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.
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.
