The Cryptic Dwarf Crocodiles of Africa

 

A dwarf crocodile (Osteolaemus tetraspis) at the Wildlife Adventure Zoo, Emmen, Netherlands. By Eric de Redelijkheid, retrieved via Wikimedia Commons. CC BY-SA 2.0

In the winter of 2014, I was lucky enough to make a getaway from the cold clutches of an Ohio February to visit northeastern Florida. While there, we visited the historic city of St. Augustine. We did visit the famous lighthouse, but I had eyes for only one attraction: the St. Augustine Alligator Farm. Spanning more than 7 acres, the park hosts many animal species, including all 24 of the world's crocodilians! Tantalized by the opportunity to not only indulge in my love for crocodilians but to check them all off my list in one go, I paid the park a visit.

It was there that I first saw the dwarf crocodile, although my recollection is dim; I think I was more interested in seeing the gharials. It is easy to see how Osteolaemus, the world's smallest crocodilian, could escape the eye. If I'd had some idea of just how fascinating this animal really is, I might have paid it more mind. Although I did not know it at the time, it also doomed my ambition (at least in the short term) to see every crocodilian species. Even though this goal seems achievable in comparison to that other archosaur group (birds, whose literal thousands of representatives one would need several lifetimes to really check off), nothing is ever that simple. More on that further below; let's discuss the intriguing lifestyles of these animals first.

Watch the Eyeshine

O. tetraspis (illustrated as "Crocodylus frontatus", a junior synonym) by G. H. Ford for the Zoological Society of London, 1862. Retrieved from Wikimedia Commons. In the public domain.

Before discussing what we know about these animals, it behooves us to clarify that there are two recognized species of Osteolaemus. The type is O. tetraspis (described in 1861 by Edward Drinker Cope, with whom dinosaur buffs will be familiar), best known from Cameroon, Gabon, and Nigeria, and the other is O. osborni (named in 1918 after Henry Fairfield Osborn, with whom dinosaur buffs will also be familiar), which is native to the Congo Basin. For a long time, there was considerable doubt over whether the latter was distinct from the former - for the better part of a century, in fact. Today, some workers still treat them as subspecies. The common name of O. tetraspis is simply the dwarf crocodile, though a few alternatives exist. The latter is sometimes called Osborn's dwarf crocodile. I'd personally prefer to use the name dwarf crocodile for the genus collectively, but such are the cards I have been dealt.

Because of this taxonomic history, I must go into this section with an apology beforehand. O. osborni is comparatively less well studied, and so most of the information we have about the lifestyles and ecology of dwarf crocodiles comes exclusively from O. tetraspis. Although they are very similar animals, we should not assume that every detail below is actually the same for both, but alas, it is the best that we can do for now.

Visually, both species are difficult to distinguish. They're both small, large-eyed animals, and, dare I say, quite cute. O. osborni is smaller on average, never exceeding 4 ft (1.2 m) in length, while O. tetraspis can be up to 6.2 ft (1.9 m). As with all crocodilians, they are well-protected by bony scutes which cover most of their body. Among all the crocodilians, the dwarf crocodiles are among the most terrestrial. They are often found moving at night through the wet forests they call home, opportunistically hunting prey including crustaceans (their favorites), fish, frogs, and various invertebrates. In a few instances, plant matter has been found in dwarf crocodile stomachs, but it is probably a result of accidental ingestion in most cases. However, it has been suggested that dwarf crocodiles may ingest palm nuts on purpose, using their hard shells as gastroliths. During the dry season, food is scarce throughout most of their range. Fortunately for the dwarf crocodiles, they, like most crocodilians, can generally endure long periods of poor food accessibility until the rains return.

The historical range of this genus is a wide one, spanning most of Western and Central Africa from the Gambia to northern Angola, and east across the Congo Basin to the border with Uganda. Anecdotal evidence from hunters suggests that dwarf crocodiles do not all nest at the same time. At any old time of the year, they will heap dead leaves and other plant matter against a tree trunk and lay between 10 and 14 eggs in the resulting mound.

