Monday, November 8, 2010

Tianyusaurus, An Ancient Fruit-eating Lizard?


Several months ago I was watching a pair of tegu lizards (Tupinambis teguixin) feed on some over ripened mangos. It was quite messy, and the lizards walked away from the meal with mango smeared over their labial scales and faces. The macroteiids (family Teiidae) and microteiid lizards (family Gymnophthalmidae) are known only from the Western Hemisphere. Macroteiids are medium to large lizards that are omnivorous, Ameiva feed on fruits, but they seem to take insects more often; while the larger tegus are mostly predators and scavengers, but also opportunistic frugivores. The teiids are mostly Neotropical, but have expanded their distribution into North America (the Racerunners, Aspidoscelis); similarly the microteiids are Neotropical but without representation in North America. As their name suggest, they are tiny. Microteiids tend to be leaf liter swimmers with reduced limbs, consuming minute insects as they wiggle their bodies through grass or decomposing vegetation, but some are arboreal and semi-aquatic. The macroteiids are likewise diverse, several are semi-aquatic, while others are semi-arboreal, some inhabit tropical forests and savannas and yet others are desert-dwellers. In 2007, Randall Nydam and colleagues did a phylogenetic analysis that placed the fossil teiid-like lizards from the Cretaceous of Asia and North America in a new monophyletic group they named Boreoteiioidea and found it to be the sister taxon to the living Teiioidea (Teiidae + Gymnophthalmidae). Boreoteiioidea is from the Greek boreas, meaning north and is in reference to the northern hemisphere distribution of the Boreoteiioidea and the close relationship of this new taxon to Teiioidea. One implication of Nydam et al's work is that the macroteiids and microteiids have no pre-Tertiary fossils, and they suggested that Teiioidea and Boreoteiioidea diverged from a common ancestor by the Early Cretaceous, with the Teiioidea entering South America while the Boreoteiioidea radiated throughout North America and subsequently dispersed to Asia and Europe.

Tianyusaurus was described by Lü Junchanga and colleagues (2008) on the basis of a partial skeleton from a single individual (see photo). The remains were discovered in Upper Cretaceous sediments of the Qiupa formation in the Chinese province of Henan. In 2010, Mo et al. described three better preserved skulls that had been recovered from the 66 million year old Nanxiong formation in Jiangxi Province. Their phylogenetic analysis placed Tianyusaurus in the Boreoteiioidea. Of interest is the presence of a fully developed lower temporal bar and a fixed quadrate in Tianyusaurus, character states not associated with lizards (this has implications not discussed here), and suggest this lizard had a exceptionally large gape. Tianyusaurus teeth were also unusual, the premaxilla had 6–7 small teeth, the dentary had about 33 teeth and the maxilla had about 24 teeth. Surprisingly, the maxilla had large canine-like teeth with the teeth behind them compressed and expanded on the tips with crowns that had four cusps that were asymmetrical. The cusps on the dentary teeth were more symmetrical. The authors suggested that unlike other boreoteiioids with deep robust skulls and jaws used to crop tough plant material, Tianyusaurus ate something quite different; their food required a large gape and the ability to exert a penetrating force in the early stages of the bite. Mo, et al. hypothesized that Tianyusaurus ate turgid, fleshy fruits, an idea consistent with the differentiation of the upper and lower teeth and the exceptional gape. The asymmetrical upper teeth perforated the food with the apical cusp and then cut the food with the oblique cusped blade as the jaws close. The dentary teeth were shaped to hold the food in the mouth, so that the fruits would not slip out of the mouth.

Some angiosperms increased the size of their fruits in the Late Cretaceous, but other plant lineages like ginkgos, conifers, cycads, and seed ferns may also have had their seeds surrounded with fleshy tissue. The purpose of most fruits is to attract animals that will eat the fruit and the seeds it contains and carry them away from the parent plant so that the plant's embryos are dispersed. A few lizards do this today, but mostly its a service provided by endotherms (Tiffney, 2004).

Mo, et al. (2010) note that boreoteiioids are first reported in the Neocomian of Japan (about 130 MYA), represented by Kuwajimalla kagaensis a species that was about 130–150 mm in body length, and believed to have eaten foliage. Kuwajimalla had lanceolate denticulated teeth that are convergent with living Iguana teeth. Thus, Kuwajimalla kagaensis is the oldest known herbivorous squamate (Evans and Manabe, 2008) and suggests an Asian origin for the boreoteiioids, rather than the North American origin proposed by Nydam et al. Boreoteiioids represent the largest known radiation of herbivorous lizards, and the only Late Cretaceous clade of terrestrial lizards thought not to have survived the K-T extinction. 

Today, only 3% of living lizards are herbivorous, but this does not appear to have been the case in the past. Why so few living lizards are frugivorous, today is unclear, but it may be that they cannot compete with endotherms. Birds and mammals tend to be more mobile and capable of getting into trees and exploiting fruit before it falls to the ground and decomposes, and possibly before lizards can gain access to it. To the right are the teeth of two modern plant eating-lizards, the modern Iguana iguana and the Marine Iguana, Amblyrhynchus cristatus.




Literature

Evans S. E. and M. Manabe. 2008. A herbivorous lizard from the Early Cretaceous of Japan. Paleontology. 51:487–498.

Junchang Lü, Ji Shu'an, Dong Zhiming, Wu Xiaochun. 2008. An Upper Cretaceous lizard with a lower temporal arcade. Naturwissenschaften 95:663–669

Mo J, Xu X, Evans S. E. 2009. The evolution of the lepidosaurian lower temporal bar: new perspectives from the Late Cretaceous of South China. Proceeding of the Royal Society, Biological Sciences 277:331-336.

Nydam R. L., J. G.. Eaton, J. Sankey. 2007 New taxa of transversely-toothed lizards (Squamata: Scincomorpha) and new information on the evolutionary history of ‘teiids’. Journal of Paleontology. 81, 538–549.

Tiffney, B. H. 2004. Vertebrate dispersal of seeds through time. Annual Review of Ecology, Evolution and Systematics 35:1-29.


No comments:

Post a Comment