Pikaia

A fossil animal from the Cambrian period has been defined. Upon close examination, although it measured only 3 centimeters, it was definitively identified as a vertebrate. The discovery of Cambrian vertebrates is a major event in every time. Vertebrates of that era had soft bodies and are rarely preserved in the fossil record. Thus, the events of the Cambrian period, which shaped the plans for vertebrate evolution, are of paramount importance. This discovery has made it possible to observe one of the intermediate stages in the process of body anterior development—cephalization.

What did the first vertebrate look like? This question has become a theoretical long duration only topic. Reliable vertebrate remains were only known from the Ordovician period, when their structures had already become well-formed (see the official image of Sakabambaspis). It was clear that some early vertebrates existed during the Cambrian period, but paleontology no definitive thing could be made about their appearance.

Only comparative anatomy models remained. In the first half of the last century, Alexey Nikolaevich Severtsov reconstructed the common ancestor of all chordates with almost no paleontological data evidence, naming it “primitive headless” (Acrania primitiva). Its most striking feature was the strong segmentation of its muscles, and these segments—myomeres—extended all the way to the anterior end of the body. This matches exactly what we observe in the most primitive chordate, the lancelet. Such a muscle structure could only have been necessary for swimming, so Severtsov depicted the overall body shape of the “primitive headless” as fish-like. The central nervous system consisted of a dorsal nerve tube, beneath which lay a flexible notochord. The anterior part of the digestive system—the pharynx—was perforated by paired gill slits. In the lancelet, the number of gill slits is very high (90 pairs), but this is clearly a secondary increase; Severtsov believed that in the “primitive headless,” the number of gill slit pairs did not exceed 20. “Primitive headless” organisms were microphagous, feeding on small organic particles carried by water flow. Water entered through the mouth into the pharyngeal cavity, where food particles were filtered out, and water exited through the gill slits (Figure 2).

To test this model, Cambrian chordate fossils were required. However, significant difficulties were encountered in this area during the 20th century. It was only in 1979 that British paleontologist Simon Conway Morris identified Pikaia, previously thought to be a segmented worm and long known from Cambrian deposits, as definitively a chordate animal: its body exhibited characteristic striated muscle segments—myomeres—which are found only in chordates (see Pikaia—The Most Primitive Chordate?, “Elements”, 16.03.2012). For a time, Pikaia was the only known Cambrian chordate. Unfortunately, this situation did not imply that Pikaia represented the common ancestor of chordates. Pikaia possesses a series of mysterious features that likely indicate an early specialization in an unusual direction (J. Mallatt, N. Holland, 2013. Pikaia gracilens Walcott: A chordate or already specialized in the Cambrian?).

The Mysteries of Pikaia

Problems begin with Pikaia’s external appearance (Figure 3, A). At its head, Pikaia has a single pair of appendages resembling those of a soft-bodied animal. Each appendage receives a nerve. Apparently, the function of these appendages was sensory. What did these appendages resemble? We can, of course, compare them to the whiskers of modern hagfish or the tentacles of microsnails. However, in both hagfish and microsnails, there are always multiple pairs of whiskers or tentacles around the mouth, whereas Pikaia has only one pair. On the other hand, externally similar sensory extensions are found in many invertebrates: snail tentacles, onychophoran and arthropod antennae like. But what is the connection between Pikaia and these? This remains a mystery.

Continuing our examination of Pikaia, we observe a very small mouth and extremely tiny gill slits. This is also problematic. Because all primitive chordates, including the earliest vertebrates, were undoubtedly filter feeders: they passed large volumes of water through the pharynx, expelled the water through the gill slits, and captured small organic particles using a specialized mucus system, then directed these particles to the intestine. Tunicates, lancelets, and the larval form of the modern lamprey, the ammocoete, feed in this manner; moreover, this filtering nutrition mechanism is so similar across all these groups that it strongly suggests a common origin (J. Mallatt, 2023. The origins of vertebrates explained by larval lampreys (ammocoetes): Response to Miyashita et al., 2021). However, Pikaia could not have fed this way. Its tiny mouth and microscopic gill slits were not large enough to pass the required volume of water. It must have fed in another way. Since no hard elements resembling teeth were found in Pikaia’s oral apparatus, it likely crawled along the seabed and sucked up small food particles one by one.

