Sunday, August 19, 2012

The Lossiemouth Sandstone Formation of Scotland

            The yellowish sandstone of the Lossiemouth Sandstone Formation can be found on the coast near Elgin, in the Moray council area, north east of Scotland (Fig 1). Fossils were collected there in different quarries, around the town of Lossiemouth, at the end of the 19th and beginning of the 20th century, notably by the ardent collector William Taylor of Lhanbryde (1849-1921). The sand from the sandstone was probably carried by wind and deposited over a fluvial area, indicating that the location during the Late Triassic was a sand dune desert with narrow strips of lowland vegetation around rivers. The sandstone did not preserve any index fossil such as pollens, plants or invertebrates, and no radiometric dating could be performed, leading to some uncertainties on the exact age of the rocks. However, based on the correlation of the Lossiemouth vertebrate fauna with those of the Maleri Formation of India, the Santa Maria Formation of Brazil, the Ischigualasto Formation of Argentina, a Late Carnian age is generally attributed to the sandstone. The fauna is represented by eight taxa of reptiles, including six archosaurs.

Fig. 1.- Location of the Lossiemouth Sandstone Formation today (left) and during the Late Triassic (right).

Saltopus elginensis Huene, 1910 ("Elgin's hopping foot"), is known from a single badly preserved skeleton. The exact affinity of this small (60 cm long) bipedal predator has been debated for decades. It was at some point classified as an early theropod dinosaur but more recent analysis put it within the dinosauriformes but outside the Dinosauria clade as a sister taxon to them (It means Saltopus is not a dinosaur but one of its closest relatives). Since the skull in the fossil is missing, the diet of this animal is unknown.

Another interesting animal from the Carnian of Scotland is Scleromochlus taylori Woodward, 1907 ("Taylor's hard fulcrum") (Fig 2). This rather strange 1.8 m long creature looks a bit like a lizard on long slender legs. It was related or even has been considered ancestral to the pterosaurs, a group of archosaurs that will eventually  conquer the air. Several skeletons of this animal are known.

Fig 2.- Reconstruction of Scleromochlus taylori.

The 4 meter long and probable top predator of its time, Ornithosuchus longidens ("long-toothed bird crocodile") (Huxley, 1877) (Fig. 3) belongs to another group of facultative bipedal carnivorous reptiles distantly related to crocodiles, the Ornithosuchians. They were equipped with sharp teeth and a row of bony scales (osteoderms) along their back and tail. Ornithosuchus was once thought to be ancestral to the theropod dinosaurs, but details of its skeletal anatomy such as the braincase and the configuration of the ankle show that it was a Crurotarsi like the Rauisuchians, the Aetosaurs, crocodiles and phytosaurs and not an Avemetatarsalia (Dinosaurs and Pterosaurs).

Fig. 3- Reconstruction of Ornithosuchus longidens.

Erpetosuchus granti Newton, 1894 ("Grant's Snake Crocodile") was a small (60 cm long) agile quadrupedal predator that might have hunted small lizards and amphibians (Fig 4). Erpetosuchus was the Lossiemouth fossil the most closely related to modern crocodiles and is known from at least four specimens. Considering the location of Scotland at that time (Fig 1), it is no surprise that an additional fossil referable to the same animal was discovered in the New Haven Formation of Connecticut, United States.

Fig. 4.- Reconstruction of Erpetosuchus granti.

Herbivores are represented by several groups of reptiles. The rhynchosaurs had stocky bodies with a broad skull and a powerful beak. They may have fed on tough vegetation, such as the seed ferns which were abundant at that time. In Europe, Rhynchosaurs were represented by the 1.3 meter long  Hyperodapedon gordoni Huxley, 1859 ("Gordon's best pestle tooth") from Scotland (Fig. 5). It is known from at least 35 individuals, making it the most abundant vertebrate fossils of the formation. The fossils came into various sizes, reflecting ages, that can be grouped into two general types (morphotypes) possibly indicating sexual dimorphism. The genus Hyperodapedon had a worldwide distribution, with several species described from India, Brazil, Argentina. Zimbabwe, Madagascar, Tanzania and the United states. They had therefore been used to correlate different formations across the globe, serving as an "index" fossil.

Fig. -5. - A pair of Hyperodapedon gordoni.

Aetosaurs were heavily armored  archosaurs, with a body covered with plate-like scutes (osteoderms) and spikes. These vegetarian animals were also distantly related to crocodiles and are now thought to have been fully terrestrial animals. The carnian representives in Europe include the genera Stagonolepis and Paratypothorax, with Stagonolepis robertsoni Agassiz, 1884 ("Robertson's pitted scale") being the Lossiemouth species (Fig. 6). It measured about 3 meters in length. The remains of Stagonolepis were originally mistaken for fish scales, thus the generic name.

Fig. 6.- Stagonolepis robertsoni.

Procolophonids were small lizard like creatures belonging to the ancient lineage of reptiles called Parareptilia. By the end of the Triassic they have adopted a vegetarian diet before being wiped out by extinction at the end of the period. Leptopleuron lacertinum Owen, 1851 ("Lizard slender ribs") is a typical procolophonid that measured about 30 cm in length (Fig 7). It might have lived in burrows. This little critter was the subject of a bitter rivalry between famed paleontologists Richard Owen and Gideon Mantell at the end of the 19th century, both wanting to be first to describe the animal. Mantell christened the fossil Telerpeton elginense but Owen was quicker to publish so the name he gave has priority according to the international rules of nomenclature.

Fig. 7.- Reconstruction of Leptopleuron lacertinum.

The rhynchocephalians (and more restrictively, the sphenodontians) were once a successful and diverse group of lizard-like mesozoic reptiles with both aquatic and terrestrial forms. The only modern surviving member of the rhynchocephalians is the tuatara (Sphenodon) from New Zealand, generally presented as a true "living fossil". The Lossiemouth sandstone has yielded the species Brachyrhinodon taylori Huene, 1910 ("Taylor's short nose teeth") (Fig. 8) which was very similar to the tuatara in shape, but smaller, measuring some 25 cm in length, and with probably a similar lifestyle. Both Leptopleuron and Brachyrhinodon are known from numerous specimens.

Fig -8.- Reconstruction of Brachyrhinodon taylori.

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