A dead dwarf crocodile and monkey by a roadside in Gabon. Photographed by Wikimedia Commons user Barada-nikto. CC BY-SA 4.0

Osteolaemus was last assessed by the IUCN (as a single species) in 1996, and badly needs another assessment. The IUCN found the genus to be Vulnerable in its range due to habitat loss and human persecution. Sadly, the story is the same as with many other African animals: although dwarf crocodiles are docile creatures and pose little threat to humans, they are hunted extensively for the global bushmeat trade, as well as for leather. While eating these animals is not an evil unto itself, the current rate of growth in this trade can only be regarded as unsustainable and dangerous for the survival of these animals.

Ironically, both the scientists trying to save them and the hunters trying to kill them search for the dwarf crocodiles by using the same method: using a spotlight to search for their eyeshine in the water. One comes with notepads and radios, desperately trying to get accurate numbers for these shy animals; the other comes with guns and machetes to make that job all the harder.

The Curious Cave Crocs

Two O. tetraspis with very different lifestyles. Left: a cave-dweller. Right: a forest-dweller. By Olivier Testa, first published in Shirley et al. (2016) and uploaded to Wikimedia Commons by the photographer. CC BY-SA 3.0

The nocturnal, forest-stalking behavior I described above is interesting enough, but there is an even stranger lifestyle practiced by a few dwarf crocodiles in Gabon. There, one finds the Abanda cave system, where complex karst galleries and caverns have been carved into the Cretaceous limestone by plentiful tropical rain. The dwarf crocodiles calling these caves home not only have a very different ecology to their forest-dwelling counterparts, but sometimes even look strikingly different.

A study of these crocodiles revealed what seemed to be a separate population, most likely descended from founding individuals who either fell into the caves, or entered by way of tunnels that were later sealed off. There, in a microclimate dominated by the activities of several species of African bats, the crocodiles have seemingly made a home for themselves away from the light of the sun by subsisting almost entirely on bats and cave crickets.

Although we tend to think of caves as rather cold places, they actually retain temperature very well, particularly in the tropics where there is little seasonal variation. The body heat of the bats, as well as, nauseatingly enough, the exothermic heat from truly epic quantities of guano decomposing, both work to keep the Abanda caves at a comfortable temperature for the dwarf crocodiles that call them home.

As for the bright orange coloration seen in certain adult crocodiles in these caves, it doesn't appear that the color occurs naturally at all. Rather, after years, even decades, of pickling away in the bat guano covering the floors of their home, the alkalines simply erode the thick skin of the crocodiles. In a few instances, some were even noted to have bits of the skull visible through the skin.

This may seem like a pretty rotten life for any animal to lead, but the cave crocodiles actually seem to be doing comparatively well for themselves. Although not enough studies have been made to determine the size of their population, the ones surveyed so far seemed to be eating far better than the forest-dwelling crocodiles above, simply by virtue of having a far more abundant, and regularly available, food supply. It just goes to show that no matter how bizarre (or disgusting), all of the lifestyles that we see in nature really do serve some purpose.

The Species Problem

O. osborni in the Republic of the Congo. Photographed by Marius Burger and retrieved via Wikimedia Commons. CC0

In the middle of Osteolaemus' western range, there is a region called the Dahomey Gap. This expanse of forest-savanna mosaic habitat stretches to the Atlantic, splitting the great Guinean forests into two halves: the Upper in the west, and the Lower in the east. Although dwarf crocodiles look much the same on both sides of this gap, genetics tell us that the two separated populations are much more different than they appear. Part of the reason why I'm so reluctant to call O. tetraspis (which is found east of this gap) simply the "dwarf crocodile", as if it was the definitive article, is that there are not actually two species of Osteolaemus. There are, at minimum, three, and very possibly even four. As of this writing, only O. tetraspis and O. osborni have official names, but the presence of a third species, living west of this gap, has been known to science since 2009. What's going on here?

Cryptic species are animal species which, while appearing outwardly very similar or identical to their relatives, are genetically distinct. Crocodiles seem to be particularly prone to hosting cryptic species, perhaps due to the fact that they are, as a rule, quite morphologically conservative. It appears that this has been fooling zoos for decades as well; despite all being housed, displayed, and even bred together, surveys of European and American zoos suggest that as many as four different Osteolaemus species are actually represented. Most are O. tetraspis, one individual at a zoo in Santillana, Spain is an O. osborni, and the Upper Guinea forest species (Osteolaemus sp. nov. if you'd like, since it still has not been named) is especially prominent in American zoos. Bizarrely, four individuals in various European zoos do not seem to belong to any of these three species, forming a genetically distinct group that seems to be most closely related to the Upper Guinea forest species. No wild individual or museum specimen has ever been found to belong to this genetic group, and since no solid information for the provenance of these four animals exists, it is simply a head-scratching mystery as to where this apparent fourth Osteolaemus species might actually live in the wild.

