Platylepas mediterranea, Collareta & Reitano & Rosso & Sanfilippo & Bosselaers & Bianucci & Insacco, 2019

Collareta, Alberto, Reitano, Agatino, Rosso, Antonietta, Sanfilippo, Rossana, Bosselaers, Mark, Bianucci, Giovanni & Insacco, Gianni, 2019, The oldest platylepadid turtle barnacle (Cirripedia, Coronuloidea): a new species of Platylepas from the Lower Pleistocene of Italy, European Journal of Taxonomy 516, pp. 1-17 : 5-9

† Platylepas mediterranea sp. nov.

urn:lsid:zoobank.org:act:B197B06B-33A0-4E86-B229-5B5AFAF86E48

Figs 1–3

Diagnosis

Wall truncated conical; bi-lamellar; transverse growth lines forming principal sculpture of outer face of compartments; longitudinal septa and canals visible through outer wall, becoming more numerous peripherally; carinolatus three-lobed; sheath extending for less than half of total height of compartment, appressed, not hollowed out from behind, exhibiting distinct transverse grooves; accessory ribs present, corresponding to distinct sulci on outer face of compartments; inner face of compartments smooth, devoid of ridges or septa; radii narrow, sub-straight, and relatively robust; alae thin; inter-compartmental ornamentation absent.

Etymology

From Mare Mediterraneum, the Latin name of the Mediterranean Sea, in allusion to the region where the holotype was found.

Material examined

Holotype

SICILY • a single right carinolateral compartment (CL1 or CL2); Cala S. Antonino, Cape Milazzo Peninsula; 38°15′54.9″ N, 15°14′12.7″ E; epibathyal deposits (“Marne Calcaree Gialle”); Lower Pleistocene (Gelasian); sole known specimen to date; MSNC 4562 .

Description

MSNC 4562 is an isolated, ca 7 mm tall, near-complete right carinolateral compartment (CL1 or CL2), the external surface of which has been moderately decorticated by wear ( Fig. 2a ). For this reason, we describe it here starting from the better-preserved and more informative internal surface ( Fig. 2b ).

When viewed internally, MSNC 4562 exhibits a rather short sheath, extending for less than half of the total height of the compartment ( Fig. 2b ). The sheath is strongly appressed, i.e., it runs subparallel to the external face of the compartment; its median portion bears a low and broad longitudinal ridge which corresponds to the location of the upper part of the midrib prop (see below). The sheath exhibits a few distinct plicae (here interpreted as growth lines) that extend subparallel to the gently curved (i.e., slightly S-shaped) basal margin of the sheath ( Figs 2b , 3a ). The fossa for the accommodation of the ala (hereinafter referred to as the alar depression) is shallow and its margin is rounded and roughly J-shaped. A few very weak, transversely oriented grooves are observed also within the alar depression. The sheath is massive and not hollowed out from behind. However, a very shallow depression is seen beneath the basal margin of the sheath on the radial side of the midrib prop; conversely, at the alar side of the midrib prop the sheath is solidly filled from behind, and its basal margin is just marked by a horizontal ridge forming a sort of “step” on the inner surface of the compartment ( Figs 2b , 3a ). Below the sheath, the inner surface of the compartment is strongly folded inwards, thus generating a prominent longitudinal midrib prop ( Figs 2 b–f, 3a). This hook-like structure distinctly projects inwards and downwards; it is not filled internally. The transverse section of the midrib prop is roughly drop-shaped ( Fig. 3f ). The midrib prop runs closer to the alar margin of the compartment than to its radial margin. Therefore, it divides the inner surface of the compartment below the sheath into two areas: the smaller one is distinctly concave and roughly shaped as a longitudinally-elongated semi-cylinder; the larger one is laterally bounded by the sutural edge of the radius ( Fig. 2b, f ), with a broad accessory fold parallel to the midrib prop. This accessory fold is directed downwards and starts a couple of millimeters below the basal margin of the sheath ( Figs 2b , 3a ). No longitudinal septa occur below the sheath, either close to its base or at the periphery (i.e., close to the preserved portions of the basal margin of the compartment), as this portion of the inner surface is substantially smooth.

