The Tribe Serpulini is comprised of the genera Serpula Linnaeus, 1758, Spiraserpula Regenhardt, 1961, Cementula Regenhardt, 1961, Hydroides Gunnerus, 1768, Crucigera Benedict, 1887, and Floriprotis Uchida, 1978. Cementula is either predominantly or exclusively known from Mesozoic fossils (depending on definition of its relationship to Cenozoic and Recent species of Serpula and Spiraserpula). The the other five genera are extant, and fossils as old as Middle Permian have been referred to Serpula. Unfortunately, in the 19^th and first half of the 20^th centuries geologists and palaeontologist often used "Serpula“ in a wider sense, including many or all genera of the Family Serpulidae.

Hydroides norvegica Gunnerus, 1768.

Tube white (sometimes bluish in Recent specimens), small to medium-sized, posterior attached tube portion straight, irregularly curved or coiled, forming an irregular loop or spiral. Anterior tube portion straight or slightly curved, rising freely above the substrate. Length of both tube portions may vary considerably. In the attached tube portion, a few species lack any longitudinal ornamentation or demarcation, but some bear a single median longitudinal line, ridge or low keel, whereas most species bear two or three longitudinal ridges. Often, two longitudinal elements are located at the left and right borders of the more or less flattened upper side, with the third element, if present, in the middle of the upper side. In the free anterior portion, the longitudinal elements become weaker or disappear. Some species bear either fine transverse striae or rough wrinkles, corrugations and/or peristomes. Cross-section in the attached tube portion, apart from narrow to moderately wide basal flanges, circular or, more characteristic, trapezoidal with flattened upper surface, in the free tube portion generally more rounded, subquadratic or circular. A granular overaly may be present. Operculum two-tiered, composed of basal funnel of fused radii and distal verticil (crown) of chitinized spines. (modified from ten Hove and Kupriyanova 2009; Kupriyanova et al. 2020, 2023a).

It is difficult to assign fossils unambiguosly to Hydroides because most species are distinguished by well-differentiated opercula and less so by differences in chaetal and tube structure (e.g., Pillai 1972; Ippolitov et al. 2014; Kupriyanova et al. 2023a). However, the chitinous opercula of Hydroides are unknown from the fossil record so only well-preserved tubes can be identified with any confidence. The mineralogy of recent Hydroides tubes varies between predominantly calcitic and predominantly aragonitic (Vinn et al. 2008). Tube microstructure has been studied in only few of the many species of Hydroides and other genera of the Tribe Serpulini. In this tribe, single-layered to four layered tubes are found with 6 different kinds of microstructure (see Vinn et al. 2008). The number of layers and the types of microstructures are moderately different between species of the same genus and also moderately different between the genera of the Tribe Serpulini and thus can be useful for distinguishing between species and genera. Microstructure studies of more species of Serpulini are necessary to get a clearer picture. The only fossil Hydroides tube whose microstructure has been studied is an unnamed medium-sized tricarinate Hydroides species from the Early Miocene (early – middle Burdigalian) Vinjhan Formation of India (Ippolitov and Desai 2015).

Holotype, UWBM 118855, single tube attached to the gastropod Ranella washingtoniana (Weaver, 1912), collected as float at UWBM loc. 232 by D. Starr, ca. 1970s. Paratypes, UWBM 25241, originally attached to a gastropod, Murex sp., collector and date unkown, loc. B2292; UWBM 118856, attached to Murex sopenahensis Weaver, 1912, collected by D. Penttila, 26 August 1972, loc. B9613. Three referred specimens: UWBM 66096 and 66097, both attached to the gastropod Murex sopenahensis, float from UWBM loc. A9522, collected by D. Nannalee, 1967–1970; and UWBM 118854 (not illustrated), single broken tube on a worn Murex sopenahensis covered with an unidentified bryozoan, loc. 00549, collector unknown.

UWBMIP loc. 232 (equivalent to 00549, A9522, B2292, B9613); The "Big Bend“ locality on the Cowlitz River, east of the town of Vader, Lewis County, Washington State, USA. GPS coordinates are 47.4074°N, 122.9328°W, datum WGS84 (World Geodetic System). Cowlitz Formation, Middle Eocene, approximately 40–43 Ma (Nesbitt 1995).

Tube thin, subtly subquadrangular in cross section in early growth stages, with easily discernable growth lines; early peristomes delicate, flange-like; later growth stages with outer tube surface with flattened upper side; flattened region with low but with distinct borders and marked by a central faint longitudinal thread, more prominent in later growth stages; ventral margins of tube widened; entire tube with visible growth lines; tube surface cross-section in later growth stages subquadrangular with distinct flattened upper surface at or near aperture quadrate but in final growth stage more rounded, with a rugose surface covered with numerous vesicles and circular cross-section; lumen is circular and smooth; no preferred direction of coiling, coils open, not tightly cemented to each other except when they cross.

