From sea to sand – ancient marine reptiles from the deserts of Saudi Arabia

Benjamin Kear (Australia)

The hot, dry deserts of modern Saudi Arabia are not renowned as a source of ancient marine reptile fossils. Indeed, only a few years ago, virtually nothing was known beyond a few unidentified scraps of bone recovered by petroleum geologists searching for oil. However, recent exploration by teams of both Saudi and international palaeontologists have led to some exciting new finds that are helping to piece together the 190 million year long story of marine reptile evolution in the Arabian Peninsula.

What are marine reptiles?

The term ‘marine reptile’ is actually rather ambiguous and does not refer to a specific group. Rather, it applies to any wholly or partly aquatic reptile that makes, or has made, its home in the ocean. Examples of modern marine reptiles include sea turtles (chelonioids), sea snakes (related to terrestrial venomous snakes or elapids), the marine iguana (Amblyrhynchus cristatus) and the salt-water crocodile (Crocodylus porosus). The latter is typically estuarine, but commonly ventures into coastal marine areas.

However, the zenith of marine reptile diversity occurred during the Mesozoic or ‘Age of Dinosaurs’, when in excess of ten major radiations, including representatives of those living today (that is, turtles, snakes, lizards and crocodiles), made the transition to life in the sea. Despite having quite different evolutionary origins, all these groups manifested similar adaptations (albeit specialised to varying degrees) towards moving, feeding and reproducing in an aqueous environment. Key amongst these features were:

• Elongation of the body and deepening of the tail, sometimes with further modification to form a fish-like fin for propulsion.
• Reduction and restructuring of the limbs into flippers for moving or stabilising the body in water.
• Propensity towards giving birth to live young. Although certainly not evident in all marine reptiles (turtles and crocodiles being notable exceptions), this last trait, termed viviparity, appears to have evolved independently in many Mesozoic groups and represents a final break in their reproductive ties to the land.

Marine reptile remains have been found on every continent including Antarctica, and range in age from the Permian Period of the late Palaeozoic (about 250 milion years ago) to today. However, it was during the Triassic Period of the earliest Mesozoic (about 240 milion years ago) that marine reptiles first achieved a global distribution. The Mesozoic marine reptile record is dominated by several principal groups: ichthyosaurs, which were fast-swimming fish-like predators; sauropterygians, an order incorporating turtle-like placodonts, long-necked plesiosaurs and their ancestors the nothosaurs; mosasaurs, essentially giant aquatic monitor lizards; turtles, including chelonioids and other unrelated forms; and crocodiles, represented by both estuarine and specialised marine lineages.

Of all these myriad types, only marine turtles and crocodiles managed to survive the great extinction event at the end of the Cretaceous Period of the Mesozoic (65 million years old), and even proliferated to some degree in the early Cainozoic (that is, during the Paleocene to Eocene of 65 to 50 milion years ago). Nevertheless, when compared to their Mesozoic radiations, neither sea turtles nor marine crocodiles ever again achieved the same peak levels of diversity.

Marine reptile fossils from Saudi Arabia

The sedimentary geological history of Saudi Arabia is one of almost continual marine inundation. This is recorded in a series of fine-grained sandstones, shales and carbonate sediments successively ‘draped’ around the eastern margin of a Proterozoic basement structure called the Arabian Shield. Today, this area is dominated by a dramatic mountainous region known as the Hijaz that extends from central Arabia to the edge of the Red Sea. However, during the Mesozoic, this same crustal block formed a north-eastern portion of the Gondwanan supercontinent and bordered the vast Tethys Ocean, stretching from the equator to the South Pole.

The warm northern coastal margins of the Tethys seem to have provided rich habitats for marine reptiles. Among the oldest evidence of these is found in the Jilh Formation of Saudi Arabia. This mid Triassic (235 million years old) tidal flat deposit crops out in a series of escarpments running for over 1,000km from just east of the northern city of Ha’il to As Sulayyil in the south. The Jilh Formation produces mainly isolated bones and teeth derived from skeletons broken up by current and/or wave action. Almost all of the remains belong to early sauropterygians, mainly placodonts and nothosaurs. Placodonts were bizarre, turtle-like animals with a protective ‘shell’ composed of bony ossicles and a mouth full of domed teeth used for crushing hard-shelled invertebrates.