On the external surface of the compartment, the midrib prop and the accessory fold correspond to two shallow, yet obvious sulci which distinguish three longitudinally-elongated lobes ( Fig. 2a ). Although MSNC 4562 is moderately eroded externally, its trilobate aspect is well recognized from the apex of the compartment downwards ( Fig. 2e ). The external surface of the compartment is ornamented by longitudinal septa, which define longitudinally elongated tubes or canals between them. MSNC 4562 is therefore tubiferous and bi-lamellar, the outer lamina being produced by the distal coalescence of the septa that extend from the inner wall. However, due to overall abrasion of the terminal T-shaped flanges of the longitudinal septa, a pristine outer wall is only locally preserved ( Fig. 3b ). Some septa bifurcate downwards so that the interseptal distance remains roughly constant along the whole height of the compartment. This septate sculpture is affected by prominent transverse growth ridges along which the external surface of the compartment widens distinctly ( Fig. 2a , c–d). The intersection of the longitudinally elongated septa and the transversely oriented growth ridges is occasionally marked by small knobs on what remains of the outer lamina ( Figs 2a , 3b ).

The radius is rather narrow and partially preserved. In radial view, it is sub-straight ( Fig. 2d ) and its basal termination weakly projects inwards. The sutural edge of the radius is relatively robust. The denticulation pattern of the sutural edge is largely eroded; where it is preserved, it appears simple, being in fact comprised of squat, blunt, transversely-oriented septa that originate from the inner margin of the radius and project outwards ( Fig. 2d ). The ala is thin and distally tapers moderately. The sutural edge of the alar portion of the compartment exhibits short, triangular, wedge-like complementary septa which project inwards from a main septum running along the outer margin of the compartment ( Figs 2c , 3a ). Therefore, judging from the architecture of the radial and alar sutural margins, the space between two adjacent compartments would have been completely filled.

Comparisons and remarks

MSNC 4562 unambiguously belongs to the monotypical subfamily Platylepadinae based on the presence of a large and prominent midrib prop – observable on the inner surface of the compartment – which corresponds to a distinct sulcus on the external surface of the shell. The general morphology of MSNC 4562 strongly suggests a truncated conical (i.e., volcano-like) rather than peltate shell. Based on its size, a truncated conical shell that measured ca 14 mm in carinorostral diameter could be reconstructed; a slightly larger size estimate (ca 17 mm) could be proposed if the shell were more depressed. These size estimates are largely consistent with measurements of mature specimens of extant P. hexastylos .

On the whole, MSNC 4562 is reminiscent of the extant species P. hexastylos in various aspects of the shell: the structure of the sutural margins of MSNC 4562, for instance, is rather close to that of P. hexastylos . In addition, the outer aspect of MSNC 4562 is similar to that of a specimen of P. hexastylos from Sicily as illustrated by Pilsbry (1916: pl. 67, fig. 4) and to that of numerous other truncated conical individuals of P. hexastylos in which the outer lamina is partially or mostly worn out, thus revealing the septate, bi-lamellar nature of the compartment wall (e.g., Darwin 1854; Pilsbry 1916; Monroe & Limpus 1916; A.C., pers. obs.). However, various characters preserved in MSNC 4562 distinguish this fossil from the extant P. hexastylos . First, whereas in P. hexastylos the shell is very light and thin, MSNC 4562 appears to have been slightly more robust. Secondly, whereas in P. hexastylos the sheath is generally smooth and only occasionally displays weak growth lines, that in MSNC 4562 exhibits evident transverse grooves. Thirdly, in P. hexastylos the sheath is hollowed out from behind, but that in MSNC 4562 is appressed and filled from behind. Fourthly, P. hexastylos bears septa that extend onto the internal surface of each plate up to the level of the basal margin of the sheath or even terminate within the hollowed out portion behind the sheath; in turn, the internal surface of MSNC 4562 is devoid of septa and substantially smooth. Fifthly, whereas compartments of P. hexastylos appear as typically bilobate externally, the external aspect of MSNC 4562 is trilobate – a character that reflects the presence of an accessory rib on the inner surface of the compartment. Although compartments belonging to adult individuals of P. hexastylos occasionally exhibit an accessory rib and an incipient third lobe developing near the base of the shell, the trilobate nature of the external surface of MSNC 4562 is recognizable even close to the orifice, thus suggesting that it represents a structural character rather than an incidental feature incurred during growth. This hypothesis could find some support in the above-mentioned observation that the inner face of MSNC 4562 is smooth and devoid of those ridges and septa which in extant P. hexastylos aid in both fixing the phoront crustacean to its substrate and preventing the collapse of the thin barnacle shell (e.g., Ross & Newman 1967). In this respect, the accessory rib observed in MSNC 4562 could be interpreted as a functional substitute of the fimbriated periphery of the compartments of P. hexastylos , substantially contributing to the stabilization of the relatively robust platylepadid shell to which MSNC 4562 belonged by providing firm anchorage to the underlying host tissue. If this interpretation is correct, a complete adult shell of the platylepadid species from Cala S. Antonino might have displayed up to 18 lobes on its external surface (e.g., three lobes on each CL, four on R, and two on C).