Although this situation is probably one of the most complicated, dwarf crocodiles are far from the only animal to have one of these cryptic species complexes. More than anything else, this shows us just how little we still know about the wild populations of many animals, particularly in remote habitats like the forests of Western Africa. This poses issues for conservation on a few different levels. In zoos, Osteolaemus of different species have unwittingly been allowed to breed for decades, producing a number of hybrids who cannot contribute to repopulation efforts in the wild. In the field, these revelations mean that we know even less than we thought about their numbers, which species of dwarf crocodile are most threatened, and even where to find many of them. We have gone from having one Vulnerable species to, very possibly, having four Endangered ones.

I don't view this as being all negative. We are learning more about these animals all the time, and at least the scope of the problem has become obvious to us. It is my hope that further work on the dwarf crocodiles will tell us more about what makes these species different, more about their unique ecology, and even more things to love about them.

If you are interested in learning more about the cave-dwellers of Gabon, Shirley et al.'s 2016 paper "Diet and body condition of cave-dwelling dwarf crocodiles (Osteolaemus tetraspis, Cope 1861) in Gabon" has much more information. There is also a documentary featuring the scientists themselves at work, "Cave Crocs of Gabon", streaming on Amazon Prime, which is pretty solid, if that's more your speed. As for seeing crocodilians in the flesh, the St. Augustine Alligator Farm, mentioned at the beginning of this article, is an excellent place to do that.

My previously-threatened Jurassic World: Dominion review is still due for whenever I happen to watch the movie. Unless that happens first, the next article will be about Maastrichtian pterosaur diversity. Until next time!

A resting Osteolaemus tetraspis (probably...) photographed at the St. Augustine Alligator Farm, 2014, by the author.

Sources:

Eaton, Mitchell J. (2010). "Dwarf Crocodile Osteolaemus tetraspis." Crocodiles. Status Survey and Conservation Action Plan. Third Edition, ed. by S.C. Manolis and C. Stevenson. Crocodile Specialist Group: Darwin: 127-132.

Eaton, Mitchell J., Andrew Martin, John Thorbjarnarson, and George Amato (2009). "Species-level diversification of African dwarf crocodiles (Genus Osteolaemus): A geographic and phylogenetic perspective." Molecular Phylogenetics and Evolution 50.3: 496-506.

Franke, Franziska A., Fabian Schmidt, Christin Borgwardt, Detlef Bernhard, Christoph Bleidorn, Wolf-Eberhard Engelmann, and Martin Schlegel. (2013). "Genetic differentiation of the African dwarf crocodile Osteolaemus tetraspis Cope, 1861 (Crocodylia: Crocodylidae) and consequences for European Zoos." Organisms Diversity & Evolution 13: 255-266.

Pauwels, Olivier S. G., Brady Barr, Mei Len Sanchez, and Marius Burger. (2007) "Diet records for the dwarf crocodile, Osteolaemus tetraspis tetraspis in Rabi Oil Fields and Loango National Park, southwestern Gabon." Hamadryad 31.2: 258-264.

Schmidt, Fabian, F. A. Franke, M. H. Shirley, K. A. Vliet, and V. L. Villanova. (2015) "The importance of genetic research in zoo breeding programs for threatened species: the African dwarf crocodiles (genus Osteolaemus) as a case study." International Zoo Yearbook 49.1: 125-136.

Shirley, Matthew H., Brittany Burtner, Richard Oslisly, David Sebag, and Olivier Testa. (2016) "Diet and body condition of cave-dwelling dwarf crocodiles (Osteolaemus tetraspis, Cope 1861) in Gabon." African Journal of Ecology 55.4: 411-422.