The limbs of placodonts formed rudimentary paddles and were probably used for moving along the bottom rather than active swimming. In contrast to placodonts, nothosaurs were adapted for catching mobile prey in the open water. Like their more famous descendents the plesiosaurs, nothosaurs had long necks, fang-like teeth, and four propulsive paddles. However, lateral undulations of the body and tail provided the primary means of locomotion. Nothosaurs came in a variety of body sizes, ranging from less than 20cm to over 4m in length (a large species of Nothosaurus with teeth over 4cm high is found in the Jilh Formation). It follows that their prey preferences must have been diverse, comprising everything from small fish and cephalopods right up to other marine reptiles.

In addition to sauropterygians, ichthyosaur bones have also been recovered from the Jilh Formation. Ichthyosaurs were fast swimming pursuit predators equipped with a streamlined body form, stabilising dorsal fin and fish-like tailfin. Their large heads had enormous eyes, suggesting a visual hunter. Narrow jaws and numerous sharp teeth imply a diet of fish and cephalopods. Many Triassic ichthyosaurs were small, including the Jilh Formation specimens, which are mixosaurids less than 1m long. However, other Triassic ichthyosaurs are known to have been giants, reaching over 20m in length and going down in the record books as the largest marine reptiles of all time.

Jurassic marine reptile fossils are very rare in Saudi Arabia and currently include only a single, unconfirmed report of marine crocodile vertebrae from near the Yemeni border. Conversely, younger Cretaceous assemblages are much better represented and show that highly diverse faunas once occupied late Mesozoic seas in the Arabian region. Two Cretaceous units have so far yielded identifiable remains: the Adaffa Formation (74 million years old), which crops out north of Duba on the Red Sea coast; and the Jalamid Formation (65 million years old), a phosphorite-rich deposit exposed on the northern border with Jordan. Unfortunately, the Jalamid Formation material has yet to be formally described, but is known to include mosasaurs, plesiosaurs and marine turtles. These ubiquitous, Late Cretaceous forms also occur in the Adaffa Formation, alongside small aquatic lizards and marine crocodiles.

Saudi Arabia’s mosasaurs are epitomised by Prognathodon, an enormous 11m long predator with spike-like teeth up to 8cm long. Mosasaur skeletons from elsewhere in the world show that Prognathodon had short paddle-like limbs and a long, deep tail that was probably used for undulatory swimming much like that of a crocodile. Mosasaurs are thought to be close relatives of modern day snakes and monitors. Interestingly, a small, aquatic, monitor-like lizard called Pachyvaranus is also known from the Adaffa Formation.

Plesiosaur fossils are generally scarce in Saudi Arabia, but include remains attributable to elasmosaurs. These quintessential plesiosaurs are typified by the popular image of the Loch Ness monster, with immensely long necks, small heads and a mesh-work of teeth that would have functioned as an effective fish trap. Elasmosaurs swam using a complex system of four propulsive flippers and may have grown to over 11m in length. However, the Adaffa Formation specimens seem to represent a smaller-bodied species (only around 6m long) that would have constituted viable prey for large mosasaurs like Prognathodon.

The record of Cretaceous marine turtles in Saudi Arabia includes limb and shell fragments (some suggesting carapace lengths of over 1m) of bothremydids. These extinct turtles were more closely related to modern freshwater species rather than true sea turtles (chelonioids), and enjoyed a particularly diverse radiation throughout the Afro-Arabian region in the Late Cretaceous. Morphologically, bothremydids were characterised by the retention of weight-bearing limbs rather than flippers and, therefore, probably had a seafloor-oriented lifestyle. This is consistent with the group’s distribution throughout mainly coastal palaeoenvironments.

Also particularly common in Late Cretaceous Afro-Arabia were marine dyrosaurid crocodilians.

These long-snouted fish-eaters had deep, laterally flattened tails for propelling themselves through the water. However, the group is most notable for managing to survive the mass extinction event at the end of the Cretaceous (65 million years old). Indeed, well-preserved dyrosaurid fossils have been found in early Tertiary deposits (about 56 million years old) of the Umm Himar Formation to the east of the Islamic holy city of Makkah. The eventual extinction of dyrosaurids may be linked to the explosive diversification of primitive whales around 50 million years old (during the Eocene of the early Tertiary).

Perhaps, these two groups competed for comparable ecological niches. Whatever the cause, after their disappearance, the only remaining large-bodied marine reptiles seem to have been sea turtles. One of these, a primitive leatherback Arabemys, has been found in the early Tertiary (about 56 million years old) Aruma Formation near Turaybah (east of Makkah). The last great reptilian marine predator, the salt water crocodile is unrelated to dyrosaurids, but does provide a faint glimpse of what these highly successful ancient animals must once have looked like.

Ben Kear is a vertebrate palaeontologist with La Trobe University in Melbourne, Australia and Research Associate of the South Australian Museum in Adelaide.