Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
8. The Phylum Chordata
The phylum of chordates, which first appears in the Early Cambrian period by about 530 mya, 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.
Fig 1: Tunicate, showing juvenile and adult forms.
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.
Fig.2: Shankouclava
In 2003, Chen et al. reported an Early Cambrian fossil tunicate named Shankouclava (fig.2), 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 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 Cambrian fossils including Pikaia from the Burgess Shale in British Columbia, and Yunnanozoan from the Maotianshan Shale deposits in Yunnan province, China.
Fig.3: Lancelet
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.
Fig.4: Find sites of Early Cambrian notochords of the Chengjiang fauna in Yunnan Province, China.
One of the more primitive of the possible chordates is the Lower Cambrian fossil species Yunnanozoon lividum (fig.5), which may have been a cephalochordate, something like a lancelet. 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.
Fig.5: Yunnanozoon lividum. A) shows the whole form, and B) and C) show details of gill rays near the mouth (after Chen and Huang 2008)
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.
Fig.6: Haikouella lanceolata (Chen et al. 1999)
In 2004 Chen disccovered ten new well-preserved samples of Yunnanozoon which clearly showed gill rays or filaments near the mouth (fig.5). 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 a cartilaginous vertebrate.
Other fossil chordates from s the Chengjiang fauna include more fish-like chordates including Haikouichthys, Myllokunmingia (fig.7), 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 (fig.7), 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 Lake. Somewhat 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,
Fig.8: Pikaia gracilens (after Conway Morris and Caron 2012).
Hagfish (class Myxini) 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.
Fig.9: Hagfish
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 hagfish are 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.
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
Glossary