The idea that non-avian dinosaurs might have been able to climb trees is (I assume) not all that familiar to people outside the field of dinosaur research, but within the field of dinosaur research it has become an increasingly familiar idea within recent decades. Thanks to the discovery of such theropods as Microraptor and Epidendrosaurus, we do now have small forms exhibiting some features suggestive of a tree-climbing (or scansorial) way of life. Perhaps surprisingly however, the idea that dinosaurs might have climbed trees goes back a long way, and well pre-dates the dinosaur renaissance of the 1960s and 70s…
Rightly or wrongly, the idea of scansoriality in non-avian dinosaurs has been considered a ‘fringe’ idea, and it’s partly for this reason that, prior to 2000, nobody had attempted any sort of review on the thoughts that had been published about the subject. As someone interested both in ‘fringe’/’non-standard’ theories and in Wealden theropod dinosaurs, I had the opportunity during the late 1990s to do something about this, and I published two articles on the subject (Naish 2000a, b) and even gave a conference presentation on it (the special relevance that Wealden theropods have for this subject will become clear in a moment). The more formal of my articles was published in the journal Archaeopteryx, a venue I chose because it seemed like an appropriate one for this sort of paper (back in those days we didn’t worry about such things as pdf availability, online access or impact factors… at least, I didn’t anyway). The problem with this otherwise fine idea is that Archaeopteryx is very hard to get hold of, and its contents often remain all but unknown to the majority of researchers. I asked Greg Paul, for example, why he didn’t cite Naish (2000a) in Dinosaurs of the Air, and he told me that he would have done had he been aware of it [image below is Gerhard Heilmann’s ‘fighting proaves’ reconstruction. Note the trunk-climbing ‘proaves’ in the background. From Early Bird].
The oldest reference to scansoriality in a dinosaur that I was able to find came from William Fox, the Isle of Wight curate and amateur fossil collector, who in 1866 proposed that Calamospondylus oweni from the Lower Cretaceous Wessex Formation of the Isle of Wight might have been in the habit of ‘leaping from tree to tree’ (Fox 1866). The Calamospondylus oweni specimen that Fox was writing about now seems to be lost, and hence we can’t be sure what type of animal it was, but there are various reasons for thinking that it was a theropod (Naish 2002). However, it’s not entirely accurate to regard Fox’s ideas about Calamospondylus as directly relevant to modern speculations about tree-climbing dinosaurs given that, if Fox imagined Calamospondylus oweni as resembling anything familiar, it was probably as a lizard-like reptile, and not as a dinosaur as they are currently understood.
During the early decades of the 20th century the idea of tree-climbing dinosaurs became reasonably popular as Othenio Abel, Gerhard Heilmann and others used comparisons with birds, tree kangaroos and monkeys to argue that the small ornithopod Hypsilophodon (also from the Wessex Formation of the Isle of Wight) was scansorial. Heilmann produced some beautiful life restorations of tree-climbing hypsilophodonts, though by the time he wrote his seminal The Origin of Birds in 1926 (first published in 1916 as Vor Nuvaerende Viden om Fuglenes Afstamning) he had come to disagree with this idea and now regarded Hypsilophodon as terrestrial. Anyway, William Swinton favoured the idea of a scansorial Hypsilophodon, concluding that ‘it would be able to run up the stouter branches and with hands and tail keep itself balanced until the need for arboreal excursions had passed’ (Swinton 1936a, p. 577), and in a 1936 review of Isle of Wight dinosaurs mentioned the idea that small theropods might also have used their clawed hands to hold branches when climbing (Swinton 1936b).
During the 1970s, Peter Galton was able to show that all of the claims made about the forelimb and hindlimb anatomy of Hypsilophodon supposedly favouring a scansorial lifestyle were erroneous, and that this animal was in fact well suited for an entirely terrestrial, cursorial lifestyle (Galton 1971a, b). Nevertheless, for several decades Hypsilophodon was consistently depicted as a tree-climber, and even people my age (early 30s) will recall reconstructions whereby small ornithopods were shown poised in trees, opposable hallux gripping the branch like a thumb, and sometimes pictured alongside hypothetical proto-birds (as shown here).
Deinocheirus the slothosaur
Something else very interesting happened during the 1970s. An entirely new dinosaur was brought into the tree-climbing story: the immense Late Cretaceous coelurosaur Deinocheirus mirificus, known only from its arms and shoulder girdles (and from a few ribs too). Generally imagined as a gigantic ornithomimosaur (ostrich dinosaur), Rozhdestvensky (1970) proposed that Deinocheirus was a sloth-like climbing form with shortened hindlimbs. I’m reliably informed that Rozhdestvensky even went so far as to produce a reconstruction of a sloth-like Deinocheirus clambering about in a tree, but I couldn’t find this in the literature and ended up creating my own hypothetical climbing Deinocheirus (Naish 2000b).
