Temporal range: Middle Ordovician - Late Permian, 460–252 Ma
|Reconstruction of the mycteroptid Megarachne servinei.|
Stylonurina is one of two suborders of eurypterids, an extinct group of merostomatan arthropods commonly known as "sea scorpions". Members of the suborder are collectively and informally known as "stylonurine eurypterids" or "stylonurines". They are known from deposits primarily in Europe and North America, but also in Siberia.
Compared to the other suborder, Eurypterina, the stylonurines were comparatively rare and retained their posterior prosomal appendages for walking. Despite their rarity, the stylonurines have the longest temporal range of the two suborders. The suborder contains some of the oldest known eurypterids, such as Brachyopterus, from the Middle Ordovician as well as the youngest known eurypterids, from the Late Permian. They remained rare throughout the Ordovician and Silurian, though the radiation of the hibbertopterids (a group of large sweep-feeding forms) in the Late Devonian and Carboniferous is the last major radiation of the eurypterids before their extinction in the Permian.
The Stylonurina contains a wide variety of different genera and species. They are all unified by possessing transverse sutures on the ventral plates and lacking a modified podomere 7a on appendage VI.
The suborder is divided into four major superfamilies; the Rhenopteroidea, Stylonuroidea, Kokomopteroidea and Hibbertopteroidea. The most primitive of these, the Rhenopteroidea, includes several previously enigmatic genera, such as Brachyopterus, Kiaeropterus and Rhenopterus, all united by a rounded posterior margin to the metastoma and the prosomal appendage III bearing single fixed spines. Brachyopterus is also the currently oldest known genus of eurypterid, being from the Middle Ordovician.
The least well-supported group is the Stylonuroidea, contaning the problematic genera Stylonurus and Stylonurella, partly due to the incomplete nature of the only known specimen of Stylonurus powriei which does not preserve the anterior prosomal appendages or any details of the ventral structures. Specimens of other members of the group are similarly incomplete, with Stylonurella spinipes not preserving the metastoma or pretelson and telson and Pagea sturrocki not preserving any dorsal structures.
The superfamilies Hibbertopteroidea and Kokomopteroidea are sister groups, united by a median ridge on the carapace between the lateral eyes and a distal thickening to the podomeres of the prosomal appendages. Though sometimes classified as an order separate from Eurypterida itself, the hibbertopterids are clearly recovered as stylonurine eurypterids in the latest analyses of the group.
Convergent evolution of sweep-feeding
Sweep-feeding strategies evolved independently in two of the four stylonurine superfamilies, the Stylonuroidea and the Hibbertopteroidea. In both superfamilies, the adaptations to this lifestyle involves modifications to the spines on their anterior prosomal appendages for raking through the substrate of their habitats. Rhenopteroids, kokomopterids and parastylonurids retained primitive prosomal appendages II-IV that were unsuited for sweep-feeding and likely adopted scavenging, whilst the hardieopterids may have been benthic bottomdwellers living partially buried in the substrate.
Stylonuroids have fixed spines on appendages II-IV which could have been used as dragnets to rake through the sediments and thus entangling anything in their way, whilst the hibbertopteroids, which have some of the most extreme adaptations, likely were more selective and specialized. Hibbertopteroids possess modified blades on their anterior prosomal appendages that feature sensory setae. The tactile function of these might have allowed hibbertopteroids to select prey from the sediments in a way that stylonuroids could not.
Historically, the phylogeny and systematics of the Stylonurina have been far less well understood and less resolved than that of the Eurypterina. Many historical analyses were limited in scope or resolution and the unique hibbertopterids have even on occasion been suggested to be a separate, but closely related, order to Eurypterida, but have lacked an analysis either proving or disproving such an idea. Lamsdell et al. (2011) performed the first major and comprehensive analysis of the suborder, which permitted a substantial systematic revision and comparisons to other eurypterid clades.
The phylogenetic analysis established that Stylonurina indeed was a monophyletic group compromised of four clades: the Rhenopteroidea, Stylonuroidea, Kokomopteroidea and Hibbertopteroidea. These superfamilies in turn contain the following families (and one genus incertae sedis):
Suborder Stylonurina Diener, 1924
- Stylonuroides (incertae sedis) Kjellesvig-Waering, 1966
- Superfamily Rhenopteroidea Størmer, 1951
- Family Rhenopteridae Størmer, 1951
- Superfamily Stylonuroidea Kjellesvig-Waering, 1959
- Superfamily Kokomopteroidea Kjellesvig-Waering, 1966
- Superfamily Hibbertopteroidea Kjellesvig-Waering, 1959
- James Lamsdell. "Systematic list of known eurypterid species". eurypterids.co.uk. Retrieved January 13, 2011.
- "Fossilworks: Stylonurina". fossilworks.org. Retrieved 2017-10-03.
- James C. Lamsdell, Simon J. Braddy & O. Erik Tetlie (2010). "The systematics and phylogeny of the Stylonurina (Arthropoda: Chelicerata: Eurypterida)". Journal of Systematic Palaeontology. 8 (1): 49–61. doi:10.1080/14772011003603564.