A fossil Anolis lizard
in the collections of the American Museum of Natural History is the second
anole preserved in amber from the Dominican Republic (Miocene Epoch) to
be studied. The fossil exhibits skeletal characters indicating that
it is a juvenile member of a large clade of anoles that includes many extant
Hispaniolan species and is characterized by close association between the
clavicles and the lateral processes of the interclavicle. Standard
external characters that can be scored in the fossil are identical to those
of two of the four extant species in the A. chlorocyanus group;
however, because relatively few characters can be scored in the fossil,
and because the polarities of the relevant characters are currently unknown,
the fossil can be only tentatively referred to that group. The skeletal
and external characters of the new fossil are also nearly identical to
those of A. dominicanus, another Dominican amber anole; however,
the latter fossil is relatively poorly preserved, and its currently known
characters are identical to those of several extant species, including
two species of the A. chlorocyanus species group. Body proportions
and lamella counts of both known amber fossils indicate that these lizards
are trunk-crown anoles, that is, members of an ecologically and morphologically
defined class of anoles specialized for life high on the trunks and in
the crowns of trees. This finding is consistent with referral of
the fossil anoles to the A. chlorocyanus species group, all extant
members of which are trunk-crown anoles.
Approximately 12 specimens of Anolis lizards preserved in amber are known. Most are from the Dominican Republic and most are now in private hands. Recently, the American Museum of Natural History purchased an exceptionally well preserved specimen from the Dominican Republic that is approximately 20 million years old. Given the extensive research on the evolutionary diversification of Caribbean Anolis lizards, we decided to investigate this specimen. Another reasonably well-preserved specimen is in the collections of the Naturhistorisches Museum Basel. This specimen, previously described by Olivier Rieppel in Nature in 1980, was also studied.
We closely examined the specimens for information on phylogenetic relationships. A detailed description of the specimens is provided. The specimens are highly similar to Hispaniolan species in the Anolis chlorocyanus species group, however, we cannot exclude the possibility that they are more closely related to other species extant on Hispaniola today.
Caribbean anoles have differentiated into a set of distinct "ecomorphs," which are species that are behaviorally and morphologically specialized to use different parts of the environment. Members of the same ecomorph class are found on each island in the Greater Antilles (with several exceptions). Nonetheless, phylogenetic studies indicate that members of the same ecomorph class on different islands are not closely related and thus have evolved convergently on each island (see Losos et al., 1998).
Because of the exceptional preservation of the specimens, we were able to accurately measure the morphological features that distinguish the ecomorph classes; namely, limb length, tail length (on one of the specimens), snout-vent length, and number of subdigital lamellae on the fourth toe of the hindfoot. Our goal was to investigate whether these specimens were morphologically similar to any of the ecomorph classes or, alternatively, whether the specimens could not be considered a member of any of these classes.
To test these hypotheses, we measured specimens of the four common ecomorphs in Hispaniola: A. chlorocyanus (trunk crown anole), A. distichus (trunk anole), A. cybotes (trunk-ground anole), and A. semilineatus and A. olssoni (grass anoles). These species, or their close relatives, are common throughout almost all of Hispaniola. We did not examine members of two ecomorph classes, the crown-giant and twig anoles. However, it was clear that the amber specimens did not conform to these ecomorphs: crown giants are much larger than all other ecomorphs and twig anoles are extreme in their short limbs and tails. Based on their size (< 35 mm snout-vent length), the amber specimens were clearly juveniles. Thus, we measured an ontogenetic series of specimens for each species.
Our results were clearcut:
both specimens grouped definitively with the trunk crown anole, A. chlorocyanus.
Hence, at least based on morphology, it appears that these specimens were
trunk-crown anoles (however, given that trunk-crown anoles have evolved
repeatedly in the Greater Antilles, it would be premature to conclude that
these morphometric similarities are indicative of a close relationship
between the amber specimen and the A. chlorocyanus group).
These results indicate that the ecomorph phenomenon is an ancient one;
at least one ecomorph type has existed in the Caribbean for 20 million
years. Examination of additional specimens would be interesting to
determine whether evidence for the occurrence of other ancient ecomorphs
exists (one could suggest that, based on habitat use, trunk-crown anoles
are particularly likely to be encased in amber, which is formed from the
sap exuded on the trunks of Hymenaea trees; hence, failure to find
evidence of other ecomorphs would not constitute strong evidence that they
did not exist).
to Losos curriculum vitae