The Devil frog and Gondwana

It’s worth noting recent news reports of a fossil from Madagascar: the Beelzebub frog (ie Beelzebufo), about the shape and size of a "squashed beach ball".

Noteworthy specifically because of its resemblance to South American frogs, and consequent claim that South America and Madagascar had Gondwanan connections until well into the late Cretaceous. (Madagascar only diverged from the fast-moving India comparatively recently).

There are two competing claims: the traditional view is that South America and Madagascar diverged about 120 million years ago; a more radical one (supported by this finding) gives a more recent 80mya (around the same time New Zealand split from Antarctica).

Most news reports gloss over the incongruity of Madagascar being directly connected to South America, but some indicate the path - via Antarctica.

The relevant scientific paper is at PNAS - but it's sufficiently recent that it requires subscription or payment.

SB Mammal: notes for a review, pt 1

Further notes to an earlier discussion on the discovery of a unique New Zealand mammal.  To recap:

The SB mammal (or "waddling mouse") is represented by a set of fossils found in New Zealand in December 2006. Found were two partial jawbones (mandibles) and a partial femur, dated to 19mya. Quite small - the bar on the photo represents 2mm. By the looks of the front incisors (picture C), it would have been quite mean - if it was bigger.

(jawbone views from the paper, referenced below.)

The reason the femur and mandible are linked is simply one of parsimony - nothing else like them has been found. A fair assumption, although it remains falsifiable.

Some of why it intrigues me so much:
- the proliferation of unique NZ bird life (including several flightless birds) had been suggested to be due to the lack of native terrestrial mammals
- it's a "ghost lineage" - no links to living mammals, yet it survived until very recently (19mya)
- how did it get there? - NZ split from Antarctica about 85 mya
- in particular, it has been characterised as non-therian, suggesting it was an egg-layer, unrelated to any other mammalian species extant in the Miocene epoch. Archaic, out of time.


Links:
The paper itself: Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific

APP (Acta Palaeontologica Polonica), a Polish publication in English, has a particularly significant reference from the paper: In quest for phylogeny of Mesozoic mammals - this links to the abstract and full 78 page text by three very significant names in the field: Luo, Kielan-Jaworowska, and Cifelli.

Also recommended: An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution (Luo, Ji, Wible, Yuan), available from Science magazine with free registration.

The only vaguely useful blog discussions I've found on this mammal are at Tet Zoo, and one at WebWeaver's World.

News reports mainly rehash, but perhaps the one at New Scientist is best.

Evolution: NZ prehistory: another narrative

The University of Waikato has an interesting and detailed narrative on New Zealand's natural history, based on its geological and fossil record. I'll summarise some of the main points here. (A reminder that this whole exercise is aimed at better understanding the circumstances surrounding the appearance of a non-therian mammal fossil in the south of NZ.)



Speaking specifically about the parts of New Zealand that are above water now, parts of NZ first rose from the sea in the Mesozoic era (dinosaur times), in the Triassic period (250 million years ago to 201 mya). Fossils are found in this period of early forms of kauri, rimu, and kahikatea trees; no terrestrial vertebrates.



In the Jurassic period (145mya to 201 mya), NZ was uplifted as part of a larger land mass ("Tasmantis"); the narrative strongly implies it was connected to Gondwana. In any case, it says that due to NZ's subsequent isolation, it is likely that this point saw the arrival of "archaic frogs, large land snails, tuatara, and peripatus".



Under the Cretaceous (65mya to 145 mya) heading, it mentions a single vertebra of an upright carniverous land dinosaur. It also mentions waterways developing around 120mya between Gondwana and the land mass that included NZ. A consensus from other sources is that that split was final by about 82mya. The dinosaur extinction event was 65mya; the iridium boundary marker has been noted in NZ.



Other points of note:

- Oligicene epoch (23-37mya): two thirds of NZ was submerged (it notes the continued survival of frogs, tuatara, snails, peripatus and ratites (flightless Gondwanan birds);

- Miocene (5-23mya): bat, gecko and takahe ancestor appeared from Australia...



And we're back where we started: the SB mammal from around 19mya: Miocene, but non-therian. Appearing from Australia relatively recently (despite no equivalent fossil found there), or surviving from Gondwanan days.

Evolution: Gondwana vs New Zealand

Gondwana* was a large land mass that tended to reside in the southern hemisphere from Cambrian times to mid-Jurassic times (c.500 million years ago to c.160mya). It consisted, in the main, of Antarctica, Africa, South America, India, Australia, and New Zealand. For part of its existence, it was united with its northern equivalent into the supercontinent Pangea.

Through this time, Gondwana experienced a number of climate changes, due in part to the movement of Earth's techtonic plates which formed then ultimately broke up the continent.
Africa moved away first, followed by South America and India (the latter has been a particularly fast mover, crashing spectacularly into Asia to form the Himalayas).



Of course, disintegration took many millions of years, so the dates are approximate. The dispersal (and later speciation) of fauna is not entirely stopped by such splits. For some time after complete separation, islands are typically scattered between the land masses. This more or less allows for some island hopping from one land mass to another, typically through "rafting" - the carriage of fauna across on masses of vegetation, often after storms. Richard Dawkins treats this well in The Ancestor's Tale, and makes the point that even if such an event sounds unlikely, given the time scales involved - millions of years - it's unlikely that it wouldn't happen. And all it takes for a population to be established is one pregnant female.


I always thought that New Zealand broke away from Australia, because they seem to be such a neat fit. However, New Zealand is usually cited as splitting from Antarctica/Australia earlier. Ultimate separation was achieved around 82mya. Dawkins says Australia was finally sufficiently free of Antarctica to obviate island hopping around 55mya, although estimates vary a fair bit; Wikipedia suggests it was still freeing itself at 40mya.
The point about this is that New Zealand was isolated for substantially longer than Australia. At the time of the dinosaur-extinction K-T meteor - 65mya - New Zealand was already isolated, and Australia probably was not.

Various sources suggest New Zealand eroded since isolation, by up to 80% - some claim it disappeared below the waves altogether for a time, although this seems unlikely. The question is around the sustainability of populations in isolation. This is mainly relevant for large terrestrial tetrapods. Conventional wisdom is that the number of unique flightless birds in New Zealand reflects the absence of terrestrial predatory species until human introduction in the past thousand years.

There remains a few species in New Zealand's islands that are "anomalous" to the global narrative, including:

• the living Tuatara - only lizard-like - actually a distant lizard relative called a sphenodont.
  
• fossil dinosaurs and crocodilia from well before NZ's isolation
  
• and the newly-discovered sb mammal or "waddling mouse" from NZ's very recent evolutionary past, c.16 million years ago (main discussions here and here), interpreted as non-therian, ie egg-laying.

These need to be drawn into the narrative one way or another.


Related discussions:

• The evolution of moas, kiwis and other ratites;
• More detail on New Zealand's evolution as a land mass;
• Extinctions in New Zealand at early human arrival;
  
• The emergence and survival of early mammal forms, here and here.
  

*Gondwana means "land of the Gonds"; the term Gondwanaland is both redundant and obsolete.


Reference

Dawkins, R (2004): The Ancestor's Tale. Phoenix, London.