Ossirarus kierani Clack & Smithson 2016

Ossirarus kierani Clack & Smithson, 2016 Holotype. UMZC 2016.3. A single block containing scattered skull and postcranial remains. Locality. Ross cliffs, Burnmouth, Scottish Borders Region, Scotland. National grid reference NT 964606. Horizon. 340.5 m above the base of the Ballagan Formation. CM palynozone, mid-Tournaisian, Mississippian. Emended diagnosis. Autapomorphies: tabular elongate triangle forming a conspicuous tabular horn with a convex lateral margin. Derived characters present in several stem amniotes: tabular-parietal contact; exoccipital separate from basioccipital; multipartite gastrocentrous vertebrae with widely notochordal centra. Plesiomorphies and characters of uncertain polarity: jugal with extensive postorbital component, with anteriorly placed shallow contribution to orbit; preopercular and intertemporal present; cleithrum with long, narrow, curved stem and expanded dorsal blade; diamond-shaped interclavicle lacking parasternal process; humerus with elongate and oblique pectoralis process comparable with the ventral humeral ridge of elpistostegalians and Acanthostega; brachial foramen piercing posterior edge of humerus at the base of entepicondyle as in Mesanerpeton; radius c. 60 % the length of humerus; neural arches as unfused bilateral halves. Description. Skull. General skull preservation. The bones are generally well preserved. They are disarticulated and have drifted apart slightly, so that sutural overlap areas are often very clear. The pre-orbital region is missing, and the lower jaws and other tooth bearing bones are not preserved apart from a fragment of maxilla or premaxilla (Fig. 2). We estimate that the skull was 54 mm long and the preserved region made up approximately two thirds of its length. Apart from the tabular and squamosal, which show some ornamentation, the skull roofing bones are essentially smooth. The lateral line system is represented by pores in the jugal, postorbital, postfrontal and preopercular; there are no open lateral line canal grooves. Cheek region. Much of the right cheek is preserved. It comprises the jugal, postorbital, an incomplete right squamosal and a preopercular. The left cheek is represented by the posterior part of the jugal. The jugal is a large bone. It is c. 25 mm long, including an extensive area overlapped by the quadratojugal, and has a maximum depth of c. 7 mm behind the orbit margin (Fig. 2). The orbit margin is shallow and below it the bone is relatively deep and posteriorly elongated. It most closely resembles the jugal of Acanthostega (Clack 2002) and colosteids (Smithson 1982; Hook 1983). It lacks the tall, vertical orbital margin seen in Diploradus (Clack et al. 2016), Pederpes (Clack and Finney 2005) and Whatcheeria (Lombard and Bolt 1995; Rawson et al. 2021). The suture with the lacrimal is vertical and judging by the shape of the orbit margin sits under the centre of the orbit. The area behind the orbit is gently concave. It is unclear if this depression is natural or the result of crushing. The dorsal edge of the bone behind the orbit margin is damaged but it appears to have overlapped the ventral edge of the postorbital. The posterior portion of the jugal bears numerous fine ridges and furrows and was probably overlapped by the quadratojugal. This area is large and represents approximately one sixth of the area of the jugal. A much smaller area of ridges and furrows on the posterodorsal edge of the jugal probably formed part of the area overlapped by the squamosal. The incomplete left jugal is represented by the posterior portion bearing the ridges and furrows of the quadratojugal overlap area. The postorbital is almost rectangular in outline with a gently concave orbital margin anteriorly. Its sutural contacts with surrounding bones are well preserved. Ventrolaterally, there is a shallow step from the smooth external surface to an area of fine ridges and grooves marking the area of overlap by the jugal. The ridges and grooves continue onto the posterior edge marking the area of overlap by the squamosal. These ridges and grooves are also found at the anterolateral corner of the postorbital where it was overlapped by the postfrontal. The medial margin of the postorbital is damaged but appears to have formed a thin lamina that overlapped the lateral edge of the intertemporal. The squamosal is incomplete and appears to have broken into several pieces, most of which have been lost. Two fragments make up part of the posterior edge of the bone and a third formed the anterodorsal portion of the squamosal between the jugal and skull roof, behind the postorbital (Fig. 2). The external surface of the bone bears a fine reticulate ornament in contrast with the smooth surface of the jugal. The preopercular lies behind the jugal (Fig. 2). We initially thought it was part of the quadratojugal but the presence of a lateral line pore and the extent and orientation of the area of sutural overlap, as well as comparisons