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Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources


The Phylum Chordata

    The phylum of chordates is named for the notochord, a flexible rod that fits inside a hollow nerve cord. The notochord runs from the head to the tail, lengthening and stiffening the body so that it can be flexed from side to side by the muscle blocks for swimming; it gives the body muscles something to pull against.

    The phylum chordata contains three distinct groups of animals: tunicates, lancelets, and vertebrates.  They share four unique characteristics during at least part of their life cycle: a) the notochord. b) the hollow nerve cord containing the notochord, running along their back or dorsal side. This differs from the nerve cords of invertebrates in being located on the back, not the front (ventral) side, and in forming as a hollow tube. c)  pharyngeal gill slits, openings in the pharynx (throat region) through which water passes for filter feeding and gas exchange.  d) a post-anal tail, resulting from the chordate digestive tract ending before the posterior end of the body.     
  
    The three groups of the chordate phylum, tunicates, lancelets, and vertebrates, belong to  three subphyla:  1) the subphylum Urochordata (“basal- chordates”) includes present-day tunicates, named for their tough outer tunic or skin.These small animals are marine filter feeders and are typically sessile (attached to the substrate), although some species float around freely.  Adult tunicates feed on organic debris and microorganisms drawn in by a siphon, which passes through the pharyngeal gill slits and is then swept into the digestive tract.   
 
     Tunicate larvae look somewhat like tiny tadpoles, with a head and a tail, and a well-developed notochord and a dorsal, hollow nerve cord.  These structures help the larval tunicates to swim around freely, and therefore disperse the species.  However, as the larvae begin to mature they attach themselves to a substrate, anchored by three adhesive papillae on the head. They then reabsorb their tail and notochord, and live as stationary, filter-feeding adults.

    In 2003, Chen et al. reported an Early Cambrian fossil tunicate named Shakouclava, of which eight specimens were found in the Maotianshan Shale at Shankou village, near Kunming, South China. These, the first fossil tunicates known, were originally deposited some 530 mya in a shallow marine delta environment. Body lengths of the individuals range from 2 to 4 cm. Like today’s adult tunicates, the fossils (fig.2) lack evidence of a notochord and show a simple, U-shaped stomach, linked to the pharynx at one end and to an out-siphon at the other end.  The fossils also show evidence of a an oral siphon with apparent oral tentacles, a large branchial basket, and a thick tunic-like skin. Like some modern tunicates such as Clavelina, it also had stolons or anchoring strips at the proximal end, indicating it was sessile and rooted to the ocean floor.  Each of the eight specimens was found alone, suggesting the animal was solitary and not colonial (Chen et al 2003).

    2) the subphylum Cephalochordata (“head-chordates”). This includes present day lancelets, and a few known known Cambrian fossils including Pikaea from the Burgess Shale in British Columbia, and Yunnanozoan from the Maotianshan Shale deposits in Yunnan province, China.

    Lancelets are slender, eel-like filter feeders. They appear pointed at both ends, giving rise to another name for them, amphioxus (amphi = “both”, oxy = “sharp”).  Lancelets have a distinct, funnel-like mouth surrounded by oral cirri or tentacles,  somewhat like those found on fossil chordates such as Pikaia. The lancelet has a series of muscle segments called myomeres located both sides of the notochord, also found in the Cambrian chordates.

    Though lancelets will swim short distances, they spend most of their lives buried in bottom sediments, with only the mouth exposed.  Here they live as filter feeders, drawing water in the mouth, and passing it through the gill slits.  As in tunicates, food particles are captured through the gill slits, and then swept into the digestive tract.

    3) the subphylum Vertebrata:  The cartilege-like notochord is the precursor of the bony vertebral column, which supports strut-like limbs, and protects the vital dorsal nerve cord.  Vertebrates include all known fishes, amphibians, reptiles, birds, and mammals, both fossil and extant.

Cambrian notochords.

    The earliest fish-like creatures were notochords from the Early and Middle Cambrian (535-520 mya). This, and several other early fish-like fossils were all found in the Lower Cambrian Maotianshan shales at Ercaicun in Yunnan, southern China, dated at 535-520 mya.This deposit has exceptional fossil preservation, and is classified as a Lagerstätten (from German lager, “storage” and stätte, “place”), as is the Burgess shale in British Columbia.  The Chinese deposits have revealed thousands of soft-bodied fossils, known as the Chengjiang fauna.

