In the hypothetical alternative geological epoch known as the Squamozoic, giant iguanian lizards known as uromastyxians dominate big-bodied herbivore niches across Africa, the Middle East and Asia. Short-headed, stout-limbed and often decorated with long, branched tail spikes, the uromastyxians evolved from the (real world) Uromastyx species, variously known as spiny-tailed lizards, mastigures or dabbs (and highly popular in the pet trade). If you haven’t heard of the Squamozoic before and don’t know what it is, go here. It’s a frivolous exercise in speculative zoology. Anyway, it’s curious and sometimes gratifying when speculation is mirrored by fact, and today we get to see just such an event in action. For you see, those big, Squamozoic uromastyxians are real: they really did evolve, but in our own world, not in an imaginary parallel timeline…

As revealed today in Proceedings of the Royal Society B, Jason Head of the University of Nebraska-Lincoln and a team of his colleagues have discovered the fossils of a remarkable new iguanian lizard that they’ve named Barbaturex morrisoni. Good news: the paper is open access! Acrodont teeth (= teeth that are fused to the underlying jaw bones) and other features demonstrate without doubt that Barbaturex is a member of the iguanian clade Acrodonta.

More specifically, Head et al. (2013) find Barbaturex to be most closely related to the spiny-tailed Uromastyx species, with Leiolepis being the next closest relative. Their favoured cladogram shows this (Leiolepis + (Barbaturex + Uromastyx)) clade as the sister-group to the more diverse iguanian assemblage that includes chameleons, sailfin lizards, the Australasian dragons, and the agamas and their kin (Head et al. 2013).

Based on its dentition (it has tightly occluding, triangular teeth that would have performed a shearing bite in life), we can be confident that Barbaturex was herbivorous (though the possibility of opportunistic animalivory can’t be ruled out). This is consistent both with its large size (herbivory is more efficient at large body size) and its phylogenetic relationships, since it’s surrounded in the phylogeny by herbivorous and omnivorous taxa (Head et al. 2013).

Incidentally, the name is pretty neat. Barbaturex means ‘bearded king’, with the ‘bearded’ bit referring to the presence of a series of sagittally aligned knobs that are spaced along the ventral surface of the mandible (in life, did these anchor some sort of fleshy display structure or structures?). The species name honours Jim Morrison, “vocalist and lizard king” (Head et al. 2013, p. 2 of preprint). Awesome.

Why Barbaturex is remarkable

Barbaturex is from the middle Eocene Pondaung Formation of Myanmar, a geological unit well known for its early primates and numerous other placental mammals (e.g., Ducrocq et al. 2000, Holroyd & Ciochon 2000, Marivaux et al. 2003, 2005, Jaeger et al. 2004, Remy et al. 2005, Holroyd et al. 2006, Métais et al. 2007). Important fossil turtles, snakes and members of other groups are known from the Pondaung Formation as well.

What makes Barbaturex remarkable is its size. Though the fossils are fragmentary (virtually all are incomplete jaw segments), they show that this lizard was about 1 m long in SVL (= snout to vent length). We don’t have a complete skeleton, nor indeed any part of the tail, so a complete length can only be guessed at. Seeing as some of the related iguanians are comparatively short-tailed, Barbaturex might ‘only’ have been 1.4 m long or less, but it was more likely rather longer (1.8-2 m long or so in total).

And what makes Barbaturex doubly remarkable is that it lived alongside a diverse assemblage of herbivorous, omnivorous and carnivorous mammals where – get this – it was one of the largest members of the fauna. Head et al. (2013) estimate that Barbaturex had a mean body mass of 26.7 kg (the estimated range is from 18.9 to 36.9 kg), in which case it was larger than the smaller Pondaung perissodactyls and artiodactyls, and somewhere in the middle of the known size range for all Pondaung ungulates. It was also larger than, or similar in size to, most of the predatory mammals from the assemblage (Head et al. 2013).

Convention would have it that big, terrestrial reptilian herbivores can't exist alongside ungulates and other large mammals since predation and competition essentially prevents their evolution. Yet here we see a big-bodied herbivorous lizard living alongside a diverse array of contemporaneous ungulates and carnivorous mammals. This pattern is fundamentally unlike that seen in modern ecosystems where even the largest herbivorous lizards are substantially smaller than even the smallest ungulates (Head et al. 2013).

Why so big?