Given that Deinocheirus has been estimated at between 6 and 12 tons (its arms were about 2.4 m long), it seems unlikely to say the least that it really was a climber. Admittedly, these mass estimates assume that it was built like the ostrich dinosaurs we know of from better remains, whereas if it was the short-legged slothosaur that Rozhdestvensky imagined it may well have weighed much less. However, there’s no reason at all to take such an idea seriously and it’s a grotesque speculation unsupported by evidence. In support of the climbing hypothesis, Rozhdestvensky pointed, not only to the large claws and sloth-like forelimb proportions, but also to the fact that the palms of Deinocheirus must have faced inwards (we now think that this configuration was common to pretty much all theropods). He also proposed that Deinocheirus had a small, lightly constructed skull, and that it might have fed on fruits and leaves ‘and also, possibly, on the eggs, or even nestlings both of birds and pterosaurs nestling [sic] in trees, or on any other small animals’ (Rozhdestvensky 1970, p. 123). Furthermore, when out of the trees and walking on the ground, he imagined that ‘these dinosaurs must have walked relying on their long forelimbs – more precisely, on the hind part of the hand, as do the sloths’ (Rozhdestvensky 1970, p. 124).
In one of the first analyses of theropod forelimb function, Osborn (1917) had noted that the forelimbs of the ostrich dinosaur Struthiomimus superficially resembled those of two-toed sloths, and this led him to propose that they were used for ‘drawing down the smaller branches’ while foraging. Osborn’s paper included comments from William King Gregory: Gregory considered the possibility that the sloth-like arms might indicate a climbing habit, but he concluded that Struthiomimus couldn’t really be a climber because it was too big, and with hindlimbs suited for terrestrial running, not for climbing (Osborn 1917). This all sounds pretty sensible. However, Rozhdestvensky (1970) chose to take notice of the forelimbs alone, and proposed that ostrich dinosaurs like Struthiomimus were climbers after all, which doesn’t seems at all likely given their ratite-like hindlimb anatomy. Nevertheless it was resurrected by Chatterjee (1997). More on Chatterjee’s scansorial theropods in a minute.
Hypothetical little climbers
In recent decades, Greg Paul has been undoubtedly influential in arguing that small theropods were capable climbers, and he not only argued for and illustrated scansorial abilities in coelurosaurs (Paul 1988: an Ornitholestes climbing in a tree is illustrated therein), he also proposed that as-yet-undiscovered maniraptorans were highly proficient climbers and included the ancestors of birds. The hypothesised existence of small arboreal theropods that are as yet unknown from the fossil record later proved integral to George Olshevsky’s ‘birds come first’ (BCF) hypothesis (Olshevsky 1991, 1994, 2001a, b). Olshevsky argued that all dinosaurs, and in fact all archosaurs, descend from small, scansorial ancestors, and that it is these little climbing reptiles which are the direct ancestors of birds.
The existence of hypothetical scansorial dinosaurs was also proposed by Nessov (1995) who thought that therizinosauroids – the mostly large to gigantic coelurosaurs often likened to ground sloths – might have gone through an arboreal phase in their evolution. Supposedly, these small, tree-climbing sloth dinosaurs were necessary (in the theoretical sense) given the absence from the fossil record of early, primitive members of the therizinosauroid lineage. Implied by Nessov’s hypothesis is the idea that the large, ground sloth-like therizinosauroids must have descended from tree sloth-like ancestors, but there isn’t really any reason to agree with this. Furthermore, we do now have fossils of basal therizinosauroids (like Beipiaosaurus Xu et al., 1999, Nothronychus Kirkland & Wolfe, 2001, Falcarius Kirkland et al., 2005, and Suzhousaurus Li et al., 2007) and they don’t match Nessov’s predictions.
