This is a paper I wrote for one of my classes. I wanted to share it on here so everyone could read it. The attached image was taken by me. It’s a recreation of what Arthropleura may look like, taken at a local museum.
During the Carboniferous Period (about 360-300 million-years-ago), the Earth was quite a different place. Due to a massive abundance of conifer trees, the oxygen levels in the air were as much as 32.5% (the current levels are only about 21%). Thanks to this oxygen rich environment, arthropods, normally limited in size by a very poor breathing system, were able to grow to unprecedented levels of monstrosity. It’s quite normal to find fossils from this time period of dragonflies with close to three-foot wing-spans. While these dragonflies may have been the largest flying invertebrate ever to call Earth its home, it can’t hold a candle to a fossil known as Arthropleura, which may just be the largest land-dwelling arthropod of all time.
Few fossils of Arthropleura exist, and none are complete, but enough exists to clearly understand many of the basics about this creature. For example, it’s very simple to discern that Arthropleura belongs to the Subphylum Myriapoda. It has many body segments (as many as 30 have been estimated), and a pair of legs attached to each. Fossil tracks have been found in areas like Nova Scotia and Scotland that have the left and right feet as much as 19.7 inches apart. This measurement coupled with a fossil specimen measuring approximately three feet long have allowed an estimation that Arthropleura could grow to lengths of 6.6 feet (or more). From other fossils, we know that Arthropleura had “armored” plates on each segment, but further study has shown these plates to only be a few millimeters thick, showing that they were not used as armor, as an arthropod this large, in this time period, likely had no predators. Older interpretations of fossils showed a more round head, but it was more recently discovered that this was the first body segment, and the head was tucked underneath. The first pair of legs is tucked under the head and has been modified with fang-like pincers on the ends.The general consensus in the paleontological world is that Arthropleura belongs to its own Class (Arthropleuridae), but is derived from primitive millipedes (Class Diplopoda). Often cited reasons for this connection are the shape of the body “armor,” the positioning of the head being tucked under the first body segment, and a fossil specimen found to have partially digested plant matter in its body cavity at the time of its death. All three of these arguments are quite weak, when examined in any detail.
The observation of body armor matching modern species of millipede is a fairly worthless statement, on the whole. The appearance of convergent evolution happens frequently in the fossil record, and in modern fauna. Even so, modern millipedes that possess plated armor have extremely thick exoskeletons, as much a 1.5mm on fairly small animals. Arthropleura’s “armor” was only a few millimeters thick, which when scaled up to the size of these animals, would be extremely thin, offering no protection at all.
The positioning of Arthropleura’s head appears to be a case of paleontologists looking for information to support their claims, rather than the present information forming a conclusion. On most fossils, the animal will not be preserved in its natural resting position; therefore, the fossil’s head positioning cannot be taken at face value. Upon my own examination of the fossils that involve Arthropleura’s head, I’ve noticed a very prominent feature of millipedes completely missing: on millipedes, the first body segment behind the head is enlarged to 2-4 times the length of the other segments. On Arthropleura, the first body segment matches the size of the other segments almost exactly (there will be slight variations as you progress down the body, of course). Minus this very pronounced feature, needed to curve a myriapod’s head into that placement, it seems the head is only placed there on the fossil due to the animal falling apart, or being crushed by substrate.
The most convincing evidence that Arthropleura was a descendent of diplopods was a small amount of plant matter found near one of the fossils. This was widely accepted for many years as proof that Arthropleura was herbivorous. A more recent discovery has shown that that association was made in error and the two fossils have nothing to do with one another. Any claims that Arthropleura has a known diet are, at this point, simply conjecture.
Upon study of Arthropleura’s fossil remains coupled with several articles on the subject, I’ve come to a firm conclusion that Arthropleura was much more closely related to centipedes (Class Chilopoda) than it was to millipedes (Class Diplopoda). I still believe that it deserves its own Class (Arthropleuridae), but the systematics of these animals need revision. Several features stand out to me as clearly chilopod features: antenna and head positioning, legs, skin thickness, and fangs.
One feature that leaps out at me immediately is the head of Arthropleura. While it is curled under the body in the fossils it exists in, as stated earlier, the first body segment above it is no longer than any of the other segments, thus giving insufficient ability for the neck to stay curved in a downward position. The head itself is flat and oval shaped. Also, the placement of the antenna are more forward-facing, and larger at the base. The connection points for the antenna are between the eyes, as opposed to slightly above them. Each of these features is distinctly chilopodian in nature.
Another trait that simply can’t be ignored is the number of legs that Arthropleura possessed and their placement. In Arthropleura, much like modern and primitive centipedes, each body segment has only one pair of legs (millipedes have two per segment). Aside from simply the number of legs, Arthropleura’s legs were placed outward, moving out from the body at 90°, then downward towards the ground. This is true of all known centipedes, but only about 5-10% of millipedes (the rest have legs that move out from the body at almost straight down from the body to the ground like brush bristles). These two key leg traits point strongly to chilopod roots.As discussed earlier, the “armor” of Arthropleura was much to thin to give any protection at all. Instead, it likely acted much like the thin skin on centipedes: to allow water to diffuse easily in and out. With the extremely humid conditions during this time period, it’s not a large leap to think that Arthropleura was using this same survival strategy that modern centipedes in extremely humid climates use.
The most damning evidence that Arthropleura was of chilopod decent and not diplopod were its modified legs that contained fangs. This exact feature is present in all known centipedes, but not a single millipede. Both use a chemical defense mechanism, but millipedes are quite different. Millipedes release a cyanotoxin through pores near their legs along the entire length of their bodies. Centipedes do not possess this ability, but are able to inject strong venom through fangs that are a modified pair of legs on the first body segment behind the head.Occam’s Razor states that the simplest answer is likely the correct one. As such, it makes much more sense for Arthropleura to evolve from an animal possessing so many traits it shares– namely an animal from Chilopoda– than it does to evolve from something that it shares very few traits with– an animal from Diplopoda– which would require it to evolve to be nearly identical to something unrelated. Presented all the evidence, along with my expert knowledge on myriapods, I’d like to present that Arthropleuridae is best classified as a class of animals derived from Class Chilopoda.
