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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Monstersauria vs Goannasauria

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Heloderma suspectum, the Gila monster. Photo by Dave Hone.

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The Gila monster Heloderma suspectum and its close relative the Mexican Beaded lizard H. horridum are the only two extant members of Helodermatidae, the gila monster clade. It's been agreed for a considerable time that, among living lizards, helodermatids are most closely related to monitor lizards (varanids) and to the weird Bornean earless monitor Lanthanotus borneensis. All of these lizards (and their close fossil relatives) are grouped together in Platynota. The name Varanoidea has also been used for Platynota, but it's been restricted specifically to the monitor lineage within Platynota at times, and hence, annoyingly, means different things to different authors.

Beaded lizard, photographed by Ltshears, from wikipedia. Released into public domain.

Helodermatids are stocky, proportionally short-limbed, short-tailed lizards with deep, robust snouts, dermal armour, osteoderms fused to the skull surface, venom grooves in the dentary teeth, and venom glands located in the lower jaw. Their teeth are formidable - check out the fantastic image below from digimorph. Gila monsters are normally 30-50 cm long in total while Beaded lizards can reach 70 cm or even nearly 1 m (Pianka & Vitt 2003). They prey on a diversity of invertebrates and small vertebrates, do a lot of digging for prey, and are sometimes regarded as specialised nest-seekers: they eat the eggs of other lizards as well as those of snakes and birds, and also prey on rodent nestlings. Gila monsters have been known to locate hen's eggs buried 15 cm below the ground surface (Pianka & Vitt 2003). Needless to say, their olfactory and vomeronasal senses are highly developed. The venom is used for self-defense - not in subduing prey. They are surprisingly good climbers (albeit slow, cautious climbers) and may regularly forage in trees, bushes and even on cacti.

In general, these are slow, sluggish lizards that may spend about 95% of their time resting and hiding in burrows, but they are known to have one of the highest aerobic scopes of all lizards - this is higher in males than in females and is presumably a sexually selected trait related to the prolonged and metabolically costly wrestling that males engage in (Beck et al. 1995). In Beaded lizards at least, this behaviour is pretty spectacular and similar to the wrestling practised by some monitors: the animals lever off the ground with their heads and tails, forming an arch shape. The aim is then to push the opponent onto his back before biting him on the jaws (Beck & Ramírez-Bautista 1991). While we know that males engage in 'sexual wrestling', relatively little is known about social behaviour in general and more work is needed.

Gila monster skull, with numerous osteoderms, from digimorph (c).

Unambiguous fossil helodermatids are known from the Eocene-Oligocene of France (Eurheloderma) and the Oligocene and Miocene of the USA (Lowesaurus and H. texanum). A number of additional taxa from the Cretaceous, argued by some authors to belong together as the 'gobidermatids' (Lee 1997), have at times been included within Helodermatidae but are here excluded from that clade following Conrad (2008). In proportions and probably overall appearance, unambiguous fossil helodermatids seem to have been similar to the extant ones, and Eurheloderma and Lowesaurus definitely have venom grooves on their dentary teeth (Nydam 2000).

Skull of Cretaceous monstersaurian Estesia, photographed at the AMNH by Ghedoghedo. Released into public domain.

A number of additional fossil taxa group closer to helodermatids than to other platynotans, and yet don't necessarily look helodermatid-like. Estesia mongoliensis from the Late Cretaceous of Mongolia, for example, had a shallow snout compared to gila monsters and probably looked superficially more like a monitor. Anatomical characters that link it and other species with helodermatids include tall, narrow neural spines and a distinctive form of tooth implantation.

Platynotan phylogeny plotted against time, from Conrad (2008). The clade shown at the top (the one including Heloderma) is Monstersauria. The one in the middle (including Varanus) is Goannasauria. See Conrad (2008) for a huge amount of additional detail. Mosasaurs and kin may or may not be part of Platynota.

In differentiating this 'gila monster lineage' from other groups within Platynota, Norell & Gao (1997) decided to name it. They went with Monstersauria. Their original definition was node-based (Gobiderma + Heloderma), but Conrad (2008) more recently co-opted the name for the entire gila monster branch (an intention hinted at by Norell & Gao (1997), though not expressed in their node-based definition). So, Estesia, the 'gobidermatids', and of course helodermatids proper are monstersaurians.

Monstersauria is the sister-group to the lineage that includes monitors, Lanthanotus and a large number of fossil taxa. Perhaps surprisingly, this lineage has no name unambiguously associated with it, and one was needed. Varanoidea has been applied to it by some authors, but the use of this name for the entire gila monster + monitor clade seems more appropriate. Conrad (2008) therefore came up with a new name for the monitor branch, and opted for Goannasauria. So, Varanoidea - the entire gila monster + monitor clade within Platynota - consists of two major lineages, Monstersauria and Goannasauria.

And this, again, is why I suck at producing 'picture of the day' posts. Dammit.

For previous Tet Zoo articles on platynotans, see...

Mosasaurs - historically associated with monitors or even with snakes - have been included within Platynota by some authors (e.g., Lee 1997). This position is looking increasingly unlikely. That is, mosasaurs are probably not platynotans. Nevertheless, if you want to see some Tet Zoo articles on them, go to...

Refs - -

Beck, D. D., Dohm, M. R., Garland, T., Ramírez-Bautista, A. & Lowe, C. H. 1995. Locomotor performance and activity energetics of helodermatid lizards. Copeia 1995, 577-585

- . & Ramírez-Bautista, A. 1991. Combat behavior of the beaded lizard, Heloderma h. horridum, in Jalisco, Mexico. Journal of Herpetology 25, 481-484.

Conrad, J. (2008). Phylogeny And Systematics Of Squamata (Reptilia) Based On Morphology Bulletin of the American Museum of Natural History, 310, 1-182 DOI: 10.1206/310.1

Lee, M. S. Y. 1997. The phylogeny of varanoid lizards and the affinities of snakes. Philosophical Transations of the Royal Society of London B 352, 53-91.

Norell, M. A. & Gao, K. 1997. Braincase and phylogenetic relationships of Estesia mongoliensis from the Late Cretaceous of the Gobi Desert and the recognition of a new clade of lizards. American Museum Novitates 3211, 1-25.

Nydam, R. L. 2000. A new taxon of helodermatid-like lizard from the Albian-Cenomanian of Utah. Journal of Vertebrate Paleontology 20, 285-294.

Pianka, E. R. & Vitt, L. J. 2003. Lizards: Windows the Evolution of Diversity. University of California Press, Berkeley.

The views expressed are those of the author and are not necessarily those of Scientific American.

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