How and why did iguanians get to be so large in a place and time that supposedly ‘belonged’ to mammals? Head et al. (2013) favour the idea that the elevated global temperatures of the middle Eocene facilitated the evolution of Barbaturex, both by making it easier for a poikilothermic terrestrial tetrapod to grow and operate at optimal physiological capability, and by encouraging high productivity of the sort of vegetation it would have relied on as a food source. Fans of fossil squamates will of course know that Head and colleagues previously argued that the exceptionally high global temperature of the Paleogene similarly facilitated the evolution of the stupendous boid snake Titanoboa from Colombia (Head et al. 2009) [adjacent photo by jinkinpark].

This all sounds logical, and it’s pretty well supported by what we know about climatic conditions and reptile diversity across the Cenozoic. However, it’s also worth taking seriously the possibility that animals like boas and iguanians simply do have the potential to evolve large size, and to potentially compete against similar-sized mammals and other animals, when opportunity allows (and those ‘opportunities’ might be reduced predation, optimal foraging time or a habitat that promotes the evolution of gigantism).

Either way, the very existence of this intriguing giant iguanian is fascinating and let’s hope that more complete remains are discovered in future.

Iguanians of several lineages have been discussed on Tet Zoo before. See…

Refs - -

Ducrocq, S., Soe, A. N., Auung, A. K., Benammi, M., Bo, B., Chaimanee, Y., Tun, T., Thein, T. & Jaeger, J.-J. 2000. A new anthracotheriid artiodactyl from Myanmar, and the relative ages of the Eocene anthropoid primate-bearing localities of Thailand (Krabi) and Myanmar (Pondaung). Journal of Vertebrate Paleontology 20, 755-760.

Head, J. J., Bloch, J. I., Hastings, A. K., Bourque, J. R., Cadena, E. A., Herrera, F. A., Polly, P. D. & Jaramillo, C. A. 2009. Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures Nature 457, 715-717.

- ., Gunnell, G. F., Holroyd, P. A., Hutchinson, J. H. & Ciochon, R. L. 2013. Giant lizards occupied herbivorous mammalian ecospace during the paleogene greenhouse in SouthEast Asia. Proceedings of the Royal Society B 20130665

Holroyd, P. A. & Ciochon, R. L. 2000. Bunobrontops savagei: a new genus and species of brontotheriid perissodactyl from the Eocene Pondaung fauna of Myanmar. Journal of Vertebrate Paleontology 20, 408-410.

- ., Tsubamoto, T., Egi, N., Ciochon, R. L., Takai, M., Tun, S. T., Sein, C. & Gunnell, G. F. 2006. A rhinocerotoid perissodactyl from the late middle Eocene Pondaung Formation, Myanmar. Journal of Vertebrate Paleontology 26, 491-494.

Jaeger, J.-J., Chaimanee, Y., Tafforeau, P., Ducrocq, S., Soe, A. N., Marivaux, L., Sudre, J., Tun, S. T., Htoon, W. & Marandat, B. 2004. Systematics and paleobiology of the anthropoid primate Pondaungia from the late Middle Eocene of Myanmar. C. R. Palevol 3, 243-255.

Marivaux, L., Chaimanee, Y., Ducrocq, S., Marandat, B., Sudre, J., Naing Soe, A., Thura Tan, S., Htoon, W. & Jaeger, J.-J. 2003. The anthropoid status of a primate from the late middle Eocene Pondaung Formation (Central Myanmar): tarsal evidence. Proceedings of the National Academy of Science 100, 13173-13178.

- ., Ducrocq, S., Jaeger, J.-J., Marandat, B., Sudre, J., Chaimanee, Y., Thura Tan, S., Htoon, W. & Naing Soe, A. 2005. New remains of Pondaungimys anomaluropsis (Rodentia, Anomaluroidae) from the latest middle Eocene Pondaung Formation of Central Myanmar. Journal of Vertebrate Paleontology 25, 214-227.

Métais, G., Soe, A. N., Marivaux, L. & Beard, K. C. 2007. Artiodactyls from the Pondaung Formation (Myanmar): new data and reevaluation of the South Asian Faunal Province during the Middle Eocene. Naturwissenschaften 94, 759-768.

Remy, J. A., Jaeger, J.-J., Chaimanee, Y., Soe, U A. N., Mairvaux, L., Sudre, J., Tun, S. T., Marandat, B. & Dewaele, E. 2005. A new chalicothere from the Pondaung Formation (late Middle Eocene of Myanmar). C. R. Palevol 4, 341-349.