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Megasiphon
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Megasiphon
Megasiphon thylakos is an extinct species of tunicate that lived in the Middle Cambrian 500 million years ago.
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.
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.
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.
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.
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. 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. 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
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Megasiphon
Megasiphon thylakos is an extinct species of tunicate that lived in the Middle Cambrian 500 million years ago.
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.
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.
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.
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.
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. 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. 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
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