Toscanisoma multipartitum
- 演化之聲
- Mar 12
- 2 min read
Updated: 1 day ago
Age
Triassic（Carnian）
237 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Ascidiacea
Order: Cassianomorpha
Family: Cassianosomidae
Genus: Toscanisoma
Species: Toscanisoma multipartitum
Morphological description
Toscanisoma multipartitum is a colonial, sessile ascidian composed of seven or more separate tubular individuals that together form a compound solid skeleton. The outer surface of the skeleton is smooth and lacks any visible growth lines. This structure is interpreted as an endoskeleton that was secreted internally within the surrounding soft tissue mantle.
Each individual zooid is constructed from more than ten irregular skeletal plates. The outer margins of these plates are smooth, whereas the inner margins show a zigzag pattern. This arrangement likely provided a certain degree of flexibility to the structure. The basal portion of the only known specimen is missing, but it is inferred that the base functioned as an attachment surface. From this base, seven tubular zooids branch outward in different directions.
The cross sections of these zooids vary in shape and include nearly circular, elongated, and flattened forms. The smallest and most complete zooid measures about 2.5 cm in length. The entire colony is estimated to have reached approximately 5–6 cm in height and about 7–8 cm in width. No skeletal spines embedded within the skeleton have been observed.
Etymology
The genus name Toscanisoma honors Maria Luigia Toscani, who donated the fossil specimen used in the study. The suffix soma derives from the Greek word meaning "body".
The species name multipartitum comes from Latin and means "composed of many parts".
Biological description
Toscanisoma multipartitum was discovered in the Dolomite Mountains of northeastern Italy. Its skeletal structure is highly unusual within the animal kingdom and is extremely rare in the fossil record. Similar skeletal structures are known only in certain ascidians from the Permian and Triassic periods.
The plate-like skeletal elements may indicate a relationship with some modern ascidians, such as species of the genus Pyura. Although living members of these groups lack rigid skeletons, they possess soft plates that can be moved by muscles.
The fossil plates show complex interlocking sutures along their inner margins, indicating that the plates were capable of limited flexibility. Because ascidians are filter feeders, they draw seawater into the body through the branchial siphon and expel it through the atrial siphon after filtering food particles. Therefore, it is likely that at least one or two plates at the top of the colony could be opened or closed by muscular action, allowing the siphons to extend outward. A similar mechanism exists in certain modern ascidians with flexible plate-like structures.
（Author: Bai Leng）
Reference
Jobst Wendt (2020). A rare case of an evolutionary late and ephemeral biomineralization: tunicates with composite calcareous skeletons. Journal of Paleontology.
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