Sharks of Whale Valley: Or should that be whales of Shark Valley?
Dr Charlie Underwood (UK)
Leaving behind the noise and pollution of Cairo, the drive across the monotonous buff desert comes almost as a relief. After passing through the lush farmlands of the Fayum Oasis and back out onto the desert plains, the first sign of the fossils to come is unexpected and indicated by the desert surface changing from pale brown to silver-grey. Looking closer at the shiny silver desert surface, fossils became visible, being millions of giant nummulite foraminifera covering the desert surface, each polished to a metallic sheen by millennia of sand blasting. However, it was not forams that we had been invited to study. Rather, it was far larger and more impressive specimens we had come to see and the appearance of dramatic sandstone cliffs on the horizon heralded some of the most extraordinary fossiliferous rocks that I, for one, have ever seen.
The fossils of the Fayom are by no means a recent discovery – they have been the source of vast numbers of important finds for over 120 years. The earliest collections were made during a series of expeditions by Georg Schweinfurth, from 1879 to 1886, and it was during these that the first fossil whales were recovered from the Eocene rocks north of Lake Birket Qarun. Despite the success of these expeditions, Schweinfurth never ventured to the western end of the region and never saw the most impressive assemblages of fossil whales.
As the twentieth century dawned, the Fayum became a busy place, with a number of expeditions of German, and then British and American, palaeontologists working in the area. Although the abundant whales of the western Fayum were discovered as early as the 1902/1903 field season, little was collected and the focus remained on the area north of Birket Qarun, where land mammals, such as the early elephant, Moeritherium, and the double-horned, Arsiniotherium, were collected. In 1906, Henry Osborn, of the American Museum of Natural History, re-visited the whale-rich area to the west and coined the term “Zeuglodon Valley” as a result of the abundance of the whale Zeuglodon (now known to be the same as Basilosaurus).
Osborn also noted the problems with working at the site – it was remote (three days camel ride from the other areas they were exploring), arid and the fossils very large. Together, these made collecting very difficult. Therefore, over the succeeding decades, emphasis again focussed on the area north of Birket Qarun, with land mammals and, in particular, early primates being the main target.
It is probably as much down to the invention of four-wheel drive vehicles as the renewed interest in whale evolution that caused the changes in palaeontological interest that opened up Zeuglodon Valley, now renamed Wadi Al-Hitan (Valley of Whales in Arabic), to study. Over the last 30 years, this has been led by Philip Gingerich of the University of Michigan Museum of Paleontology and his co-workers, who have recorded about 300 fossil whales in the area, as well as fossils of seacows, turtles, crocodiles, fish and many others.
As the importance of the site became more evident, so did its accessibility and it became a popular day out from Cairo, often culminating in tourists picking up a couple of whale bones as souvenirs. Therefore, to protect the site, it was proposed as a UNESCO World Heritage Site and gained that status in 2005. It was then incorporated into the far larger Wadi Rayan protected area. With the protection of the site secured, it was developed for public access and many of its fossils are now laid out where they were uncovered (or at least close by), and is now a feature of many tourist excursions from Cairo.
Visiting Wadi Al-Hitan
The public area of the park is dramatic in every sense. Wadi Al-Hitan sits in a broad valley, enclosed by the steep sandstone cliffs of a large and elongate mesa to the southeast and a low escarpment to the northwest. Late Eocene rocks are superbly exposed in the cliffs, buttes and in parts of the valley floor. Once you have made it past the souvenir shop and café, a series of sandy paths wind around the valley, passing the fossils and other points of interest. The majority of the valley floor and walls are composed of the yellow, shallow marine sandstones of the Birket Qarun Formation. These are soft and easily eroded, and numerous small buttes, carved by wind and sandblasted into dramatic shapes, dot the valley.
Towards the top of the cliffs, and capping some of the largest buttes, alternating hard and soft beds can be seen. These comprise the Qasr el-Sagha Formation, which are made up of shelly limestones and interbedded sandstones and mudstones, and represent a more restricted or shallower environment than the Birket Qarun sandstones. Below the Birket Qarun sandstones are mudstones of the Gehannam Formation, although these are rarely seen in the park.