A long vessel running along the ventral side of Pikaia is interpreted as the ventral blood vessel (see Pikaia—The Most Primitive Chordate?, “Elements”, 16.03.2012). The issue here is that a similar ventral vessel extending the entire body length is typically found in invertebrates. However, in vertebrates and lancelets, the ventral vessel always forms a capillary network—a liver portal system. These capillaries then converge into the hepatic vein. However, in Pikaia, the ventral vessel is continuous, and no structure resembling a hepatic portal system has been found; the nature of this vessel remains unknown.

Complete, the only serious argument compelling us to accept Pikaia as a chordate is its myomeres. Myomeres with segmented axial muscles are difficult to match with any other group, and muscle organization in this manner is unique to chordates. However, Pikaia’s myomeres are not entirely without problems. A myomere is a block formed by muscle fibers arranged in a sequence. Each myomere’s muscle fibers attach to myosepta, which are strong partitions connecting to the notochord sheath. Sequential contraction of myomeres propels the animal forward. Now, examining the lancelet, we see that its myomeres intersect at sharp angles and each myoseptum forms a V-shaped contour. Why so complex? Why are the boundaries of the myomeres not straight?

Canadian biologist Thurston Lacalli explains this (T. Lacalli, 2012. The Middle Cambrian fossil Pikaia and the evolution of chordate swimming): The overlapping of myomeres balances mechanical loads on the notochord and its sheath. Consider the opposite: suppose the myomere boundaries were perfectly straight. Then, at the points where the myosepta attach to the notochord, narrow, tensioned regions would form, and as the muscles continuously pull on these areas, the segments of the notochord between the myosepta would remain motionless. Because the myomeres contract asymmetrically, left and right, the notochord would be constantly bent, the sheath wrinkled, and especially during strong loading, it could tear. However, if the myosepta are angled (Figure 3, B and C), the areas where they attach to the notochord become much broader, and the loads are distributed more evenly across nearly the entire length of the notochord. The sharper the angle, the stronger this effect becomes.

Now consider Pikaia. Its myomere boundaries are nearly perfectly straight and exhibit only slight undulations (Figure 3, A). This means that all the load on the notochord is concentrated on the vertical myoseptal connections. To prevent the notochord sheath from tearing at these points, these loads must be sharply limited. From this, Lacalli concludes that Pikaia’s myomeres were weak and its swimming speed was much lower than that of a modern lancelet or fish. Probably its muscle fibers were “slow,” physiologically incapable of rapid contraction: there was no need for them to do so, because the skeleton did not permit it.

In the meantime, large sea predators such as Anomalocaris already existed during Pikaia’s time; Pikaia could not escape them quickly (see Missing ring Found: Anomalocaris and the Mysterious Cambrian Monsters, “Elements”, 16.02.2009). Therefore, Lacalli suggests that Pikaia led a hidden life, spending most of its time resting on the seabed and using its appendages to search for small food particles on the bottom.

However, Pikaia’s most mysterious feature is the “dorsal organ” located just beneath the dorsal covering of its body. This organ is quite thick (in Lacalli’s words, “sausage-like”) and its nature is entirely enigmatic. It cannot be the notochord, because above the notochord there must be a nerve tube, yet here there is no space for a nerve tube. The “dorsal organ” consists of a closed tube containing some cell extraneous mass. It was probably a hydrostatic organ with a semi-fluid interior, used for flexibility or buoyancy (or both). But where did it come from? Thurston Lacalli proposes several possible script scenarios for the origin of this organ (T. Lacalli, 2024. The Cambrian fossil Pikaia and the origin of chordate somites), but since we know nothing of its embryology, choosing among these scenarios is impossible. The most outlandish theory is that this “dorsal organ” was simply a nerve tube that expanded by filling with fluid and provided mechanical support to the body in addition to neural functions. Before going this far, we can recall the dorsal fin of lancelets; its skeleton is entirely hydrostatic: it consists of a series of small, closed chambers containing pressurized fluid. However, in Pikaia, a similar structure is a single large one extending the entire body length. Unfortunately, no modern animal has a comparable organ.

Looking forward, Pikaia is not the oldest known chordate. Moreover, it is younger than some vertebrates. Pikaia lived at least 15 million years after Haikouichthys, the oldest known “normal” vertebrate. Considering the high rates of evolution during the Cambrian period, Pikaia had ample time to specialize in the direction it chose. Perhaps it had already diverged significantly from the common chordate ancestor. In any case, Pikaia has raised more questions than it has answered.