A well-preserved respiratory system in a Silurian ostracod

Ostracod crustaceans are diverse and ubiquitous in aqueous environments today but relatively few known species have gills. Ostracods are the most abundant fossil arthropods but examples of soft-part preservation, especially of gills, are exceptionally rare. A new ostracod, Spiricopia aurita (Myodocopa), from the marine Silurian Herefordshire Lagerstätte (430 Mya), UK, preserves appendages, lateral eyes and gills. The respiratory system includes five pairs of gill lamellae with hypobranchial and epibranchial canals that conveyed haemolymph. A heart and associated vessels had likely evolved in ostracods by the Mid-Silurian.


Introduction
Ostracod crustaceans, originated about 500 Mya [1], are abundant as fossil shells from the Ordovician to the Holocene and have colonized all aquatic environments. Most are benthic/nektobenthic, with pelagic species (exclusively Myodocopa) known from the Silurian onwards ( [2] and references therein). Of the respiratory strategies [3][4][5][6] in living species only one involves respiration by gills. Only eight ostracod species (six Myodocopa) with soft parts preserved are known from the Palaeozoic, most documented from single specimens ( [7] and references therein). The Herefordshire Konservat-Lagerstätte (approx. 430 Mya), UK, has yielded unparalleled evidence of the palaeobiology and evolutionary significance of diverse Silurian invertebrates, including four ostracod species ([8] and references therein). Here, we report a new Herefordshire ostracod that reveals the ancestry of the respiratory/circulatory system within the group.
spiritus, breath of life, plus copia, abundance, alluding to the gills; auritus, eared, fancied resemblance of the posterodorsal lobes. From the Wenlock Series, Herefordshire, England.
Diagnosis: Cylindroleberidid with an elongate, reticulate carapace with a rostrum, adductorial sulcus, posterodorsal lobe, second maxilla bearing an epipod and five pairs of gills.
Description: Carapace maximum length is 7500 mm; maximum height (2950 mm) and width (3200 mm) lie anterior of an adductorial sulcus (figure 1a,l; as). There is a beak-like rostrum above a gently concave rostral incisure (figure 1a,j; ro, ri). Valves gape at about 208; a posterior gape is present (figure 1i; pg). The narrow adductorial sulcus occurs at one-third valve length and curves gently forward to below valve mid-height. Each valve has a prominent posterodorsal lobe (figure 1a,b,i,l; pl). A weak marginal ridge occurs in the right valve, demarcating the reticulate lateral surface from the smooth ventral (contact margin) surface (figure 1j,k; cm, mr); valve overlap was probably left over right.
The first antenna (antennule: figures 1a-c and 2a-d; a1) originates close to the sagittal plane. It has a long, stout proximal part ( presumed podomere) geniculate with a shorter stout middle part ( presumed podomere) and a transversely compressed, longer, weakly curved distal section ( podomeres not evident). Two slender setae project ventrally, at the junction of the proximal and middle podomeres and from the distal section, respectively. Four other setae, preserved only in the right limb, project from the distal podomere. A pair of pedunculate lateral eyes originate posterodorsally to the first antenna (figures 1c and 2b,c,l; le). A tiny projection between the lateral eyes may represent a medial eye (figure 2c; me?).
The second antenna (antenna: figures 1a-c and 2a-c,e; a2) arises laterally to the first. The basipod is large, almondshaped, globose; a lateral depression demarcates an anterior and a smaller posterior part. The exopod is stout, strongly curved to the posterior and projects beyond the carapace. An array of long, closely spaced curved setae in the distal half of the ramus (more evident in the right limb) may reflect the podomere distribution in living cylindroleberidids (in which podomere 1 lacks setae; podomeres 2 -8 each bear one seta and podomere 9 has multiple setae; figure 2o). The endopod is stout and a quarter as long as the exopod. It is strongly geniculate proximally, at a presumed podomere boundary. The longer distal part terminates in a long, straight seta and a shorter tightly reflexed seta.
as rv p1 p1 g4 g3 g2 g1 li a7 is le a2ex a2ba  [11], gill lamella (from [5] rsbl.royalsocietypublishing.org Biol. Lett. 14: 20180464 The mandible (figures 1c and 2a,f; ma) has a broad flat limb base ( presumed basipod and coxa) positioned over a small conical labrum and the presumed site of the atrium oris. Its inner edge, although poorly preserved, is serrated, suggesting enditic processes. Only a stout, proximal part ( possible podomere) of the endopod is preserved. The exopod is slender, geniculate at about mid-length ( possible podomere boundary) and slightly longer than the preserved part of the endopod.
The limb base (presumed basipod and proximal endite) of the first maxilla (maxillula: figures 1c and 2a,g; mx1) is broad, flat and projects adaxially over the presumed site of the atrium oris; its inner edge is poorly preserved. The ramus ( presumed endopod) is large, blade-like, and consists of subequal proximal and distal parts (possible podomeres) joined at a marked geniculation. An exopod is not discernible.
The second maxilla (fifth limb: figures 1c and 2a,h; mx2) is poorly preserved. The limb base bears a stout ramus (conventionally the exopod in myodocopids; but see [13]) and a large laterally projecting lamellar epipod with a curved ventral margin. A small sixth limb lies against the inner base of the fifth appendage; it consists of a short, flat ramus bearing a row of three short setae ( figure 2i; a6). A seventh limb arises below mid-height anterior of the gills. It is vermiform, about 3500 mm long with a v-shaped termination (figures 1c and 2a,c,j; a7).

