| Helmetia Temporal range: | |
|---|---|
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| Holotype specimen of Helmetia expansa | |
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| Diagrammatic reconstruction | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | † Trilobitomorpha |
| Subclass: | † Conciliterga |
| Genus: | † Helmetia |
| Species: | †H. expansa |
| Binomial name | |
| †Helmetia expansa Walcott, 1918 | |
Helmetia is an extinct genus of arthropod from the middle Cambrian. Fossils of the type species Helmetia expansa have been found in the Burgess Shale of Canada. A putative second species Helmetia? fastigata was found in the Jince Formation of the Czech Republic. [1]
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Fossils are both rare and poorly known; the genus was described by Walcott in 1918 and has not been reexamined, though it was briefly reviewed in the 1990s and has been included in a number of cladistic analyses. Alongside other helmetiids (e.g. Kuamaia ), it is a member of Conciliterga, [2] a group which had been resolved by multiple phylogenetic analysis as one of the closest relatives of trilobites within Artiopoda. [3] [4] [5]
The most complete specimen of Helmetia is 19 cm long. The whole animal is broad and flat with a thin exoskeleton. [6] The leaf-shaped dorsal exoskeleton (tergite) was divided into a trapezoid cephalon (head shield), a thorax with six segments, and a triangular pygidium (tail shield) with 5 marginal spines. Unlike trilobites, the margin of the head shield is concave, ending in a spine on each frontal corner. There is an oval anterior sclerite with two median eye-like frontal organs at the anterior center of the head shield, behind which are two stalked lateral eyes hidden underneath the head shield. [6] [7] Other ventral structures are not well described, and due to that it is originally considered as swimming suspension feeder which only had filamentous limbs (exopods). [6] However, remains suggest that it had endopods (walking legs) like other artiopods, and possibly have around 15 pairs of them: 3 or 4 for head (the fourth pair are located at the cephalon-thorax boundary), 1 for each thoracic segment and at least 5 for pygidium. [2] The first pair of appendages (antennae) are yet to be discovered. [5]
