| Megasiphon thylakos Temporal range: | |
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| Type specimen of M. thylakos | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Tunicata |
| Genus: | † Megasiphon Nanglu et al., 2023 |
| Species: | †M. thylakos |
| Binomial name | |
| †Megasiphon thylakos Nanglu et al., 2023 | |
Megasiphon thylakos is an extinct species of tunicate that lived in the Middle Cambrian 500 million years ago. [1]
The barrel-shaped main body extends apically into two long siphons of similar size, which have millimetric circular transverse muscle bands. The siphons have an axis of inclination of approximately 25° with respect to the longitudinal axis of the organism and are associated with longitudinal muscular bands extending from the upper region. [1]
The siphon with more extensive disaggregated muscle fibers and thicker muscle bands may be the oral siphon, while the other siphon is probably the auricular siphon. However, it is difficult to unequivocally differentiate between the two siphons based on the holotype. [1]
In modern representatives of ascidians, the contraction of the circular and longitudinal musculature causes the ascidian to squirt or shrink. Its presence in Megasiphon indicates that these behavioral traits typical of tunicates had already evolved approximately 500 million years ago. [1]
The holotype is very similar to phlebobrachs in morphology; in contrast, Megasiphon lacks any clear similarity to stalked stolidobranchs or motile thaliaceans, and its morphology is incompatible with tadpole-like appendiculars. The new species differs greatly from Shankouclava . [1] [2]
Since external morphology alone may not be able to resolve the phylogenetic placement of Megasiphon relative to extant tunicates, it is more optimal to propose two alternative phylogenetic placements, which has profound evolutionary implications.
Megasiphon may be a crown tunicate as a sister group to the clade uniting the modern Ascidiacea and Taliacea (Acopa). This location would indicate that the divergence between appendicularians and all other tunicates had already occurred earlier than indicated by the molecular clock estimated at 450 million years ago, pushing the divergence back approximately 50 million years. [3] Other primitive ascidians such as Burykhia and possibly Ausia that date back 550 million years push the origin of the Acopa clade back even further than indicated by the molecular clock. [4] This proposal follows the argument that an olfactorian and tunicate ancestor was motile, and that the sessile nature of ascidians is a derived character arising after appendicularians and vertebrates diverged from the modern ascidian lineage.
Motile animal
Sessile animal, with motile larva.
| Chordata |
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Another possibility is that, as with Shankouclava , Megasiphon may represent a stem-tunicate, a position that would indicate that a sea squirt-like body plan is ancestral to the entire Tunicata group, and the tadpole-like appendicular motile is a derived state. Supported by developmental biology and the report of massive gene loss in appendicularians, [5] [6] this hypothesis would imply that the first non-appendicular tunicates were probably sedentary-pelagic ascidian-like organisms, with a two-phase life cycle that included larval metamorphosis. The uniphasic and pelagic lifestyle would then be synapomorphic for thaliaceans. [1] Using the molecular clock to indicate Megasiphon as a stem-tunicate because the divergence between Appendicularia and Acopa occurs 450 million years ago, implies that the 555-million-year-old Burykhia is possibly a stem-olfactorian, because the divergence among modern tunicates and vertebrates it occurs approximately 547 million years ago. [3] Added to this is a study that indicates that vertebrates (and all Bilateria in general) descend from a sedentary ancestor in the adult stage but a pelagic ancestor in the larval stage, appealing to Hox genes, deep homologies, and metamerism without segmentation. [7] The unusual morphology of Shankouclava makes its relationship with the rest of the tunicates somewhat ambiguous because the oral siphon is not very defined and presents tentacle-like structures reminiscent of ambulacraria and lophophorates, it lacks an atrial siphon, and does not resemble phlebobranchs such as Megasiphon and Burykhia; Therefore it is possible that it is actually a more basal group of Olfactores. [8] [9] [10] [7]
| Chordata |
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The Megasiphon authors postulate the organism as a stem tunicate, rather than a stem Acopa, as more plausible because the position of Megasiphon as a main group tunicate does not require drastically changing the estimate of the molecular clock nor the origin of Ascidiacea given by the same, would instead only set minimum divergence estimates for the origins of Tunicata at about 500 million years ago; second, an evolutionary scenario involving sedentary-pelagic sessile ancestry appears to be more likely for Olfactores and Tunicata when consider other lines of evidence.
A chordate is a deuterostomal bilaterian animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.
The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleotide sequences for DNA, RNA, or amino acid sequences for proteins.
