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?
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 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:
For more ornithischians:
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.