The ideas promoted by Rozhdestvensky and Nessov have essentially remained restricted to obscure, specialised literature and there hasn’t been much opportunity for them to become widely known (articles by Naish notwithstanding). However, Sankar Chatterjee – perhaps best known for his publications on the alleged Triassic bird Protoavis – has brought wide attention to ideas about scansorial theropods by publishing them in a book and in well known, mainstream journals, and by speaking about them at conferences. Among maniraptoran theropods, Chatterjee (1997) proposed that the stiffened dromaeosaurid tail functioned as a prop, functionally analogous to the stiffened rectrices of woodpeckers and other trunk-climbing birds, and he also noted that the opisthopubic pelvis and strongly hooked manual claws allowed these dinosaurs to be proficient climbers. Rather than restricting tree-climbing to those theropods closest to birds, however, Chatterjee (1997, 1999) went on to propose that scansoriality was primitive for all coelurosaurs, and he has even promoted the downright unlikely idea that such animals as compsognathids and ornithomimosaurs were tree-climbers.
In a similar but far less well known hypothesis, Svend Palm (1997) proposed that small, climbing coelurosaurs used their hooked manual and sharp pedal claws to climb the vertical trunks of cycad-like plants, regarding the crowns of such plants as providing ideal hiding and nesting places for such dinosaurs (Palm 1997). Palm compared his little scansorial proto-birds with the non-dinosaurian proto-birds imagined by Gerhard Heilmann, and – like Palm – Heilmann had reconstructed his hypothetical animals as well able to ascend vertical trunks [one of Palm’s hypothetical climbing proto-birds is shown below, from Palm (1997)].
While there are a few additional mentions of tree-climbing dinosaurs here and there in the literature, that’s essentially it. As discussed here, most of these ideas can be discounted for being extremely unlikely (e.g., scansorial ornithomimosaurs) or for being too speculative (e.g., hypothetical early therizinosauroids). But they can’t all be discounted in this way, and we should still take seriously the idea that such theropods as dromaeosaurids might have been capable climbers. Furthermore, as mentioned earlier we now know of at least a couple of small theropods that may even have been specialised climbers. To look at the climbing abilities of these animals further we’d need to carefully examine their anatomy and compare them with what we know about living animals. I might do that some time, but not right now. Some comments on this subject can already be found in Naish (2000a, b). Goats in trees, wolverines, big-headed turtles, pygmy parrots and so on.
Refs – –
Chatterjee, S. 1997. The Rise of Birds. The Johns Hopkins University Press, Baltimore.
– . 1999. Feathered coelurosaurs and the early evolution of birds. Journal of Vertebrate Paleontology 19 (supp to 3), 37A.
Fox, W. 1866. Another new Wealden reptile. Athenaeum 2014, 740.
Galton, P. M. 1971a. Hypsilophodon, the cursorial non-arboreal dinosaur. Nature 231, 159-161.
– . 1971b. The mode of life of Hypsilophodon, the supposedly arboreal ornithopod dinosaur. Lethaia 4, 453-465.
Naish, D. 2000a. Theropod dinosaurs in the trees: a historical review of arboreal habits amongst nonavian theropods. Archaeopteryx 18, 35-41.
– . 2000b. 130 years of tree-climbing dinosaurs: Archaeopteryx, ‘arbrosaurs’ and the origin of avian flight. The Quarterly Journal of the Dinosaur Society 4 (1), 20-23.
– . 2002. The historical taxonomy of the Lower Cretaceous theropods (Dinosauria) Calamospondylus and Aristosuchus from the Isle of Wight. Proceedings of the Geologists’ Association 113, 153-163.
Nessov, L. A. 1995. Dinozavri severnoi Yevrazii: Novye dannye o sostave kompleksov,. ekologii I paleobiogeografii. Institute for Scientific Research on the Earth’s Crust, St. Petersburg State University, St Petersburg.
Olshevsky, G. 1991. A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Publications Requiring Research, San Diego.
– . 1994. The birds first? A theory to fit the facts. Omni 16 (9), 34-86.
– . 2001a. The birds came first: a scenario for avian origins and early evolution, 1. Dino Press 4, 109-117.
– . 2001b. The birds came first: a scenario for avian origins and early evolution. Dino Press 5, 106-112.
Osborn, H. F. 1917. Skeletal adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus. Bulletin of the American Museum of Natural History 32, 133-150.
Palm, S. 1997. The origin of flapping flight in birds. Svend Plam, Ballerop [available online here].
Paul, G. S. 1988. Predatory Dinosaurs of the World. Simon & Schuster, New York.
Rozhdestvensky, A. K. 1970. Giant claws of enigmatic Mesozoic reptiles. Paleontology Journal 1970 (1), 117-125.
Swinton, W. E. 1936a. Notes on the osteology of Hypsilophodon, and on the family Hypsilophodontidae. Proceedings of the Zoological Society of London 1936, 555-578.
– . 1936b. The dinosaurs of the Isle of Wight. Proceedings of the Geologists’ Association 47, 204-220.