Gigantic rhizodonts in Scotland’s lochs: The one that got away

James O’Donoghue (UK)

Every angler dreams of reeling in a prize catch – a 40lb pike perhaps, or a whopper of a salmon. Record-breaking fish fire the imagination as few other creatures can, and the lochs of Scotland have inspired many a fishy tale. However, even the tallest of these stories pale into insignificance when compared with the primeval occupants of the lochs.

Had you cast a line there 340 million year ago, you could have ended up as bait yourself. For Scotland’s ancient lakes and rivers held a behemoth of a fish known as Rhizodus hibberti (Fig. 2), which notched up a truly staggering snout-to-tail length of seven metres. It was the ultimate ‘one that got away’, a predator that was half as big again as a great white shark. To this day, it remains the largest freshwater fish ever to have lived.

Rhizodonts, the group of fishes to which R. hibberti belonged, may have been the last truly gigantic predators to live in fresh water, suggests palaeontologist Jon Jeffery, an expert on one of the most widely distributed species, Strepsodus. They also have the distinction of being the most primitive ‘tetrapodomorphs’ known. That is, they belong to the group of fishes from which tetrapods descended. Tetrapods are vertebrates that colonised land and includes all amphibians, reptiles, birds and mammals.

Even though rhizodonts were among the very first fossil fishes known to science, they were poorly understood until recently. A jaw fragment was described in 1793, as were various scales, teeth and bones in the early nineteenth century. In spite of these discoveries, taxonomic confusion was the order of the day, owing to the poor preservation of the fossils. It wasn’t until 1985 that Mahala Andrews, formerly of the Royal Scottish Museum in Edinburgh, described a complete rhizodont for the first time. This 350 to 360 million years old fossil of a 35cm juvenile Strepsodus from Foulden (near the Scottish border with England) was unique in not having broken apart after death, unlike every other specimen found up until then. The Foulden site produced two distinct sizes of rhizodont, with tiddlers of up to 50cm and others measuring 1.5m to 3.5m. Since the two sizes are mainly from different fossil beds, it is likely they are juvenile and adult forms of the same species, with the juveniles shoaling in shallow water, safe from predation by the larger fish.

The next major breakthrough came from the other side of the world, with the discovery in Australia in 1998 of the 360 to 370myrs old primitive rhizodont, Gooloogongia. This find led Zerina Johanson of the Australian Museum in Sydney and Per Ahlberg of the Natural History Museum in London to conclude that rhizodonts were the most primitive members of the tetrapod line. Other species are now known from all over the world including Turkey, the United States and even Antarctica. However, it is the Scottish forms, especially those from around Edinburgh, that are best known.

Eight different types of rhizodont are known from Devonian and Carboniferous rocks that date from 380 to 310 million years ago. Although Gooloogongia measured just 80cm, most rhizodonts were exceptionally large and grew to between three and seven metres in length, with the biggest individuals weighing as much as 2,000kg. It is a mystery why they were so gigantic compared to their prey or, indeed, to any other creatures known from this time.

The heyday for rhizodonts was the Carboniferous period between about 350 and 310mya. At that time Scotland was located just north of the equator and was covered with rivers and lakes surrounded by lush tropical rainforests. Early amphibians, weighing up to 200kg and reaching a couple of metres in length, may have been prey for rhizodonts, as might some of the largest arthropods ever to have lived, such as the 2.5m eurypterid, Cyrtoctenus. In the lakes, lungfishes, coelacanths and freshwater sharks were likely to have been eaten. Three species of rhizodont overlapped at this time, with the 4.5 to 7m-long Rhizodus hibberti and the two metre Screbinodus ornatus sharing the same deep lakes. In shallower and more vegetation-choked water was Strepsodus sauroides, which grew to five metres , although it is best known in its smaller juvenile form.

Rhizodonts had elongated bodies powered by great shoulder paddles, while their other fins were reduced in size, giving them a somewhat eel-like appearance. There is nothing remotely like them in the modern world, although crocodiles provide some clues as to their lifestyle. Like the crocodile, they were not built for chasing prey over long distances. Instead, they would have lain in wait and ambushed their next meal in a frenzied burst of violence.

