Thalassematidae Forbes and Goodsir, 1841

Botting, Joseph P. & Muir, Lucy A., 2023, A new thalassematid echiuran worm from the Middle Ordovician Castle Bank Biota of Wales, UK, Acta Palaeontologica Polonica 68 (4), pp. 571-581 : 574

Family Thalassematidae Forbes and Goodsir, 1841 Genus Llwygarua nov.

ZooBank LSID: urn:lsid:zoobank.org:act:7D02B25D-F3F1-476E-8196-387F4BAFF9EE

Etymology: From Welsh Llwy garu, a love spoon: a traditional carved wooden spoon, given as a gift of romantic intent, in Wales; in reference to echiurans (spoon worms). Pronounced “H-loui-garry-a”. Gen- der feminine.

Type species: Llwygarua suzannae sp. nov., see below. By monotypy.

Diagnosis.—As for the type species.

Remarks.—Recognition of the new genus as an echiuran is based on the presence of several key features, including the presence and form of the proboscis, diagnostic muscle-band arrangement, anterior setal pair and the presence of a differentiated caudal region. The combination of characters that have been preserved is fully diagnostic of the Echiurida ; in particular, no other worms show the same bilobed proboscis morphology or pair of anterior setae, and the musculature is also highly distinctive.

No described fossil echiurans show even an approximate similarity to the new genus. The closest similarity is to the extant thalassematids such as Ochetostoma Rüppell and Leuckart, 1828 (e.g., Fig. 2A ), but assignment of an Ordovician fossil to a living genus would be highly contentious, and the current morphological classification of genera within the family appears unreliable ( Goto et al. 2020). Some of the key characters that are applied in living taxa e.g., the number and arrangement of nephridia) cannot be assessed, so such an assignment would be based on incomplete characters. There are also some possibly unique characters in the fossil specimens described here, such as the reticulate structure of the proboscis, that support separation. Overall, diagnosis of the new species and genus is based on the limited characters that are preserved, and these cannot be used to separate it easily from extant thalassematids; although a distinct generic assignment is preferred, it remains possible that future discoveries and advances in classification of modern taxa will support a reassignment of Llwygarua gen. nov. to an extant genus.

Stratigraphic and geographic range.—Darriwilian, Middle Ordovician ( Didymograptus murchisoni Biozone ) of Castle Bank, near Llandrindod, Wales, UK.

Figures

Fig. 2. Recent thalassematid echiuran (A) and a fossil equivalent (B). A.?Ochetostoma sp., highlighting significant features for comparison with the fossils (photograph courtesy of Arthur Anker), typical length 10–20 cm. B. Holotype of Llwygarua suzannae gen. et sp. nov., from the Darriwilian (Middle Ordovician) of Castle Bank, Wales, UK, NMW.2021.3G.99 showing overall specimen in slightly oblique lateral view (B1); enlargement with anterior ventral part of trunk (B4), showing one anterior seta (white arrow) and probably second seta (black arrow); detail of proboscis in cross-polarised light (B5), showing reticulate structure and anterior projections adjacent to midline; central region of trunk with distinctive wrinkling of cuticle in response to compression (B2), and proboscis in plane-polarised light (B3), with anterior projections arrowed.