Eysyslopterus

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Eysyslopterus patteni: a small, early eurypterid from Estonia

Eysyslopterus is a genus of eurypterid, an extinct group of aquatic arthropods. It belongs to the family Adelophthalmidae, the earliest branch of the adelophthalmoids. The only species known in this genus is E. patteni, described from a single fossil carapace (head shield) found in the Rootsikula Formation in Saaremaa, Estonia. This fossil dates to the Ludlow stage of the Late Silurian period, about 422 to 419 million years ago.

Discovery and naming
- The fossil was found by William Patten and described in 1934 by Størmer as Hughmilleria patteni.
- In 1961 it was placed in the subgenus Nanahughmilleria patteni by Kjellesvig-Waering, and later discussions kept it close to Hughmilleria.
- In 2008, Tetlie and Poschmann established a new genus, Eysyslopterus, for patteni because of distinctive features, especially the position of the eyes. The name combines Eysysla (the Viking name for Saaremaa) with -opterus, a common suffix for eurypterid genera. The species name honors William Patten.

What is known about the creature
- Only one specimen is known, and it preserves just the prosoma (the head shield) with no limbs. It is housed as AMNH 32720 at the American Museum of Natural History.
- The preserved carapace is broad and parabolic (roughly U-shaped). It measures about 19.5 mm long and 22 mm wide.
- The surface shows a gentle arch with deep transverse furrows forming ornamentation. The eyes are bean-shaped (reniform) and about 4.5 mm long. The ocelli (simple eyes) lie in the posterior half of the prosoma.
- The eyes are positioned closer to the margin rather than mid-shape, a feature that makes Eysyslopterus a basal member of the Adelophthalmidae family.
- Overall body length is estimated at around 8 cm (3.1 inches), making it a small eurypterid compared with some of its later adelophthalmid relatives.
- Because only the head is preserved, details about limbs and tail are unknown.

Habitat and significance
- The Rootsikula Formation represents a nearshore lagoonal environment where Eysyslopterus lived alongside other eurypterids such as Erettopterus, Eurypterus, Mixopterus, and Strobilopterus, in a mud-dominated, dolomite- and limestone-rich setting.
- Eysyslopterus is considered the most basal adelophthalmid genus due to the eye position. This placement helps scientists understand how eye placement evolved within this group. Some researchers have questioned its exact placement, noting that more fossil material is needed for a firmer conclusion.

In short, Eysyslopterus patteni is a small, early member of the adelophthalmids known from a single head shield fossil from Silurian Estonia, important for its distinctive eye placement and its role in understanding the early evolution of this group.