Centrichnus dentatus, Uchman & Wisshak & Madeira & Melo & Sacchetti & Ávila & Ávila, 2025

Centrichnus dentatus View in CoL isp. nov.

Figs. 3–8 View Fig View Fig View Fig View Fig View Fig View Fig .

Zoobank LSID: urn:lsid:zoobank.org:act:.

Etymology: From Latin dentatus , toothed; in reference to the marginal series of pits formed by the verrucids anchoring structures.

Type material: Holotype INGUJ249 P204, a distinct two-step depression (marked “h” in Fig. 3A View Fig 1 View Fig , A 2, A 5, A 6) in an oyster shell with well-expressed collar . Paratypes all other specimens in INGUJ249 P204 Figs. 3A View Fig 1 –A View Fig 4, 4A, B View Fig ) and INGUJ249 P115 ( Fig. 4C View Fig ), isolated or partly overlapping depressions showing different depths and more or less developed bounding grooves or series of pits .

Type locality: Pedra-que-Pica palaeosite, Santa Maria Island, Azores, Portugal .

Type horizon: Zanclean , lower Pliocene, coquina of the Touril Complex .

Material.— Type material, 19 small bivalve shells or valve fragments ( DBUA-F 95-12 , 99-15 , 18 , 20 , 30 , 180-1 , 185-2 , 10 , 17 , 24 , 25 , 186-4 , 10 , 25 , 187-2 , 4 , 15 , 19 , 193-2 ) with a total of about 70 attachment traces, and 33 small bivalve shells or valve fragments (collective number DBUA-F 1545 : INGUJ249 P115, 116, 119–123, 125–138, 145, 150, 156, 157, 186, 192, 201–205a, b, 206), with a total of about 400 attachment traces, and one thin section ( INGUJ249 P207) crossing a trace vertically .

Diagnosis.— Centrichnus circular or elliptical in outline, bounded by a groove and/or a series of pits and having a more or less pronounced central to decentral depression surrounded by a flat area.

Description.—Regular to irregular circular or oval, smooth bowl-like depression on bivalve shells, usually 2–3 mm and occasionally up to 4 mm in diameter, encircled by a narrow groove and small pits piercing the bottom of the groove Figs. 3–7 View Fig View Fig View Fig View Fig View Fig ). The marginal groove is ca. 0.1 mm wide. The pits are trapezoid with very rounded corners in outline, ca. 0.1 mm wide (longer axis), and ca. 0.1 mm apart ( Fig. 5A View Fig ) with the longer axis commonly oriented perpendicular to the groove. The number of pits varies between individual borings and ranges from 25 to 70.

The bowl-like, central to decentral depressions are up to 1.5 mm deep, but never deeper than the trace is wide. The deeper that depression, the wider it is, and the smaller the flat area between the depression and the marginal groove/ pits becomes during ichnogeny sensu Belaústegui et al. 2016), i.e., changes of the trace during ontogenetic development of the tracemaker ( Fig. 5A, B View Fig ). On the slopes of the deeper depressions, intersections of shell layers can be visible, and, in some cases, differential dissolution has terraced these layers, which then appear as concentric rings, but with no grooves or ridges ( Figs. 4C 4 View Fig , 5 View Fig , 6B View Fig , 7B View Fig ).

In many specimens, the marginal groove and pits still contain remnants of the skeletal elements of the trace-making verrucid barnacle. These skeletons are at least as wide as the groove and reach 0.25 mm in thickness. Where the barnacle is not preserved, the marginal groove with pits becomes visible in its virtual prolongation ( Fig. 5A, B View Fig ). The thin section perpendicular to the depression shows that the shell of the barnacle is attached to the slope of the depression in its upper marginal part, the cross-cut shell layers of the bivalve, and is indented in the bivalve shells by tooth-like protrusions, which are ca. 0.05 mm wide and deep ( Fig. 8 View Fig ). In any case, shell remains are part of the basal part of the barnacle shell, which, with its dental protrusions, is anchored in the shell of the bivalve host. SEM images clearly confirm this interpretation. Therefore, these structures are not a part of the trace fossil but a part of the tracemaker.

The traces are isolated or can be fused in clusters or rows to form composite traces ( Figs. 6C View Fig 1 View Fig , 7A View Fig ). One depression can truncate the neighbouring one ( Figs. 3A View Fig 1 View Fig , A 2, 4C 1, C 3, 6C, 7B). The number of depressions in one shell may attain nearly forty.

Remarks. —The circular or oval outline of Centrichnus dentatus is similar to that of Centrichnus concentricus Bromley & Martinell, 1991 , Centrichnus circularis ( Santos et al., 2005) , and Centrichnus ellipticus ( Buckeridge et al., 2019) , but C. concentricus and C. ellipticus have distinct concentric ridges and crenulated margins, and C. circularis lacks the central to decentral depression. While shallow morphotypes of C. dentatus are very close in shape to those of C. circularis , the latter does not show the central depression, and it does not have pits in the marginal groove. Hence, these pits are the most relevant diagnostic feature of C. dentatus . Centrichnus undulatus differs by its flabellate margin and no distinct depression ( Santos et al. 2005) and C. eccentricus Bromley & Martinell, 1991 , is a more complex Centrichnus with a tier-shaped central etching and eccentric grooves, surrounded by abrasive imprints of the trace maker shell margin and hinge area (see Neumann et al. 2015 for a revision of this type ichnospecies).

Stratigraphic and geographic range.—The Touril Complex (Zanclean, lower Pliocene), Santa Maria, Azores.