    One of the more primitive of the possible chordates is the Lower Cambrian fossil species Yunnanozoon lividum, which may have been a cephalochordate, something like a lancet. Yunnanozoon, now known from about 60 fossil specimens of varying degrees of quality,  was 1.6 - 2.2 cm long (Chen and Huang 2008). The changeable history of interpreting Yunnanozoon can provide insights into the workings of palaeontology over time, vis a vis a growing sample size and variable preservation of fossil examples.    

    When the first Yunnanozoon fossils were discovered in the late 1980s, its interpretation was uncertain, and it was classified as problematica or  an unresolved taxon (Chu 1991). A few years thereafter, based on better preserved samples which revealed a notochord, branchial or gill arches, and a pharygeal cavity, Yunnanazoon was reclassified as a notochord (Chen et al. 1995). The following year, however, Shu et. al (1996) and Chen and Li (1997) reclassified Yunnanozoon as an early hemichordate, which, like tunicates, only had notochords during the larval stage. Meanwhile, new taxa from the same Early Cambrian deposits were discovered and interpreted, such as a large sample of 300 specimens of Haikouella lanceolata (“lancet-like from Haikou”), a cephalochordate with a notochord, and tentacles near its mouth somewhat analogous to those of the lancelet.  The excavators Chen, Huang, and Li (1999) proposed Haikouella to be the earliest craniate-like chordate, and noted it showed marked similarities with Yunnaozoon, indicating they were closely related.

    A few years later, Shu et al. (2003) described a new fossil species, Haikouella jianshanensis, which they interpreted as an extinct deuterostome, or hemichordate. They also stated that, as Yunnanozoon was very similar, it shared this evolutionary stage. The next year (2004), however, Chen disccovered ten new well-preserved samples of Yunnanozoon which clearly showed gill rays or filaments near the mouth. These resembled the tentacles of Haikouella which must have been used for filter feeding, and those of both the modern lancet and hagfish. Based on this evidence of gill rays, Chen and Huang (2008) reinterpreted the status of Yunnanozoon as a chordate, possibly even cartilaginous vertebrate.

    Other fossil chordates from s the Chengjiang fauna include more fish-like chordates including Haikouichthys, Myllokunmingia and a third genus named Zhongjianichthys, all placed in the same family called Myllokunmingiidae, dating from 535-520 mya. The species Haikouichthys ercaicunensis (“Haikou fish from Ercaicun”)  is about 2.5 cm or 1 in. long a distinct head showing at least six, and perhaps up to nine gills with filaments, and a neural chord. For all these reasons, it was identified by Chen, Huang, and Li (1999) as a chordate, although there only a short segment of the notochored is preserved in the single known specimen. The tail has a number of muscle segments (myomeres). and there is a prominent dorsal fin with radials similar to those of hagfish and lampreys, which angle forward toward the head. There are also 13 circular structures along the bottom representing organs that are still unidentified.

           Myllokunmingia fengjiaoa, another chordate from the Chenjiang fauna and dated at 524 mya, is about the same length (2.8 cm), but broader (6 mm) than Haikouichthys. Myllokunmingia is thought to be a vertebrate, although this is not conclusively proven (Shu et al. 2003). It appears to have a skull and skeletal structures made of cartilage. There is no sign of bone mineralization of the skeletal elements. The animal has a distinct head and trunk with a forward, sail-like dorsal fin 1.5 cm high, and a ventral fin fold (probably paired) further back. The head has five or six gill pouches with hemibranches. There are 25 muscle segments (myomeres) in the trunk. There is a notochord, a pharynx and digestive tract that may run all the way to the rear tip of the animal. The initial specimen, or holotype, is missing part of its tail which was buried in sediment.

        
Zhongjianichthys rostratus, named for paleontologist Zhongjian Yang and for its pronounced frontal or rostral lobe, was found in the Jianshan beds of the Maotianshan shale, near Dianchi LakeSomewhat similar to Haikouichthys, it is considered a slightly more advanced vertebrate. Its prominent anterior or rostral lobe has two arcuate, plate-like structures on the front side, separated by a notch interpreted as a median nostril (similar frontal plates occur on Hakouichthys). Its eyes and nasal sacs are also similar in form to those to Haikouichthys, although the placement of the eyes of Zhongjianichthys are behind its anterior lobe instead of within it, as in Haikouichthys. Another more derived feature of Z. rostratus is an absence of muscle impressions (myomeres), indicating thicker skin, as in modern lamprey and hagfish (Shu 2003).

