An interesting new paper recently published online in the Proceedings of the National Academy of Sciences reports on Miocene (16–19 million year old) mammal remains from South Island, New Zealand. The fossils, which consist of two jawbone fragments and the end of one femur, are surprising for two reasons:
- These are the first terrestrial mammals known from the island nation before the arrival of humans, excepting bats (who were able to fly there),
- The remains are tantalizingly primitive, closer in developmental “grade” to mammals that lived during the Age of Dinosaurs than most of its Miocene contemporaries.
Put together, these facts suggest that New Zealand was home to a heretofore unkown lineage of mammals that split off from the rest of the mammal-bearing world during the Cretaceous Period, when New Zealand split off from eastern Australia and began its lonely journey across the South Pacific.
The discovery of a relatively recent member of a group believed to be long extinct implies what is known as a ghost lineage—a series of ancestors whose fossils have yet to be found (or possibly were never preserved). The concept of a ghost lineage is perhaps easiest to envision in a diagram that combines a timeline with a family tree:
So here we have a grotesquely simplified family tree of mammals from the Middle Jurassic to the present day. I’ve left out quite a few critters, and I may be a bit off on some of the branching dates, but for a rough outline, it should suffice. On the right, the major players in today’s mammalian fauna, the placentals and marsupials, which split apart from each other about 125,000,000 years ago. Moving down the family tree we run into the long extinct aegialodontids, dryolestoids, and multituberculates. According to the new paper, the branch leading to the new New Zealand mammals split off before the aforementioned groups, but after earlier mammals like eutriconodonts, monotremes, and docodonts.
The dotted lines represent ghost lineages (to which I’ve added little ghosts to reinforce the point). Provided the family tree is correctly interpreted, these are paths of evolution that most certainly existed despite gaps in the fossil record. There are undoubtedly gaps in this diagram that I’ve neglected to portray, but hopefully the main points are clear.
Islands seem particularly prone to ghost lineages, perhaps because their isolated faunas often contain species whose relatives have met with extinction elsewhere in the world. Monotremes, for example, are today only found in Australia and some nearby islands. These modern-day monotremes (three species of echidna and the platypus) are island-dwelling relicts of a group of mammals that started down their own path sometime in the Jurassic Period. Geographically isolated and only distantly related to other living groups, their fossil record contains several long gaps with ghost lineages filling in the blanks. Viewed in this light, New Zealand seems like quite a likely spot to uncover a mammal with a 60 to 140 million year gap in its pedigree.
One more note about ghost lineages. The HMNH has been suffering gaps in its own record of late, and for that I apologize. (An interesting admission is that we’ve had more visitors lately than during periods of regular posting, which I hope isn’t a commentary on my writing.) Hopefully this post will mark the end of this particular ‘ghost lineage,’ and, with any luck, the Hairy Museum will regain its typical, punctuated equilibrium as we head into 2007!
Further reading:
Worthy, T. H., Tennyson, A. J. D., Archer, M., Musser, A. M., Hand, S. J., Jones, C., Douglas, B. J., McNamara, J. A., and Beck, R. M. D. 2006. Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific. Proceedings of the National Academy of Sciences USA, published online; doi:10.1073/pnas.0605684103. PDF. Press release.
Afarensis has the story, as does Palaeoblog.
And Darren Naish notes that this find inspired two earlier stories on unexpectedly long-lineaged synapsids over at Tetrapod Zoology.
—Matt Celeskey.
File under: Mammals, Neogene, Recent Discoveries.
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