This work reports the first record of a sauropterygian reptile remain from the uppermost Triassic of the westernmost part of the South Iberian Palaeomargin. The fossil bone, found in the Upper Triassic carbonate succession of Ayamonte (Huelva, Spain), corresponds to a neural arch of a sauropterygian. The carbonate succession was deposited in very shallow marine environment simultaneous with igneous activity during the Rhaetian (latest Triassic). The studied remain is isolated, disarticulated and presents fractures that evidence transport but also potential activity of scavengers. The neural arch is preserved as calcium phosphate enriched in some elements (e.g. Sr) relative to the surrounding carbonate sediment. The anatomic features do not allow a detailed taxonomic identification. This remain extends the record of sauropterygians to the westernmost end of the Tethys following the South Iberian Palaeomargin and evidences the colonization of the Algarve Basin during the extensional phase related with the progress of the rifting of Pangaea and the opening of the Tethys to the west.
Este trabajo se centra en el estudio del primer registro de un resto fósil de sauropterigio procedente del Triásico superior del sector oriental del Paleomargen Suribérico. El resto, encontrado en la sucesión sedimentaria carbonatada del Triásico que aflora en Ayamonte (Huelva, España), corresponde a un arco neural de un sauropterigio. La sucesión carbonatada se depositó en un ambiente marino muy somero simultáneamente a cierta actividad ígnea durante el Rhaetiense (Triásico superior). El resto fósil aparece aislado, desarticulado y presenta algunas fracturas que evidencian cierto transporte por corrientes, sin descartar la posible interacción de organismos carroñeros. El arco neural está conservado como fosfato cálcico con enriquecimiento en Sr, de acuerdo con los mapeos composicionales realizados sobre el resto fósil y el sedimento circundante. Los rasgos anatómicos no han permitido su identificación taxonómica detallada. El resto estudiado extiende el registro de reptiles sauropterigios hacia el occidente colonizando el Paleomargen Suribérico, en este caso la Cuenca del Algarve, conforme se producía la rotura (
Marine reptiles were diverse and abundant in the Triassic marine ecosystems of Western Tethys, both in the Germanic and in the Alpine basins, forming part of the Germanic Bioprovince and the Tethyan Bioprovince, as compiled in
The Germanic facies also developed along the Iberian Palaeomargin, today outcropping in the Algarve Basin, the Betic External Zones, the Iberian Range, the Catalonian Coastal Range and the Pyrenean Cordillera. The record of marine reptiles in these areas is also relatively rich but most of the reported remains consist of isolated elements (e.g.
In the southwestern part of the Iberian Palaeomargin the record of marine reptiles is comparatively scarce. Only at the Algarve Basin (South Portugal), in the Carnian Grés de Silves Group, fossil vertebrates have been reported from the Rocha da Pena bonebed with numerous temnospondyl remains of
The objective of this short note is reporting the first record of an isolated vertebra of sauropterygians from the Upper Triassic of the eastern part of the Algarve Basin in Ayamonte (Huelva province, Southwestern Spain).
The studied remain was recorded in the Triassic deposits that outcrops in the east side of the Guadiana River, close to Ayamonte town, and more exactly in the hill of the Parador Nacional (coord. 37º13´32´´N, 7º24´26´´W) (
A. Geological map of southwestern Iberia with indication of Ayamonte Town. B. Geological map of the surroundings of the Ayamonte (Huelva) with location of the outcrop with fossil remain (modified from
In Ayamonte, the Triassic is unconformably overlying the shales of the Lower Carboniferous. The lower part of the Triassic succession (around 15 m thick) is constituted by red siltstones and sandstones with gypsum and some microconglomerate intervals. These deposits were interpreted as related to continental or coastal deposits (
The upper part of the Triassic sedimentary succession (around 26 m thick) is constituted by a carbonate interval composed by marlstones, marly-limestones, limestones and dolostones interbedded with basic volcanic rocks (dolerites) and volcano-sedimentary deposits to the top (see detailed description in
According to the presence of trace fossils (
The studied remain appeared in the broken surface of a red limestone block and it is constituted by two halves identified as NOT/AY/001a and NOT/AY/001b. Polished slabs and thin sections of the red limestone block have been prepared and the microfacies being analysed with an Olympus SZ60 microscope at the Universidad de Jaén.
The fossil bone and hosting rock were scanned at the Universidad de Jaén using a Bruker XR-microfluorescence M4 Tornado equipped with a rhodium target X-ray tube with a high voltage of 50 kV, a current of 600 μA and pressure of 20 mbar. The spotsize of the X-ray optics was 25 μm. The maximum penetration depth from which fluorescence X-rays can still reach the detector is less than 20 μm. This low penetration allowed analyses of the sediment surface showing lateral compositional changes, especially the contrast between the fossil bone and the surrounding sedimentary rock. The geochemical compositional maps obtained for each element are represented by a range of colour intensity that indicates the relative concentration of each element.
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 (
The fossil bone is an isolated vertebral arch without centrum (
The upper part of the left transverse process presents a semicircular fracture with around 1 cm in diameter (
According to the geochemical compositional maps (
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 (
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 (
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 (
Nevertheless, considering the age of the studied fossil remain (Upper Triassic, probably Rhaetian), the fossil record of sauropterygians (
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 (
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.
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 (
Most of the specimens of sauropterygians recorded in Iberia are Middle Triassic in age (see
The studied remain corresponds to a neural arch of an Upper Triassic sauropterygian deposited in a very shallow marine environment. The studied remain is isolated, disarticulated and presents fractures, that evidence transport but also potential activity of scavengers. The neural arch is preserved as calcium phosphate and some elements such as Sr, are especially rich compared with the surrounding carbonate sediment; fact that could probably be related to substitution of Ca by Sr during the diagenesis. Although the anatomic features do not allow a more accurate taxonomic identification, the studied remain probably corresponds to the neural arch of a caudal cyamodontoid vertebra. This remain extends the record of sauropterygians to the westernmost end of the Tethys and evidences the colonization of the Algarve Basin during the extensional phase related with the progress of the rifting of Pangaea and the opening of the Tethys to the west.
Authors would like to thank Ramón Martín, Miguel Ángel Bernal and Benjamín Cabaco for informing us of the discovery of the fossil bone. Financial support through the projects PY20_00111 and RNM-200 Research Group (Junta de Andalucía, Spain) and PID2019-104625RB-100 (Spanish Govern) is gratefully acknowledged. Technical and human support provided by Centro de Instrumentación Científico-Técnica (CICT) of the University of Jaén is gratefully acknowledged. The authors thank the constructive comments of the reviewers Carlos de Miguel Chaves and Torsten Scheyer.