Shirley, Matthew H., V. L. Villanova, K. A. Vliet, and J. D. Austin. (2014) "Genetic barcoding facilitates captive and wild management of three cryptic African crocodile species complexes." Animal Conservation 18.4: 322-330

Shirley, Matthew H., Kent A. Vliet, Amanda N. Carr, and James D. Austin. (2014) "Rigorous approaches to species delimitation have significant implications for African crocodilian systematics and conservation." Proceedings of the Royal Society B 281.1776

Smolensky, Nicole L. (2014) "Co-occurring cryptic species pose challenges for conservation: a case study of the African dwarf crocodile (Osteolaemus spp.) in Cameroon." Oryx 49.4: 584-590

Crocodile Specialist Group. (1996). "Osteolaemus tetraspis." The IUCN Red List of Threatened Species 1996: e.T15635A4931429.

The Fabrosaur Heresies

A Triassic scene from the Meyers Konversations-Lexikon, a 19th-20th Century German publication (4th edition, 1885). In the public domain. Modified from the original retrieved from Wikimedia Commons.

What would your reaction be if I told you that there was a dinosaur family... that never existed? If you have much familiarity with the science of classifying organisms, then you shouldn't be very surprised at all. Our taxonomies are, after all, ephemeral little things, at best only convenient labels for the diversity we see in the natural world, and never a perfect reflection of reality. It often turns out, especially when considering long-extinct taxa like non-avian dinosaurs, that even important or well-known groups eventually turn out to be hopelessly poly- or paraphyletic, or else to be based on such little solid material that they end up being scientifically useless. With these prospects in mind, same as the last time we talked about murky archosaurian taxonomy, we'll be delving back into the Triassic, that most complicated of time periods. There we'll consider just one such problem - that there's something wrong with our fabrosaurs...

From the start of modern science's efforts to understand the Triassic period (252 - 201 Ma), unraveling the mystery surrounding the origin of dinosaurs has been a singular obsession of many researchers. Somewhere in the space of the Middle and Late Triassic, the first true dinosaurs split away from the other archosaurs, and quickly began to diversify into the familiar clades which would soon come to dominate the remainder of the Mesozoic. Among many other examples, true sauropods first emerged toward the end of this period, and it's likely that tetanuran theropods (the "advanced theropods", that is, all theropods closer to birds than to ceratosaurs) did as well.

The deepest division within Dinosauria, and the one which is the least well understood, is that between the saurischians and ornithischians, which must have occurred deep in the Middle Triassic. Despite the numerous saurischian dinosaurs known from the Triassic, however, ornithischians are comparatively sparse, which makes deciphering their early history difficult. Here and there, however, fossils from the Late Triassic and Early Jurassic have turned up to give us a glimpse at the base of the ornithischian tree. Most of these were found during the course of the 20th Century as dinosaur research began to pick up once more after the Second World War, and so it is this period which, until recently, shaped our understanding of the ornithischians' earliest days. In an era of palaeontology when it was still fashionable to call more or less any small ornithopod a hypsilophodontid, it was tempting to find a place for these basal ornithischians in the dinosaurian scheme, and what better way than to slot them into a convenient, family-level grouping?

Enter the Fabrosaurs

A replica of the Lesothosaurus diagnosticus holotype, housed at the Royal Belgian Institute of Natural Sciences, Brussels and photographed by Wikimedia user "Ghedoghedo". Original retrieved from Wikimedia Commons.

Happily, in 1972, just the right label came along. Named principally for the scrappy remains of a South African dinosaur called Fabrosaurus (after Jean Fabre, the French palaeontologist whose efforts uncovered the holotype), Fabrosauridae eventually expanded from a somewhat redundant single-species family grouping to a rather popular label for further ornithischians found from Late Triassic and Early Jurassic strata. For a long time, Lesothosaurus, from the same general time and place as Fabrosaurus (199-189 Ma, Early Jurassic), was also held to be a member of this clan. Known from much more substantial remains than the scant Fabrosaurus, Lesothosaurus informed many a Triassic diorama of scampering "fabrosaurs", running and leaping across the landscape of the imagination. Agilisaurus from the Middle Jurassic of China joined the ensemble as a late-surviving member when it was originally described as a fabrosaurid, and a smattering of suspiciously ornithischian-like teeth from the Late Triassic of North America which received the moniker Revueltosaurus claimed new ground for the group. On all fronts, geographical and temporal, the mighty Fabrosaur Empire seemed to be expanding, joining Triassic dioramas everywhere as a forerunner of sorts to the later, more advanced ornithischian groups which would populate scenes of the Jurassic and Cretaceous. Alas...