The extant species Platylepas coriacea , P. indicus and P. ophiophila are similar to P. hexastylos in shell shape and structure. MSNC 4562 further differs from P. coriacea by its narrower radius, longer sheath, and the trapezoidal (rather than near-triangular) outline of the paries. It differs from P. indicus , in which both sides of the midrib prop of each compartment bear a distinct ridge, and from P. ophiophila by not displaying three or more secondary ribs lateral to the midrib prop.

MSNC 4562resembles P.decorata as its sheath is not hollowed out from behind and in lacking longitudinal septa on the inner surface of the compartment. However, MSNC 4562 differs markedly from P. decorata by exhibiting a thinner wall, a distinctly shorter sheath, sutural edges of the compartment that lack external ornamentation, and a different aspect of the outer wall (e.g., the shell of P. decorata is monolamellar, each compartment being externally ornamented by fine longitudinal columns). Moreover, the shell of P. decorata is more or less cylindrical – a feature different from MSNC 4562, which would best match a truncated conical shell morphology.

MSNC 4562 also resembles † P. wilsoni in exhibiting transverse grooves on the sheath and a basal inner margin of the compartment without septa. However, MSNC 4562 clearly differs from this Late Pleistocene species by having a P. hexastylos -like radius (rather than a hollow tube) and by a clearly different ornamentation of the external surface of the shell (e.g., the outer wall of † P. wilsoni has a shingled appearance).

In conclusion, MSNC 4562 looks both structurally and morphologically more similar to the type species of the genus Platylepas (i.e., P. hexastylos ) than to other congeners known to date (e.g., P. decorata ). At the same time, MSNC 4562 significantly differs from all the species of Platylepas known to date, thus warranting the erection of a new species – † P. mediterranea sp. nov.

Figures

Fig. 1. Locality map, showing provenance (white star) of the holotype of the new platylepadid (MSNC 4562) described herein. All images from Google Earth.

Fig. 2. †Platylepas mediterranea sp. nov., holotype, single right carinolateral compartment (CL1 or CL2) collected at Cala S. Antonino (Cape Milazzo, Sicily, southern Italy) from Lower Pleistocene (Gelasian) deposits (MSNC 4562). a. Outer view. b. Inner view. c. Alar view. d. Radial view. e. Apical view. f. Basal view.

Fig. 3. †Platylepas mediterranea sp. nov., holotype (MSNC 4562). a. Interno-alar view. Note the strongly appressed sheath exhibiting distinct transverse grooves (arrows) and the short, triangular, wedge-like, transverse septa on the sutural edge of the alar portion of the compartment. b. Close-up of the outer surface of the compartment. The outer lamina is only locally preserved; where it is eroded, longitudinally elongated septa and canals are visible. The intersection between longitudinal septa and transverse growth ridges is occasionally marked by small knobs.