Although generally monotonous, there are two marker beds that can be seen in some exposures of the Birket Qarun Formation. High up, a bed of black mudstone indicates a temporary change to quiet seafloor conditions, while lower down, a hard pale unit, known as the Camp White Layer, forms a surface that the paths follow over much of the site. This hard bed is riddled with conspicuous burrows that were originally considered to be mangrove root traces, and marks a level above which whale and other fossils are especially abundant. The whales themselves vary from complete skeletons (two nearly complete fossils are currently exposed) to piles of desert-weathered bone rubble. Other fossils that can be seen include a huge sawfish rostrum, a turtle and a large log full of shipworm borings, while the ground is commonly covered with small nummulites.
As the paths wind around the site, small domed mud huts offer well-earned places to sit out of the sun and (often) wind. These, along with the buildings near the site entrance, are designed to blend into the landscape and have an architectural style familiar to anyone who is a fan of Star Wars films. The site at Wadi Al-Hitan is evolving all the time and there are plans to have more whales on display in the near future.
Shark-eating whales and whale-eating sharks
There are about five species of whale known from fossils in Wadi Al-Hitan, along with several other species that have been found nearby in overlying and underlying rock units. Of these five, two make up the majority of the fossils that can be seen and have been found. The smaller of these is Durodon atrox (Fig. 4), although this is hardly a tiny animal, reaching about six metres in length. This is known from many skeletons, some very complete, and the finds at Wadi Al-Hitan include both adults and juveniles. Although similar in overall shape to a modern dolphin, this whale still shows many features of its land ancestors, most obviously a robust, superficially wolf-like skull, with the large, pointed teeth of a major predator. Well-preserved skeletons have also revealed small hind limbs – useless for walking, but still large enough to have been visible on the living animal.
Durodon may have been impressive, but its skeletons are dwarfed by those of the other common whale, Basilosaurus isis (Fig, 5). Far larger in all respects than Durodon, Basilosaurus is longer still due to very elongate and cylindrical vertebrae, giving an extremely long vertebral column and contributing to a maximum length of maybe 18m. If the powerful jaws and predator’s teeth made Durodon a powerful hunter, the far greater size and vast slicing molar teeth of Basilosaurus would have made it a force to be reckoned with.
Clearly, these two whales were not the gentle giants of today, but large carnivores, with teeth able to slice through flesh, meaning that they were not restricted to hunting prey that they could swallow whole. Basilosaurus was thus equipped to tackle almost any of the fish, sharks, rays, turtles or seacows that it lived alongside. It may have even fed on other whales – the presence of only adult Basilosaurus alongside many juvenile Durodon may suggest that the great whale hunted young Durodon in their nursery waters.
However, whales were not the only large hunters in the area. Although leaving behind little in the way of skeletons, the teeth of the megatoothed shark Carcharocles sokolowi Fig. 7) indicate that another huge predator was here. These great serrated teeth suggest a shark possibly well in excess of six metres long, with a dentition capable of cutting through flesh and bone. Indeed, many of the teeth that are found show well developed ‘dings’, that is, chips from the tip of the tooth caused by contact with hard bones – something that cannot have happened were it to have been feeding on fish or other sharks. So it appears that the whales were not just hunters, but were likely to have been hunted as well. One has to start feeling sorry for the slow and fat seacows that would have been so tempting a meal for both sets of predators.
The sharks of Wadi Al-Hitan
Despite all of the fossils that are on display around the public part of the site, the commonest, and most diverse, fossil vertebrates in these rocks are not at all conspicuous. There are well in excess of 50 species of sharks (and rays, which are best thought of as flattened sharks) present at Wadi Al-Hitan, but these are not as readily seen as the whales. With skeletons of partly hardened cartilage, sharks rarely fossilise as anything more than teeth, with the rostra of sawfish (which are giant rays) being a notable exception.
Larger sharks’ teeth are very abundant in the whale-bearing rocks and, in places, litter the desert surface, but any examples near the paths are likely to have been picked up (largely illegally) long ago. In addition, most smaller sharks, and the great majority of rays, have teeth that are extremely small and are unlikely to be seen in the field. Therefore, to study the sharks’ teeth of the area, it was considered important to work both outside the public parts of the park and use collecting methods that would ensure that even the smallest teeth would be found. And this is why we came to Wadi Al-Hitan.