with the preopecular of Acanthostega (Porro et al. 2015) and Whatcheeria (Rawson et al. 2021), convinced us it is the preopercular. The bone is roughly triangular-shaped. It is c. 7 mm long and c. 6 mm high. Approximately two thirds of the surface is covered with the fine ridges and grooves that mark the area of overlap with the quadratojugal (Fig. 2) with only about one third exposed on the surface of the skull. The exposed area is roughly quadrangular, and bears a single lateral line canal sulcus in the posteroventral corner. One side of the quadrangle forms part of the posterodorsal edge of the suspensorium. The anterodorsal edge formed a suture with the squamosal and the anteroventral and posteroventral areas were overlapped by the quadratojugal. The preopercular is hypothesized to have occupied a position on the edge of the suspensorium, above the quadratojugal, in a similar position to the preopercular in Ichthyostega (Clack and Milner 2015, fig. 8), Pederpes (Clack and Finney 2006) and Whatcheeria (Rowson et al. 2021), rather than forming the posteroventral corner of the suspensorium as found in Acanthostega (Porro et al. 2015). Skull table. Much of the right side of the skull table is preserved (Fig. 3) and comprises the parietal, postparietal, postfrontal, intertemporal, supratemporal and tabular. On the left, parts of the parietal, postparietal, supratemporal and tabular are preserved. The parietals have separated along the midline and the left has drifted back relative to the right. The bones are thin and incomplete. The thickened area around the pineal is preserved on both sides. Using information from each bone gives a minimum anteroposterior length of 15 mm. The incomplete lateral edge of the parietal is thin and appears to have had a broad overlapping suture with the bones of the temporal series. On the right, the parietal appears to be partially overlying the supratemporal. The incomplete postparietals have separated and drifted backwards. Each is poorly preserved with a fractured dorsal surface and little if any true edge around the bones. The right is the more complete and appears to be approximately square in outline. They are much smaller than the parietals and have an anteroposterior length of c 8 mm. The tabular is well preserved on the right. It is a relatively large, approximately triangular-shaped bone and its surface is ornamented with pits and grooves. The anterior edge, where the tabular meets the supratemporal is straight, the medial edge which contacts the midline bones is convex, the posterolateral edge is slightly concave and extends well beyond the posterolateral corner to produce a prominent tabular horn. There is no evidence of sutural contact with the squamosal, the lateral edge of the bone is gently rounded and smooth. The areas of sutural contact with the midline bones are very clear. Along most of the medial edge there is a shallow step down from the external surface to a broad area of ridges and grooves that would have been overlapped by the midline bones. At the posteromedial corner the tabular is thickened and the ridges and grooves form a sloping shelf which extends around on to the posterior edge. This posterior shelf probably marks the area of contact with the postparietal, while the broad flat area probably formed the suture with the overlying parietal. This arrangement suggests that Ossirarus had a tabular-parietal suture and is the earliest record of this feature in early tetrapods. The incomplete left tabular shows part of the tabular horn and the two discrete areas of sutural overlap along the medial edge. The pattern of ornament is similar to that on the right. The supratemporal is well preserved on the right. It has separated slightly from the tabular and is partially overlapped by the parietal. It is incomplete anteriorly where it meets the intertemporal. The external surface is smooth. The posterior part of the lateral edge is gently rounded and shows no evidence of sutural contact with the squamosal. The anterior part is incomplete. As on the tabular, the exposed part of the medial edge bears a shallow step down from the external surface to a broad area of ridges and grooves that would have been overlapped by the parietal. On the left, the supratemporal has separated from the tabular. It is incomplete anteriorly and damaged along the lateral edge, but the straight, butt suture with the tabular is preserved posteriorly, together with an area of ridges and grooves on the medial edge originally overlapped by the right parietal. The intertemporal appears to be present on the right between the supratemporal and postorbital and partially overlain posteriorly by the parietal. It is a relatively long bone, c. 9 mm is exposed, but its width cannot be determined, because of the overlying parietal. It is incomplete with damaged edges. There appears to be a small area of sutural overlap ridges and grooves at the anterior tip of the bone. The posterior part of the postfrontal is prese