Origins of NZ skinks revealed

By Hilary Miller 18/10/2009 Most New Zealanders can name at least a dozen or so species of native bird, but how many can do the same for our native reptiles?  If you starting counting and only got as far as 1. tuatara, you’re probably not alone.  Although we are missing some of the major groups of reptiles (like snakes and alligators), we do have a diverse array of lizards.  In fact New Zealand has around 80 different lizard species in two major groups – geckos and skinks (tuatara are not lizards, they are Sphenodontids). 

Around half of our lizard species are skinks.  These are the most commonly encountered native reptiles, being the species most likely to be spotted disappearing under rocks or into long grass on a hot day, and generally being favoured by the domestic moggy.  Now new research is improving our understanding of the origins and evolution of our skink fauna, with some exciting fossil finds and the publication of a comprehensive genetic study. 

David Chapple (formerly of Victoria University of Wellington, now at Monash University) and colleagues have just published a molecular phylogeny for New Zealand’s skink fauna, which investigates the relationships among 32 of our skink species and their closest relatives from Lord Howe Island, New Caledonia and mainland Australia. 

New Zealand’s skinks were previously grouped into 2 separate generaCyclodina and Oligosoma.  Oligosoma species are found throughout New Zealand, are diurnal and have pointed heads and long limbs and toes.   In contrast, Cyclodina species are nocturnal or crepuscular, have squarer heads and bodies, and relatively shorter limbs and toes.  However, Chapple’s genetic work shows that there are in fact 8 different clades of skinks that likely evolved from a single ancestral species after it colonised New Zealand from New Caledonia.  There is no clear division between Cyclodina and Oligosoma, suggesting that the differences in morphology that separate these two genera have evolved on multiple occasions.  Chapple’s paper thus spells the end for Cyclodina as a recognised genus – all New Zealand skinks (plus their closest relative C. lichenigera from Lord Howe Island) are now under the genus name Oligosoma.

Oligosoma alani, one of nine species renamed from the genus Cyclodina

Oligosoma alani, one of nine species renamed from the genus Cyclodina

Genetic studies like this can also give us a better idea of how long ago a species diverged from its common ancestor.  By employing a molecular clock, calibrated against a couple of known timepoints (e.g. known fossil ages or timing of islands emerging), researchers can relate the number of changes in DNA sequence between two species to evolutionary time.  Chapple and colleagues used this method to estimate that skinks first colonised New Zealand 16-22 million years ago.  This date conflicts with previous studies, which suggested a much more recent arrival less than 8 million years ago, but fits with some recent fossil finds from central Otago.  The St Bathans area of central Otago is proving to be a goldmine for early miocene (16-19 million yrs ago) fossils, and a recent paper by Lee and colleagues at the University of Adelaide, Te Papa and Canterbury Museum documents several fossil lizard finds that indicate an Oligosoma-like species was present in New Zealand by 16 million years ago. 

Both the genetic work and the St Bathans fossils point to skinks colonising New Zealand not longer after the “oligocene drowning”, a period 25-35 million years ago when  much of the present-day New Zealand landmass was underwater.  Chapple suggests that after diverging from their New Caledonian cousins, ancestral NZ skink species may have survived on now-submerged volcanic islands along the Lord Howe rise and Norfolk Ridge before reaching New Zealand.  Of course, this scenario requires skinks to have dispersed across large distances of open water, which you may think would be a problem for a non-flying, terrestrial vertebrate.  But this is not as unlikely as is sounds – many of our skink species live in coastal areas, amongst material that is often swept out to sea during storms.  They have also been observed swimming in rock pools, and can stay underwater for up to 20 mins.  So its not unreasonable to think they could survive a trip across the ocean on a raft of kelp or driftwood.

Two of New Zealand's coastal skink species, O. smithii (left), and O. suteri (right)

Two of NZ's coastal skink species, O. smithii (left), and O. suteri (right)

The work of Chapple and colleagues has also resulted in the revision of a number of individual species names for NZ skinks – check out the papers for yourself if you want the details.

Chapple, D., Ritchie, P., & Daugherty, C. (2009). Origin, diversification, and systematics of the New Zealand skink fauna (Reptilia: Scincidae) Molecular Phylogenetics and Evolution, 52 (2), 470-487 DOI: 10.1016/j.ympev.2009.03.021

Lee, M., Hutchinson, M., Worthy, T., Archer, M., Tennyson, A., Worthy, J., & Scofield, R. (2009). Miocene skinks and geckos reveal long-term conservatism of New Zealand’s lizard fauna Biology Letters DOI: 10.1098/rsbl.2009.0440

For more about the aquatic abilities of skinks see also: K Miller (2007) Taking the plunge. Forest and Bird 326: 20-22.