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Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Sixteen specimens are known from the Greater Phyllopod bed, where they comprised 0.03% of the community. It resembled the lancelet and perhaps swam much like an eel.
A cephalochordate is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or primary characteristics, that all chordates have at some point during their larval or adulthood stages. These 5 synapomorphies include a notochord, dorsal hollow nerve cord, endostyle, pharyngeal slits, and a post-anal tail. The fine structure of the cephalochordate notochord is best known for the Bahamas lancelet, Asymmetron lucayanum. Cephalochordates are represented in modern oceans by the Amphioxiformes and are commonly found in warm temperate and tropical seas worldwide. With the presence of a notochord, adult amphioxus are able to swim and tolerate the tides of coastal environments, but they are most likely to be found within the sediment of these communities.
Balanoglossus is an ocean-dwelling acorn worm (Enteropneusta) genus of zoological importance because, being a Hemichordate, it is an "evolutionary link" between invertebrates and vertebrates. Balanoglossus is a deuterostome, and resembles the Ascidians or sea squirts, in that it possesses branchial openings, or "gill slits". It has a notochord in the upper part of the body and has no nerve chord. It does have a stomochord, however, which is a gut chord within the collar. Their heads may be as small as per 2.5 mm (1/10 in) or as large as 5 mm (1/5 in).
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Soluta is an extinct class of echinoderms that lived from the Middle Cambrian to the Early Devonian. The class is also known by its junior synonym Homoiostelea. Soluta is one of the four "carpoid" classes, alongside Ctenocystoidea, Cincta, and Stylophora, which made up the obsolete subphylum Homalozoa. Solutes were asymmetric animals with a stereom skeleton and two appendages, an arm extending anteriorly and a posterior appendage called a homoiostele.
Ctenocystoidea is an extinct clade of echinoderms, which lived during the Cambrian and Ordovician periods. Unlike other echinoderms, ctenocystoids had bilateral symmetry, or were only very slightly asymmetrical. They are believed to be one of the earliest-diverging branches of echinoderms, with their bilateral symmetry a trait shared with other deuterostomes. Ctenocystoids were once classified in the taxon Homalozoa, also known as Carpoidea, alongside cinctans, solutes, and stylophorans. Homalozoa is now recognized as a polyphyletic group of echinoderms without radial symmetry. Ctenocystoids were geographically widespread during the Middle Cambrian, with one species surviving into the Late Ordovician.
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References
- ↑ Jefferies, R. P. S. (1999). "The Calcichordate Theory". Science. 236 (4807): 1476. doi:10.1126/science.236.4807.1476. PMID 17793239.
- ↑ Lefebvre, Bertrand; Guensburg, Thomas E.; Martin, Emmanuel L. O.; Mooi, Rich; Nardin, Elise; Nohejlová, Martina; Saleh, Farid; Kouraïss, Khaoula; El Hariri, Khadija (2019-02-01). "Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes". Geobios. 52: 27–36. doi: 10.1016/j.geobios.2018.11.001 . ISSN 0016-6995.
- 1 2 3 Gee, Henry (1996). "Jefferies' Calcichordate Theory". Before the backbone: views on the origin of the vertebrates. London: Chapman & Hall. ISBN 978-0-585-25272-8.
- 1 2 3 Ruta, M. (1999). "Brief review of the stylophoran debate". Evolution & Development. 1 (2): 123–35. doi:10.1046/j.1525-142x.1999.99008.x. PMID 11324028.
- ↑ Taylor, Christopher. "Stylophora: Like a Tee-to-tum They Twirled Him". The Paleos Project. Archived from the original on 2010-06-05. Retrieved 2010-05-29.
- ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2011-07-19. Retrieved 2011-02-23.
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