Their formidable dental armoury shows how well adapted they were for grabbing big, slippery creatures. Two rows of teeth ran side by side in each jaw – a complete row of small teeth running alongside widely spaced and much larger fangs. This arrangement was sufficiently unusual to be analysed by Chris Burgoyne, a structural engineer at Cambridge University. As Chris wrote in 2004:

So when our beast gripped its prey the long teeth would have exerted the first pressure, causing torsion in the jawbone, which twisted inwards, gripping the victim and bringing the second rows of teeth into action,” “They were supremely well adapted for their environment.”

Most fearsome of all, however, were their huge, deeply rooted tusks (Fig. 3). Measuring up to 22cm in R. hibberti, these are among the largest carnivore teeth ever known and would have plunged deep into any creature that came within striking range. Having secured its prey, the rhizodont would have finished the job off by shaking vigorously from side to side, in much the same way as a crocodile does. Its large, fan-shaped shoulder paddles lifted its upper body off the substrate to gain greater force to dismember its kill into manageable chunks for swallowing.

It is these shoulder paddles that have attracted great interest from evolutionary biologists studying the origin of tetrapods. In 1997, Philadelphia-based scientists Edward Daeschler and Neil Shubin wrote in Nature about the discovery of a rhizodont paddle that is remarkably similar to the tetrapod limb: “The presence of digit-like structures [similar to those seen in tetrapods] in the paddle of an aquatic fish suggests that digits could have evolved for reasons other than bearing weight during terrestrial locomotion.”

The lift required of the shoulder paddles to shake prey to pieces may have been the reason sturdy tetrapod limbs evolved in the first place. Jeffery argues that these strengthened paddles could then have allowed rhizodonts to crawl onto land. “The deep set, ball-shaped rhizodont shoulder paddle socket was designed to take a lot of stress and was very mobile. Plus, you have a massive shoulder girdle for support,” says Jeffery. “I wonder if this would have allowed it to hump itself over land, although it wouldn’t have been especially agile.”

Why would rhizodonts have wanted to travel onto land? One suggestion is they lunged after land animals drinking at the water’s edge, just as crocodiles do now. But their very survival might also have rested on their ability to leave the water at certain times of the year. They lived in seasonally flooded tropics and what was a huge lake during one season may have later dried out into widely separated, small lakes. Being able to haul yourself several kilometres from one lake to another could have made the difference between life and death. What’s more, they would have needed to be able to breathe air in the same way that many very large fishes do today, such as the two metre Arapaima, which is found in the Amazon river.

It seems that the evolutionary fate of the rhizodonts was inextricably linked to the emergence of tetrapods on land. As large tetrapods evolved from being water-loving amphibians into reptiles that were free of the lakes and rivers, so the rhizodonts’ food sources dried up. “Rhizodonts were the last big fishy top predators,” says Jeffery. “As the biomass moved onto land proper and you no longer got big tetrapods living in the water, so you no longer got the big rhizodonts to prey on them.”

Which may be just as well, for Scotland’s anglers at least. With fish as big as rhizodonts around, there really wouldn’t be any need for tall stories about the one that got away.

Acknowledgements

I am grateful to Dr Jon Jeffery for the interview and for commenting on this feature, to Dr Neil Clark of the Hunterian Museum for the use of the photographs and to Megan Whatley for producing the illustration (which was inspired by a drawing by Dr Mike Coates), and to Megan Whatley and Dr Neil Clark for their generous contributions to this feature. The illustration was inspired by a drawing by Dr Mike Coates.

References

Andrews, S.M. 1985. Rhizodont crossopterygian fish from the Dinantian of Foulden, Berwickshire, Scotland, with a re-evaluation of this group. Transactions of the Royal Society of Edinburgh: Earth Sciences 76: 67-95.

Burgoyne, C. 17 February 2004. Structures have relevance to biomedical research. The Structural Engineer: 13-14.

Daeschler, E.B & N. Shubin. 1997. Fish with fingers? Nature 391: 133.

Johanson, Z. & P.E. Ahlberg. 1998. A complete primitive rhizodont from Australia. Nature 394:569-572.