The tube of H. engi displays two different growth types, depending on the available surface area for attachment. In the case of the holotype the tube was able to grow mostly straight until near the aperture of the gastropod, and then forced to turn back on itself. The paratype and referred specimens of H. engi were all attached to a gastropod with a smaller available surface area than that occupied by the holotype, and as they grew they had to make numerous turns and twists, so the tube is cemented to itself in many places, forming a pile. Because of this, in places the flattened "upper“ surface of the tube appears to be in different orientations. Maximum length preserved for the holotype of H. engi is estimated at 13.6 mm, greatest diameter (UWBM 66096) is 5.1 mm with the flattened dorsal part 2.7 mm in width. Older tubes, like UWBM 66096, also become more rugose, with the borders of the flattened dorsal surface appearing to be more raised, and eroded to almost appear beaded. Greatest diameters of other specimens are 4.5 mm (UWBM 118854) and 4.9 mm (UWBM 66097).

Because of its trapezoidal cross-section, H. engi superficially resembles some species, especially of Middle Jurassic age, of the genus Nogrobs de Montfort, 1808. However, the peristome of Nogrobs, if present, is usually quadrilobate in frontal view, and the attached tube portion of Nogrobs is usually short compared to its long free tube portion and lacking altogether (or usually not preserved) in Nogrobs from Late Cretaceous deposits. Nogrobs further differs from Hydroides by its often thick and wide but rounded longitudinal keels, of which two occur in the attached tube portion and four equally spaced in the long free anterior tube portion with a rounded quadratic cross-section. Species of Filogranula Langerhans, 1884, can also have tubes with three longitudinal elements and a trapezoidal cross-section but differ from those of Hydroides by often possessing distinctly undulating keels and strongly developed, either stellate or annular peristomes.

The lateral sides of Hydroides are slightly sloped to develop a narrow basal flange as in H. engi. A raised, non-attached free anterior portion of the tube was apparently not developed, or is not preserved, in H. engi. Hydroides? quadrata (Nielsen, 1931) from Middle Danian cold water coral reefs at Faxe in Denmark (Jäger et al. 2018) is distinguished by its smaller diameter, up to 1.6 mm. The early Miocene species identified as Hydroides cf. elegans and Hydroides sp. by Mandor et al. (2022) from Gebel Gharra at Egypt are both smaller in diameter than H. engi. Hydroides vesicularis Radwańska, 1994, and H. vicinalis Radwańska, 1994, from Middle Miocene rocks in Poland are are relatively massive and both possess a middle crest on the flat upper side. Only one specimen of H. engi (paratype UWBM 25241; Fig. 4A–A2) preserves the final growth stage, similar to that of H. vesicularis in possessesing a circular cross-section with its surface is covered by dense corrugations and vesicles. Hydroides vicinalis also possesses a sub-qudrangular cross-section but lacks vesicles on its surface (Radwańska 1994).

Named for Ronald C. Eng, invertebrate paleontology collection manager, UWBM. Ron has been involved with museum collections for many decades, and the species name honors his help with numerous paleontological studies over the years.

Fossils from the Cowlitz Formation in southwestern Washington comprise a diverse shallow marine to fresh water invertebrate fauna of more than 200 species (Nesbitt 1995). Bivalves and gastropods are the most numerous, and have received the most attention (e.g., Weaver 1912, 1943; Dickerson 1915; Weaver and Palmer 1922; Merriam 1941; Beu 1988; Nesbitt 1995 and references therein) but scaphopods, nautiloids, foraminiferans, crustaceans, and corals have also been documented (e.g., Hanna and Hanna 1924; Rathbun 1926; Durham 1942; Beck 1943; Miller and Downs 1950; Palmer 1961; Armentrout et al. 1980). Ostracods have been mentioned (Henriksen 1956: 55) but are the only major group that still awaits study. Epibionts were mentioned by Nesbitt (1995: 1064), principally bryozoans, worm tubes, and barnacle attachment scars, but these were not further investigated by her. Barnacles have been described from the Big Bend locality previously (Zullo 1966) but none were apparently preserved still attached to other organisms. We also found one other apparent epibiont, a vermetid gastropod (UWBM 118853) attached to the apical fragment of a Turritella shell (Fig. 4B); vermetids have not been previously listed from the Cowlitz Formation.

Nesbitt suggested that the epibionts might be indirect evidence of hermit crabs (Paguroidea), and this seems to be corroborated by the Murex shells overgrown with H. engi described here. Murex sopenahensis is moderately abundant both in the outcrop (Nesbitt 1995) at the Big Bend locality and in the collections from that locality at the UWBM (pers. observation). However, only those few specimens documented herein were encrusted by Hydroides and bryozoans, the others are clean, unworn shells without any indications of ever having been encrusted by epibionts. In the analysis by Nesbitt (1995) all of the Murex specimens were recovered from a mollusc assemblage dominated by infaunal and epifaunal suspension feeders. The Murex shells illustrated herein must have been occupied by the gastropod itself or some other organism because the large worms (in relation to the size of the colonized shell) did not overgrow the shell aperture. Several extant species of Hydroides have been documented as associates of hermit crabs, sometimes on Murex shells (e.g., Stachowitsch 1980; Walker 1992; Williams and McDermott 2004). The Oligocene record of Hydroides sp. from North Carolina (Feldmann et al. 2019) was also apparently part of a hermit crab association. So far, however, there is no direct fossil evidence of hermit crabs from the Cowlitz Formation.