The Potential Misidentification of Early Cambrian Chordates
Word Count: 1448

Introduction:

There is great debate regarding the potential misidentification of chordates in the fossil record. Fossils are the remains of ancient life where the gross morphology of the original tissue is preserved in rock (Butterfield, 1990). Because ancient chordates are extinct, the only way that they can be studied is through their fossilized remains. Chordates are animals that have existed since the Cambrian Explosion. They are classified based on the presence of a notochord and other features including a dorsal nerve cord and successive muscle segments, which will be henceforth classified as myomeres (Turner et al., 2010). Humans are an example of modern chordates.

The Cambrian Explosion occurred roughly 542 million years ago and was the first fossil record of many phyla, including chordates, which are in existence today; prior to this event biodiversity was limited and most organisms were single-celled bacteria (Butterfield, 2003). Organisms classified as chordates that existed during the Cambrian include, but are not limited to: Cambropustula (Muler and Hinz, 1991), Emmonsaspis cambensis  (Conway Morris, 1993), Metaspriggina M. walcotti (Conway Morris, 2008), Myllokunmingia fengjiaoa (Xian-guang et al., 2002), Pikaia gracilens (Conway Morris, 1979) and Yunnanozoon lividium (Chen et al., 1995). Taxonomy of the preceding species was determined by examining the fossilized physiology and relating it to that of modern chordates (Conway Morris, 2008). Because the preservation of soft-bodied creatures is limited, it is not uncommon to use only one synonymous characteristic for identification (Chen et al., 1995). This lack of resolution is problematic and has resulted in the possible misidentification of Pikaia (Butterfield, 1999), Emmonsaspis (Conway Morris, 1993) and others to be discussed. To accommodate for this drawback, Butterfield (1990), analyzed Pikaia’s fossilized remains to derive what the original organic material may have been. His findings suggest that Pikaia had an exoskeleton and because of this the original chordate phylum placement is questioned.

This paper will discuss early chordate physiology, similarities to modern creatures of the same phylum, preservation bias, and misidentification of previously considered chordates in the literature. The evidence that other potential chordates have been placed in the wrong phylum will also be investigated.

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Physiology of Cambrian Chordates, Similarities to Modern Chordates and Preservation Bias:

There are two primary characteristics used to diagnose chordates: myomeres and a notochord; depending on the preservation of the specimen both are not always readily identifiable (Conway Morris, 2008; Shu et al., 1999; Xian-guang et al., 2002). A notochord is a rigid cartilage backbone that provides support to the organism, comparable to a spinal chord (Rychel, 2005). Myomeres are chevron-like repetitions of muscles that occur along the notochord of chordates. High-angle cross-sections of primitive chordates will show a series of v-shapes with the opening towards the posterior end. More advanced chordates have sigmoid or w-shaped myomeres with the bottom of the w pointing towards the posterior end (Turner et al., 2010).

Cambrian chordate physiology is often compared to hagfishes, lungfishes and lampreys (Blieck et al., 2010; Turner et al., 2010), which are primitive modern-day chordates. These creatures are ideal because their anatomy is similar to ancient life; hagfish and lamprey myomeres are w-shaped where lungfish are v-shaped. This provides a point of reference for understanding the ambiguous preservation of ancient creatures, it also can provide a bias as dissimilar qualities may be ignored in favour of those which fit a hypothesis. This bias was demonstrated by Conway Morris’ (2008) rejection of Butterfield’s (1990) evidence regarding Pikaia and will be discussed in further detail in the next section.

Even with a basis for comparison the ambiguity of the fossil record often makes phylum placement difficult. Butterfield (1990) has extensively studied organic remains and concludes that even in events of exceptional fossilization, where conditions are ideal for preservation, known as laggerstratten; only material resistive to decay will be preserved with discernable resolution. Consequentially, soft organic tissue typical of chordates will likely be no more than a carbon shadow. This opinion is not universally agreed with; in a recent study of the Burgess Shale lagerstratten preservation is attributed to a rapid change in oxic conditions (Caron and Jackson, 2006). Therefore inhibiting decay and fossilizing tissue that would not normally survive in the rock record. Although soft-bodied creatures can be fossilized with detail, it is never argued that preservation will be of the same caliber as creatures with a higher preservation potential, like arthropods (Caron and Jackson, 2006).