With the history and significance of the site, being invited along as a part of a joint project between Michigan and the Egyptian Geological Museum was something I could not turn down. On arriving, it soon became obvious just how abundant shark fossils are. Even within the public park, there are places where teeth are so abundant that it would be impossible to place a saucer on the ground without covering some up. However, moving outside the public part revealed not only areas that have probably never even been prospected for fossils other than whales, but also rock units not seen in the park.
Indeed, one of the main problems with working further afield is the difficulty in working out what rock unit you are looking at, as the rocks of the main escarpment change from yellow sandstone of the typical Birket Qarun Formation to grey mudstones when traced to the northeast. With no real dip or faults, it soon became apparent that this is a lateral change in ancient environments from shallow water sands to deeper water muds. Therefore, it was possible to study the sharks from the deep water mudstones of the Gehannam Formation, the shallow water sandstones of the Birket Qarun Formation, and their deeper water lateral equivalents. In addition, sharks could be collected from both close to whale skeletons and further away (although the latter proved difficult in places where the whale skeletons were very abundant, as it became difficult to find sample sites that did not have a whale skeleton nearby).</p>
The Gehannam Formation typically forms low, undulating ground and, while the large bones of marine mammals often stand proud, shark teeth and other small fossils lie within a veneer of gypsum (from the weathering of pyrite) and desert sand. Teeth of large, midwater predators are common, with the ‘mako’ shark, Macrorhizodus, the thresher shark, Alopias, a sand tiger shark similar to Carcharias and the impressive teeth of the megatoothed shark, Carcharocles, all being common.
Searching on hands and knees, teeth of the whaler sharks, Physogaleus, Rhizoprionodon and Abdounia (all of which will be familiar to anyone who knows the northern European or American Eocene), the lemon shark, Negaprion, and the snaggletooth shark, Hemipristis, are all common. Genera usually considered limited to deep water, such as the sixgill shark, Hexanchus, are also present. In comparison, teeth of rays are rare. Surprisingly, Striatolamia, probably the commonest large shark in most northern Eocene faunas, is absent from this and all the other rock units in the area.
Within the shallower water sandstones of the Birket Qarun Formation, Negaprion becomes the dominant shark, at some sites comprising 90% of the field-collected teeth, while pelagic (that is, not living near the sea floor) sharks become rare. Rostral teeth of several species of sawfish were also found. Sieving for small teeth produced a huge diversity of species, with catsharks, nurse sharks, guitarfish, electric rays and, above all, stingrays all being represented by multiple species. One of the commonest small sharks is a previously unknown genus of weasel shark, with teeth suggesting that it was a specialist predator of octopus. In many shallow tropical seas today, a similar situation is seen, with lemon sharks, stingrays and sawfish being common and other larger sharks far less so.
Following these deposits into deeper water, the sharks of the underlying Gehannam Formation return, while, at the same time, sieving revealed minute species of each of the bullhead shark Heterodontus, sawshark Pristiophorus and houndshark Triakis. The abundant whale skeletons in the Birket Qarun Formation allowed the sharks near these to be compared to those found in the sandstones nearby. Fortuitously, a whale skeleton had recently been excavated and, while being prepared in Michigan, the surrounding matrix became available to sieve for sharks.
What we were expecting was to find lots of teeth of scavengers that would have fed on the whale carcass, but what we really found was very different. Although the species present were seen to be similar to a sample collected close by, the proportions were very different, with species otherwise more common in deeper water being commonly present, with species that feed on small invertebrates, such as the butterfly ray, Gymnura, also being more common. Therefore, it appears that the whale skeleton formed not just a food source as it rotted, but after that formed a weed covered reef on the sea floor, giving cover to deeper-water sharks that would otherwise be exposed to predators on the open shallow seafloor, as well as to the crabs and shrimps that many sharks and rays fed on.
I may be biased, but it seems to me that Whale Valley is a bit of a misnomer, with sharks being more abundant, more diverse and containing more species that have yet to be named. Despite this, I suppose I would have to concede that (to the non-specialist at least), whale skeletons do look more dramatic than sharks’ teeth and that, as mammals, we are (supposed to be) drawn towards our closer relatives, even if these relatives could really be considered as little more than cows trying to be sharks.