The Fall of the Fabrosaur Empire

The incriminating teeth of Revueltosaurus callenderi. Originally photographed by the (American) National Park Service and retrieved from Wikimedia Commons.

From the start, there were problems. The only fabrosaur with remains worth writing home about was Lesothosaurus, which placed the diagnosis of the grouping on a pretty shaky foundation. It was therefore easy (and tempting) to assign any tiny scraps of ornithischian from the first half of the Mesozoic to Fabrosauridae, making the group seem less a mighty empire, and more a kingdom of bits and pieces. As early as the beginning of the 1990s, as the Dinosaur Renaissance entered full swing, palaeontologists were already expressing doubts about the validity of the family as a whole, given the uncertain validity of Fabrosaurus itself. Bit by bit, pieces of fabrosaur material ended up being reassessed as belonging to heterodontosaurs (on which more another time) or even as prosauropods. Most incriminatingly, the ever-important Revueltosaurus material expanded to several skeletons, betraying its shocking identity as not even a dinosaur, but a crocodile-line archosaur close to aetosaurs. As a killing blow of sorts, more complex cladistic analyses after the turn of the century split up what was left, leaving Lesothosaurus as perhaps the sister taxon to thyreophorans (the "armored dinosaurs", ankylosaurs and stegosaurs), and Agilisaurus as closer to the base of the ornithopods and marginocephalians. The sun, it seemed, had set on the fabrosaurs.

Will the Fabrosaurs Rise Again?

Lesothosaurus diagnosticus. Illustrated by Jack Wood and retrieved from Wikimedia Commons.

Well, no, probably not - at the very least, not in the same way that they were before. "Fabrosauridae" is defined around its type species, Fabrosaurus australis, which most researchers now consider a dubious taxon. Even if more complete and diagnostic material of the same animal turned up, it would be difficult to assign it to Fabrosaurus with confidence given how little there already is to work with. However, all is not lost, for the Triassic ornithischians are not altogether gone. Pisanosaurus from the Late Triassic of Argentina and the slightly younger Eocursor of South Africa still firmly place the ornithischians within the Triassic period. However, the loss of Revueltosaurus means that these two taxa, plus some unnamed heterodontosaurid remains also from Argentina, are the only confirmed ornithischian remains from the Triassic period, and all are localized to a small part of the then-extant supercontinent of Gondwana. Since the earliest saurischian dinosaurs are also known from South America, it is not surprising that the ornithischians, too, found their origins in the same place. The question of why ornithischians seem to be so scarce in the northern continents during this time when compared with saurischians is, as of yet, unanswered, but more Triassic ornithischians surely remain undiscovered, and it's wholly possible that some will turn up from Laurasia and make us reconsider the early history of the ornithischians all over again.

And, who knows? Perhaps some very close relatives of Lesothosaurus, the last true fabrosaur, will turn up in Africa or elsewhere in the future. The lesothosaurs could well rise from the ashes of the fallen empire and shake off the dust to scamper anew across our Triassic scenery and feed a new generation of over-ravenous theropods. It's nice to dream...

For more confusing Triassic business:

• The Curious Case of Crurotarsi

For more ornithischians:

• The 'Duckbill' Stomp

Sources:

• Bonaparte, J. F. "Pisanosaurus mertii Casamiquela and the Origin of the Ornithischia." Journal of Paleontology 50, 5. 1976.
• Butler, Richard J., Roger M. H. Smith, and David B. Norman. "A primitive ornithischian dinosaur from the Late Triassic of South Africa, and the early evolution and diversification of Ornithischia." Proceedings of the Royal Society B 274. 2007.
• Butler, Richard J., Paul Upchurch, and David B. Norman. "The phylogeny of the ornithischian dinosaurs." Journal of Systematic Palaeontology 5. 2008.
• Irmis, Randall B., William G. Parker, Sterling J. Nesbitt, and Jun Liu. "Early ornithischian dinosaurs: the Triassic record." Historical Biology 19(1). 2007.
• Parker, William G., Randall B. Irmis, Sterling J. Nesbitt, Jeffrey W. Martz, and Lori S. Browne. "The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs." Proceedings of the Royal Society B 272. 2005.
• Sereno, Paul C. "Lesothosaurus, “Fabrosaurids,” and the early evolution of Ornithischia." Journal of Vertebrate Paleontology 11. 1991.