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Phylum Misidentification of Chordates in Literature:

In 1979 the Pikaia was reviewed and classified as a chordate due to the presence of a notochord and v-shaped myomeres (Conway Morris, 1979). However, this diagnosis was questioned by Nicholas Butterfield (1990). Butterfield argues that the reflective fossilized remains of Pikaia indicate the presence of organic material resistant to decay, which he hypothesizes to be an exoskeleton. The implication is that chordates do not possess exoskeletons, thus Pikaia cannot be a chordate. He also notes that the preservation is similar to Amiskwia, a Cambrian species belonging to the phylum incertae sedis, which is reserved for creatures of unknown taxanomic affinity. Pikaia has also been compared to the creature Tullimonstrum of incertae sedis (Blieck et al., 2010). Allthough Butterfield does not place Pikaia within any phyla he does state that is it certainly not a chordate. Following Butterfield’s reassessment of Pikaia, Conway Morris (2008) refutes his claim and argues that the physiology, namely the presence myomeres, is overwhelming and reaffirms that it is a chordate. It is interesting to note that Morris ignores the reflective preservation of Pikaia and does not attempt to evaluate Butterfield’s (1990) evidence. Clearly this debate is long from over.

Emmonsaspis cambensis was also misidentified as being a chordate due to the presence of a central stalk and branching pattern that closely resembled a notochord and myomeres respectively (Resser and Howell, 1938). Upon reviewing the physiology, the branching is likely not muscle segments due to the fact that it tapers towards the apex and the preservation of the stalk is unlike any other specimen observed (Conway Morris,1993). In Conway Morris’ (1993) description of Emmonsaspis’ mode of life he speculates that it is likely benthic and a passive filter feeder. It is also possible that the organism was colonial and housed zooids. This lack of evidence and poor preservation is inconclusive; as such Emmonsaspis’ is not placed in any given phylum (Conway Morris, 1993).

Cambropustula is one of the earliest known chordates of the conodonta class and possesses many of the physiological characteristics used for phylum placement (Muler and Hinz, 1991). In a recent review of conodonts it is argued that regardless of the characteristics that are synonymous to chordates, there are too many other physiological characteristics that are unknown to designate them as vertebrates (Turner et al., 2010), primarily the anomalous crystalline structure and composition of the creature’s teeth. The reviews focus is to explain why conodonts are not vertebrates and although it is beyond the “scope of this paper” they mention that conodonts may not have any chordate phyla affinity.

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Implications with Respect to the Phylum Placement of Other Chordates:

Any Cambrian chordate that has been classified only on the presence of a notochord or myomeres should be reviewed; at the very least it should be acknowledged that the current phylum placement may be wrong. This includes Metaspriggina M. walcotti, which was classified solely on the presence of v-shaped myomeres, no obvious notochord is present (Conway Morris, 2008). Myllokunmingia fengjiaoa’s chordate affinity was attributed to the presence of myomeres and a notochord (Xian-guang et al., 2002). However, it is important to note that the authors find the presence of myomeres sufficient to diagnose the creature as a chordate and “permitted” the further interpretation of the fossilized remains. Interpretations of less apparent characteristics include: a dorsal and ventral fin, gut, gill pouches and head structures. Yunnanozoon lividium, like Myllokunmingia, was also classified based on the overwhelming evidence of myomeres (Chen et al., 1995)

Has it not been considered that myomeres may be a logical manifestation of evolution and the most efficient means of bilateral movement? These fossils are extremely old and although the concept of finding an early chordate is attractive, the implication being it may be the origin of vertebrates and ultimately humans.

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Conclusions:

The method of comparing modern creatures, such as hagfish, to ancient life can be useful but it can also be misleading. The most notable case of this is Emmonsaspis whose affinity was placed to chordates but upon further evaluation the phylum has been rendered unknown; with limited speculation to its mode of life. We must also consider organic preservation and the implications it might have to the original material, as is the case with Pikaia and Butterfield’s unaddressed evidence. Even the widely accepted connodont Cambropustula and its overwhelming chordate characteristics are now being questioned. There is a very alarming trend that myomeres alone are sufficient to classify a creature. Has it not been considered that myomeres may be a logical manifestation of evolution and the most efficient means of bilaterial movement? These fossils are extremely old and although the concept of finding an early chordate is attractive, the implication being it may be the origin of vertebrates and ultimately humans. Paleontologists should proceed with caution before jumping to such absolute conclusions.