    From the Burgess shale in British Columbia, another deposit with excellent preservation and dating from the Middle Cambrian (520-510 mya), came the first known fossil notochord, Pikaia gracilens First discovered by Charles Walcott (1911), Pikaia was named for Pika Peak, a mountain in Alberta, Canada, and for its slender or gracile form. Based on the regular segmentation of the body, Walcott (1911) made an initial classification of Pikaia as polychaete or sea worm. A later reexamination of a much larger sample of the Burgess Shale fauna, however, led Simon Conway Morris (1979) to reclassify Pikaia as a chordate. In their recent monograph on Pikaia gracilens, Conway Morris and Caron (2012) compare Pikaia to other Cephalochordata, including both the Early Cambrian Yunnanozoon and the extant lancelet Branchiostoma. P. gracilens is now represented by a relatively large sample of 114 fossil specimens, many with excellent preservation, found in three quarries of the Burgess Shale. It is presently one of the best studied of the early notochord fishes (Conway Morris and Caron 2012). 

    In many ways, Pikaia, with a flattened body, notochord, and paired muscle segments (myomeres) attached to the notochord, resembles the lancelet. Pikaia, with  a range of sizes averaging  3.8 cm (1 ½ in.) in length, also has a pair of head tentacles, and a series of short appendages, which may be linked to gill slits, on either side of its head. Pikaia, as well as the lancelet, Yunnanozoon, and the three above-mentioned Early Cambrian Chinese chordates grouped in the Myllokunminigidae family, all lacked lateral fins, and swam using their tail fins fin in S-shaped movements, much like an eel.
 
    Hagfish (class Myxini):  Hagfish are eel-shaped creatures who live in the continental shelf waters off California. New Zealand, and East Asia. They have shown little tendency to change; fossil hagfish from 300 mya  show little or no difference in form. The Myxini are unique among living chordates in that they have a partial skull or cranium, but no vertebrae, so they are not considered vertebrates Hagfish vary between 4 cm and 1 m in length,  lack side fins, and have simple and have paddle-like tails. They feed mainly on dead fish and polychaetes (sea worms) on the ocean bottom.  

    Their vision is poor, as their eyes lack both lens and controlling muscles, and apparently cannot resolve detailed images. They also have a vestigial third eye, sometimes marked on the top of the head by a white patch. They have a single nostril which draws water in and across the gills then out through external gill openings.  They also have well developed senses of touch and smell, including four pairs of sensing tentacles arranged around their mouth, and two pairs of tooth-like rasps on top of a tongue-like projection. A total of 5 genera with 77 extant species of hagfishare recorded.  

    The skeleton is composed of cartilage, and lacks bone. Their brains lack cerebrum or cerebellum.Hagfish were traditionally placed with lampreys in a group called Cyclostomata  ("round mouth"), now considered a mixed grouping in terms of ancestry or phylogeny. A long-standing theory that hagfish and lampreys are the oldest surviving groups of fish, however, has been supported by recent DNA evidence. In an interesting contrast with both tunicates and lampreys, Hagfish do not have a larval stage, giving birth to miniature adult forms, in contrast to lampreys, which have a long larval phase, and tunicates, whose notochord is confined to the larval stage.

    Two fossil hagfish have been found, both from the Pennslvanian or Late Carboniferous (325-200 mya), and each with excellent preservation. In both cases, the close similarity to modern hagfishes suggests that there has been little change in this group over the last 300 million years The first, Myxinikela siroka, is from the Francis Creek Shale of northeastern Illinois (Bardack, 1991). This shows the paired tentacles found in living hagfish, along with internal organs, and detail of the cranium. The second fossil hagfish, Myxineidus gononorum, was found at Montceau-les-Mines, France. This specimen, preserved in a concretion, includes the pharynx and oral cavity, and the impressions of two pairs of symmetrical tooth rows.


Glossary 
 
References: 

Bardack, 1991
Chu 1991.
Shu et. al  1996
Chen and Li  1997

Chen, Huang, and Li 1999. Nature 402, pp.518-522.
Chen et al 2003
Chen, Ailin, and Diuaing Huang, 2008.
Conway Morris, Simon 1979
Conway Morris and Caron 2012
Shu, Degan 2003. A Paleontological perspective of vertebrate origin. Chinese Science Bulletin 48, pp.725-735.
Walcott, Charles. 1911





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