Discussion
A vermiform seventh appendage and lateral eyes occur only in myodocopids, and gills are known only in Cylindroleberididae (figure 2n). The morphology of appendages 1 -6, including the presence of an epipod only on the second maxilla, is also compatible with a cylindroleberidid placement. Many of the appendages resemble those of the Herefordshire Lagerstätte cylindroleberidids Colymbosathon, Nasunaris and Pauline and the associated nymphatelinid myodocopid Nymphatelina [14][15][16][17][18]. Spiricopia is unusual among cylindroleberidids in having a prominent adductorial sulcus and atypical in apparently lacking a setate comb on the second maxilla. The shell morphology of these five Herefordshire myodocopids is very variable and emphasizes that the carapace alone is an unreliable basis for classifying fossil ostracods [17].
Living cylindroleberidids use the setate comb for filter feeding. As a setate comb is not evident in Spiricopia, it likely used its furca and appendages for predation, scavenging and detritivory on or near the substrate, as do most living myodocopids [19]. Its powerful antennae suggest effective swimming.
In small ostracods (less than 3 mm long), gas diffusion occurs via the surface of the body integument and the uncalcified inner lamella integument. In larger ostracods (myodocopid and halocyprid Myodocopa) simple diffusion for gaseous exchange is supplemented by internal fluid convection via an anastomosing integumental vascular network which forms part of an integrated circulatory system [3][4][5][6]. Evidence from external shell ornament suggests that certain Silurian myodocopes had an integumental vascular network [4]. Among myodocopes, cylindroleberidids augment this with gills (external integumental folds), typically seven pairs, which, inter alia, enhance metabolism. The only fossil ostracods previously known to have gills are Colymbosathon ecplecticos, which preserves six incomplete gill pairs, and Nasunaris flata and Pauline avibella, which preserve fragmentary gill lamellae [14,16,17]. Gill-like features also occur in the Triassic myodocope Triadocypris spitsbergensis [20]. By comparison with living cylindroleberidids [4,5] (figure 1h), the swollen margin of each gill lamella in Spiricopia can be confidently interpreted as the site of hypobranchial (concave; afferent) and epibranchial (convex; efferent) canals that conveyed haemolymph in life (figures 1c-g and 2m). The ridge-like free edges previously noted in the gills in Colymbosathon [14] can be similarly interpreted. As in living cylindroleberidids [5], the epipod of the second maxilla likely functioned to rhythmically ventilate the domicilium including the gills, with a water/feeding current entering anteriorly and egressing posteriorly. The gills are posterior, the probable region for preferential oxygen exchange in myodocopids and a position that facilitates grooming by the vermiform seventh limb [4,5]. The lamellae in S. aurita provide a total surface area of about 35 mm 2 for gaseous exchange in an outer shelf/slope, purported 150-200 m deep environment [21]. Whether a diurnal activity rhythm featured in tissue oxygenation, as in some living cylindroleberidids, which migrate between the sea bottom and the water column [22], is unknown. The myodocopid S. aurita reveals a well-developed branchial system. Likely a heart and associated vascular system had evolved in ostracods by the Silurian.