Clade Bilateria, Clade Protostomia
Vermilituus gregarius gen. et sp. nov.
Etymology: Genus name from vermis (Latin) meaning worm and lituus (Latin) meaning a curved trumpet, alluding to the shape of the fossils. Species name from gregarius (Latin), meaning flock or herd.
Holotype: YKLP 13079a, b (counterparts), U-shaped tube (Fig. 1a, b), 6.5-mm long, and reaching a maximum width of 0.6 mm: the holotype is associated with Vetulicola rectangulata YKLP 13075a, b. Paratypes (with preserved shell annulation), YKLP 13084 and 13085 (Fig. 1c, f) associated with V. rectangulata YKLP 13074, and YKLP 13082 and 13083 (Fig. 1e) associated with V. rectangulata YKLP 13073.
Referred material: About 192 specimens from Ercaicun, 75 from Mafang and 10 from Jianshan associated with Vetulicola rectangulata, all in the collections of the Yunnan Key Laboratory for Palaeobiology (YKLP). In total, 17 specimens from Xiaolantian and 55 specimens from Heimadi associated with Vetulicola cuneata, all in the collections of the Chengjiang Fossil Museum (CJHMD, Supplementary data file).
Locality: Ercaicun (type locality), Mafang and Jianshan localities in the Haikou area of Kunming, and Xiaolantian and Heimadi in Chengjiang County, Yunnan Province, China (for localities see ref. 5).
Horizon: Yu’anshan Member, Chiungchussu Formation, Eoredlichia-Wutingaspis trilobite Biozone, Nangaoan Stage of Chinese regional usage, Cambrian Series 2, Stage 3. All specimens are from rapidly sedimented ‘event beds’5.
Diagnosis for genus (monotypic) and species. Small (0.8–7.2-mm long) elongated, conical tubes having three general forms, as a U-shape, J-shape or complex sinusoid, the latter being the dominant type: occasionally the tube also begins with a 360° planispiral coil before straightening. Coiling can be both dextral and sinistral and is in a single plane. The proximal end of the tube blunts (no bulb-like origin). Tubes increase in diameter very slowly, the proximal diameter being about 0.2 mm and the distal diameter reaching 1 mm. No longitudinal ornament. The transverse ornament of the tube consists of distinct annulation, there being about 12–16 annulae per mm. Most tubes are discrete, but in some cases two or more tubes cross. The tube wall appears to be very thin, and there is no evidence of internal septae, pseudopunctae or punctae. Paired crescentic structures are preserved at the open end of the tube in some specimens.
All specimens of Vermilituus gregarius are associated with vetulicolians, a group of extinct animals of disputed phylogenetic affinity that possessed a convex anterior part with frontal and lateral openings, articulating with a tail-like posterior extension (Figs. 2–6; Supplementary Figs. 1 and 2; for a summary of vetulicolians see ref. 6). The soft anatomy of these animals is largely unknown, but the anterior part of Vetulicola has been hypothesised to comprise a pharynx with gill-like structures that flexed by means of horizontal and longitudinal muscle fibres attached to four flexible plates covered by a thin outer membrane (see below).
Vermilituus gregarius occurs in four specimens of Vetulicola cuneata from the Chengjiang region (Figs. 2, 4, and 5), plus six specimens of Vetulicola rectangulata from the Haikou region (Fig. 3; Supplementary Figs. 1 and 2; Supplementary data file). Overall, at least 400 specimens of V. rectangulata and 80 specimens of V. cuneata have been collected from the Chengjiang biota (YKLP and CJHMD collections), meaning that Vermilituus gregarius is a rare associate of vetulicolians. Vetulicolian fossils occur as composite moulds where the rock splits through the specimen, each part containing components of both the external and internal surfaces (Supplementary Fig. 3). For the anterior part of Vetulicola, we interpret Vermilituus gregarius as occupying the space between the interior of the exoskeleton, and the convex surface that appears to demarcate the position of the internal anatomy (Figs. 2a–c, 3a, b, g, 4a, b; Supplementary Figs. 1b–e, 2a, b).
The number of Vermilituus gregarius per associated vetulicolian is variable, ranging from a single tube to 88 individuals (Supplementary data file), and in some cases, V. gregarius occurs in local aggregates of up to 25 individuals, for example in YKLP 13075 (Fig. 3c, e, f). In most cases where V. gregarius aggregates, the individuals are discrete, but occasionally some overlap. The overall size of V. gregarius is from 0.8 to 7.2 mm in length, with maximum diameter ranging from 0.4 to 1 mm (proximal width is circa 0.2 mm). Average tube length varies within individual Vetulicola specimens (Supplementary data file), by a minimum of 1.6 mm (specimens associated with CJHMD 00031) to a maximum of 6.4 mm (specimens associated with YKLP 13073).
Rather than representing post-mortem assemblages, or the result of Vermilituus scavenging or colonising vetulicolian carcasses, all evidence suggests that Vermilituus attached to the body surfaces of living vetulicolians. All infested vetulicolians are preserved within ‘event beds’5. This means that they were rapidly buried by sediment, and therefore post-mortem colonisation at the seabed is highly unlikely. Tubes of Vermilituus gregarius occur almost exclusively inside the vetulicolians, rather than the external body surface, and preferentially within the anterior part (Figs. 2–5; Supplementary Figs. 1 and 2). They are absent from other fossils preserved adjacent on the same slabs (Fig. 5a), and indeed have never been observed in other Chengjiang fossils in our investigations over the last three decades. Most specimens of V. gregarius occur in the anterior section of the vetulicolian body (n > 345) (Figs. 2–5, Supplementary Figs. 1 and 2), with just 4 specimens associated with the posterior section of the most-infested specimen in our collection (YKLP 13075, Fig. 3d). In this rare case, V. gregarius may have over-spilled onto the external surface of the animal or has been displaced post-mortem. Among those in the anterior part, most are located in the convex area between the central groove and the fin-like margins, with some concentrations often in the anterodorsal region (Fig. 3a, b). Only a few tubes of V. gregarius occur along the margins of Vetulicola. In one case, at least 10 U-shaped tubes grow with a posterior orientation in Vetulicola YKLP 10906 (Supplementary Fig. 1e). In Vetulicola YKLP 13075, there is a clear association of 24 V. gregarius with the central groove (Fig. 3f), each having a distinctive orientation with the narrow end of the tube pointing towards the groove.
The consistent occurrence of Vermilituus gregarius inside the anterior section of vetulicolians, combined with the observed patterns of localisation and occasional preferred orientation (Supplementary data file), argues against a chance post-mortem association, or generalist epibiontic habit. In the latter scenarios, the posterior section should also be infested. Furthermore, V. gregarius is absent from any other fossil organism in the Chengjiang biota, suggesting a highly specific relationship. The robust (possibly biomineralised) and curved tubes of V. gregarius are consistent with a sessile, attached ecology, but not with a motile scavenger that might have fed on vetulicolians after death. The size range of Vermilituus on each specimen (Supplementary data file) suggests animals growing in situ for some time, rather than colonising carrion. In addition, the lack of evidence for decay and disarticulation of infested vetulicolians combined with their preservation in event beds supports an in vivo association. In this light, the observed patterns in size, number and distribution of V. gregarius tubes also shed light on vetulicolian biology and the ecological relationship between the taxa.