest group of animals in the world, constituting 400,000 species, or about 40% of all known animals on earth. Beetles and other insects are part of the phylum Arthropoda, which encompasses animals that have an exoskeleton and jointed appendages. Besides the insects, crabs, spiders, centipedes, scorpions, and the extinct trilobites are also arthropods. Together, arthropods account for 80% of all known animals.
Genetic evidence suggests that the closest living relatives of arthropods are the Onycophorans, or velvet worms. These animals show segmentation like the arthropods, but they lack rigid bodies or limbs. Interestingly, the specialized, jointed appendages of arthropods seems to have been critical to their evolutionary success; although over a million species of arthropods are known, only about 70 living species of onycohporans have been described (Pechenik 2010).
Recently, a team of paleontologists from China and Germany published some exceptionally preserved Cambrian fossils that show a possible link between the onycohporan and arthropod bodyplans. The animal, called Diania cactiformis, has a soft body like an onycohporan, but hardened, jointed limbs like arthropods. Below is an image of one of the fossils from the publication, as well as a reconstruction of what the animal may have looked like:
Diania is not the first fossil animal to bridge the gap between onycohporans and arthropods. During the Cambrian, a number of animals generically lumped together as lobopods evolved, which had soft, worm-like bodies with varying degrees of armor on their shoulders and legs (including Hallucigenia, which wins my nomination for greatest animal name in history). But Diania shows more characteristics in common with true arthropods than any previously described lobopod.
The authors of this paper are hesitant to say that this conclusively shows that hardened arthropod limbs evolved before hardened arthropod bodies, or speculate on why complex, hardened limbs might have evolved first. Whatever the case, Diania is an incredible example that “missing links” in the fossil record do not typically look like a perfectly intermediate blend of two animals. Instead they often have an unexpected mosaic of features, and show unique adaptations suited to their distinct environments.
Works Cited
Pechenik, J. A. (2010). Biology of the Invertebrates (Sixth Edition). New York: McGraw-Hill Higher Education.
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