Ausia fenestrata is a curious Ediacaran period fossil represented by only one specimen 5 cm long from the Nama Group, a Vendian to Cambrian group of stratigraphic sequences deposited in the Nama foreland basin in central and southern Namibia. It has similarity to Burykhia from Ediacaran (Vendian) siliciclastic sediments exposed on the Syuzma River of Arkhangelsk Oblast, northwest Russia. This fossil is of the form of an elongate bag-like sandstone cast tapering to a cone on one end. The surface of the fossil is covered with oval depressions ("windows") regularly spaced over the surface in the manner of concentric/parallel rows. The taxonomic identity of Ausia is unresolved.
A tunicate is an exclusively marine invertebrate animal, a member of the subphylum Tunicata. This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals.
Thaliacea is a class of marine chordates within the subphylum Tunicata, comprising the salps, pyrosomes and doliolids. Unlike their benthic relatives the ascidians, from which they are believed to have emerged, thaliaceans are free-floating (pelagic) for their entire lifespan. The group includes species with complex life cycles, with both solitary and colonial forms.
Larvaceans, copelates or appendicularians, class Appendicularia, are solitary, free-swimming tunicates found throughout the world's oceans. While larvaceans are filter feeders like most other tunicates, they keep their tadpole-like shape as adults, with the notochord running through the tail. They can be found in the pelagic zone, specifically in the photic zone, or sometimes deeper. They are transparent planktonic animals, usually ranging from 2 mm (0.079 in) to 8 mm (0.31 in) in body length including the tail, although giant larvaceans can reach up to 10 cm (3.9 in) in length.
Ascidiacea, commonly known as the ascidians or sea squirts, is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer test or "tunic" made of the polysaccharide cellulose.
Yarnemia ascidiformis is a fossil tentatively classified as a tunicate. While Y. ascidiformis looks similar to tunicates, the oldest unequivocal tunicate, Shankouclava dates to the Cambrian period, while Y. ascidiformis is Ediacaran in age.
Ciona intestinalis is an ascidian, a tunicate with very soft tunic. Its Latin name literally means "pillar of intestines", referring to the fact that its body is a soft, translucent column-like structure, resembling a mass of intestines sprouting from a rock. It is a globally distributed cosmopolitan species. Since Linnaeus described the species, Ciona intestinalis has been used as a model invertebrate chordate in developmental biology and genomics. Studies conducted between 2005 and 2010 have shown that there are at least two, possibly four, sister species. More recently it has been shown that one of these species has already been described as Ciona robusta. By anthropogenic means, the species has invaded various parts of the world and is known as an invasive species.
Styela clava is a solitary, subtidal ascidian tunicate. It has a variety of common names such as the stalked sea squirt, clubbed tunicate, Asian tunicate, leathery sea squirt, or rough sea squirt. As its common names suggest, S. clava is club-shaped with an elongated oval body and a long peduncle for attaching to a substrate. Although native to the northwestern waters of the Pacific Ocean, since the 1900s, S. clava has become an increasingly successful invasive species outside of its native range. It is edible.
The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into four phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is thought to be a member of Deuterostomia.
Pyura stolonifera, commonly known in South Africa as "red bait", is a sessile ascidian, or sea squirt, that lives in coastal waters attached to rocks or artificial structures. Sea squirts are named for their habit of squirting a stream of water from their exhalant siphons when touched at low tide.
Styela plicata, commonly known as pleated sea squirt, is a species of tunicate in the family Styelidae. This sessile filter feeder can expel water when threatened.
Polycarpa pomaria is a species of tunicate or sea squirt in the family Styelidae. It is native to the northeastern Atlantic Ocean where it lives on the seabed at depths down to about 450 metres (1,500 ft).
Olfactores is a clade within the Chordata that comprises the Tunicata (Urochordata) and the Vertebrata. Olfactores represent the overwhelming majority of the phylum Chordata, as the Cephalochordata are the only chordates not included in the clade. This clade is defined by a more advanced olfactory system which, in the immediate vertebrate generation, gave rise to nostrils.
Phallusia mammillata is a solitary marine tunicate of the ascidian class found in the eastern Atlantic Ocean and the Mediterranean Sea.
Shankouclava is an extinct genus of tunicates. It is one of two candidates for the oldest member of this group, dating to 518 million years ago. It has been found in the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming. Each of the eight specimens found and used for description were isolated, suggesting that the genus was solitary and not colonial.
Cheungkongella is a fossil organism from the lower Cambrian Chengjiang lagerstatte, the affinity of which has been the subject of debate. It was announced as a "probable" tunicate while noting the lack of definitive Cambrian fossils from that group. However, this affinity was later disputed in a paper announcing the discovery of Shankouclava, also from Chengjiang, as the oldest known tunicate. Cheungkongella has been accepted as a distinct taxon and possible tunicate by multiple workers not involved in its discovery, but the dispute remains unresolved.
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