The fossil bone is recorded in red to purple limestone which is roughly laminated with calcisiltite to calcarenite thin layers, locally rich in organic matter and native sulfur. The sediment surrounding the fossil bone is constituted by a bioclastic packstone of peloids, lumps and thin-shelled bivalves. Bivalves are commonly disarticulated and concave-up in the bed, most of them < 1 cm (Fig. 2A). Microgastropods and coal fragments are also common in the bed with the fossil bone.

The fossil bone is an isolated vertebral arch without centrum (Figs. 2b and 2C). The maximum width of the neural arch is 46 mm and the maximum height is 44 mm. The neural arch presents a high and thin neural spine (31 mm high and 4 mm width). The square-sided transverse processes are thick (10 mm high and 21 mm width) and the distal margins are slightly rounded. The transverse processes present areas with spongy bone tissue. The neural canal or vertebral foramen is elevated and relatively circular with 8 mm in diameter. The development of the transverse apophysis sensu Alafont (1992) (maximum width of neural arch / height of neural canal) is 5.76, whereas the development of the neural canal (neural canal height*100 / neural arch height) is 18.26. Since the studied specimen is a section in a fracture surface, it is not possible to study the morphology of the zygosphene-zygantrum articulation.

The upper part of the left transverse process presents a semicircular fracture with around 1 cm in diameter (Fig. 2B and C). Other minor fractures are evidenced in other places of the transverse processes and the neural spine. The spongy bone tissue present infilling by iron oxides whereas pyrolusite is locally observable at the surface (Fig. 2D).

According to the geochemical compositional maps (Fig. 3), the bone is preserved as calcium phosphate. The Sr content is especially high in the bone and is absent in the surrounding sediment which is enriched in Si, Fe, Ba and Cu in respect to the bone.

The isolated neural arch is attributed to a sauropterygian as evidenced by the morphology of the thin and elongated neural spine with relatively reduced development of the square-sided transverse processes. These features allow to exclude the assignation to phytosaurs, because this group exhibit more robust vertebrae with thicker neural spines for supporting dorsal armor plates and thick and large transverse processes (Case, 1932; Lucas et al., 2002; Witzmann et al., 2014). Other typical aquatic diapsid reptiles of Late Triassic (Nicholls, 1999; Rieppel et al., 2000), the thalattosaurians, are also excluded due to the characteristic small neural canal of vertebrae in spite of the neural archs of thalattosaurs are of moderate height, almost in the Superfamily Askeptosauroidea (Müller, 2005).

The development of the transverse apophysis shows a ratio of 5.76 whereas this value is around 7.3 in the Family Pachypleurosauridae and ranges between 3.7 to more than 8.5 in Placodontia (Alafont, 1992). Transverse processes of the dorsal vertebrae are specially elongated in placodonts such as Placodus gigas (Rieppel, 2000). This ratio in the dorsal vertebrae documented by Segesdi & Osi (2021) ranges from 5.2 to 6.4 for Nothosaurus and is around 5 for simosaurids.

The development of the neural canal in the studied vertebra is 18.26, whereas this ratio is 27.8 in pachypleurosaurids and from 25 to 31 in placodontids (Alafont, 1992). According to the vertebrae reported by Segesdi & Osi (2021), the development of the neural canal is lower in Nothosaurus (10.9 to 12.9) and in simosaurids (8.6). However, the values proposed by Alafont (1992) comparing the development of transverse processes and the development of the neural canal, are variable along the postcranial skeleton from cervical to caudal vertebrae. Therefore, the taxonomic assignation of the vertebra within Sauropterygia is complicated. Pachypleurosaurids can be excluded because their size was much smaller than the studied specimen. In fact, this specimen was relatively large due to the size of the neural arch, probably reaching more than 2 m in length. But an adult specimen of Nothosaurus presents larger dorsal vertebrae than the studied specimen, reaching more than 8 cm height for the neural spine and 6 cm width for the neural arch (Segesdi & Osi, 2021). Therefore, the studied neural arch could correspond to a caudal vertebrae of an adult Nothosaurus.

Nevertheless, considering the age of the studied fossil remain (Upper Triassic, probably Rhaetian), the fossil record of sauropterygians (Rieppel, 1999), and the features of the neural arch, the studied remain would also correspond to a cyamodontoid placodont such as Henodus or Psephoderma. Most of the sauropterygians, included other placodonts, nothosauroids and pistosauroids, are mainly restricted to Middle Triassic (see Rieppel, 1999). Henodus was recorded from the Algarve as Carnian (Campos & Mateus, 2018; Ruciński, 2020) and only Psephoderma has been reported from Rhaetian (Rieppel, 1999). Recently, a new placodont, Parahenodus, has been described from the Upper Triassic (Carnian-Norian) of Spain (de Miguel Chaves et al., 2018). Although this genus has been described from a unique partial skull (vertebrae have not been recorded), it could also be considered as a potential candidate for the taxonomic assignation of the studied remain. If this assignation to Superfamily Cyamodontoidea is accepted, this would be a neural arch from a caudal vertebra and not from a dorsal vertebra of cyamodontoids. The dorsal vertebrae of cyamondontoids present characteristically elongated, broaded and curved transverse processes, and these features are not observed in the studied neural arch of Ayamonte. However, the neural canal of cyamodontoids is relatively high and narrow (Rieppel, 2000) distinct to the studied specimen and the taxonomic uncertainty persists.

The characteristic compact vertebrae of sauropterygians are usually well preserved but the studied specimen present numerous fractures. The typically high neural spines of the sauropterygians are commonly well preserved but commonly disarticulated respect to the centrum, as occurs in the studied vertebra. The disarticulation of centra and neural arches is typical in bonebeds (Reolid et al., 2014). The main distortions in the vertebrae are fractures in neural spines and transverse processes as here reported. The fossil vertebra is isolated and other fossil bones have not been recorded, which is an evidence of transport and dispersion of bone remains by currents and potentially scavengers. Fragmentation of the bone points to a more or less extensive bioestratinomic exposure of the bone previous to the definitive burial.

The main composition of specimen (calcium phosphate) does not seem to have been modified during diagenesis. However, the observed enrichment in Sr could be interpreted as related to the replacement of Ca in bioapatite (hydroxylapatite) via co-precipitation or adsorption. Sr is commonly incorporated into bones as a biogenic trace element (e.g. Bocherens et al., 1994; Gilbert et al., 1994; Silen & Sealy, 1995; Keenan et al., 2016; Rey et al., 2022). However, since Ba and Sr are incorporated by living organisms in the same way, and the Ba content in the studied bone is not remarkable (Fig. 3), probably part of the Sr has a diagenetic origin (Tuken et al., 2008). Sr-rich fluids may also have leached from sulfate-rich Triassic deposits or from doleritic volcanic rocks (Wey et al., 2022). Therefore, Sr was very likely incorporated via adsorption in bioapatite during diagenesis.

On the other hand, the presence of iron and manganese oxides must be related to diagenetic processes. Iron oxides are infilling small voids of the spongy bone tissue and manganese oxides (pyrolusite) are located on the bone surface but not within the bone. The parallel distribution of Fe and Cu in the spongy bone tissue could be related to the presence of original sulphides (Tusken et al., 2008) related to microbial activity associated with decay of the soft tissues (e.g. Vietti et al., 2015; Pesquero et al., 2015; Domenech-Carbo et al., 2016).

Most of the specimens of sauropterygians recorded in Iberia are Middle Triassic in age (see de Miguel Chaves et al., 2020), but this specimen was recovered from the less common Upper Triassic record. In addition, this specimen, together with the placodont remains of the Algarve Basin (Campos & Mateus, 2018; Ruciński, 2020), constitute the westernmost record of Triassic sauropterygians in the Tethys. The detrital red beds (silts and sands) of the Silves Group represent the earliest phase of sedimentation related to the initial rifting of Pangaea, dated as early Carnian (Vilas-Boas et al., 2022). The carbonate succession, dated as Rhaetian (Santos et al., 2022), represents the flooding of the Algarve Basin and the colonization by mollusks (bivalves and gastropods) and marine vertebrates such as phytosaurs (Mateus et al., 2014), the placodont Henodus (Campos & Mateus, 2018; Ruciński, 2020) and the here studied sauropterygian of Ayamonte. The presence of dolerite rocks in the succession is congruent with the extensional phase of the rifting.