Early extinctions in New Zealand

New Zealand had a special environment. From the time it split from Antarctica about 80 million years ago, it was one of the larger, more isolated land masses in the world*. The fact that it was mostly free of terrestrian mammalian predators** eems to have, of itself, fostered a unique panoply of ground-based wildlife.

A recent article in American Scientist (The Rat's Tale) paints a picture of ecological destruction that followed the first wave of predatory mammals - although the second wave (Europeans) would ultimately have been more destructive.

The article details the laborious scientific process - and debate - in establishing the date for that first wave, which last year was settled with a period range of 1290 to 1380AD for the arrival of the Maori and the Polynesian rat, kiore or Rattus exulans.

The rats ate "plants, fruits, seeds, insects, lizards, snails, eggs, and the nestlings of ground-breeding birds".

Maoris were apparently responsible for the extinction of the largely predator-free [ostrich-like] moa, and the reduction in forestation of New Zealand from about 85-90% to 25-27% (according to the article). Good evidence suggests this latter was due to significant burn-offs - for various reasons, including living space, ease of travel, and fostering food sources, specifically the bracken root, a starchy staple.

From that first wave, human and rat between them precipitated a 50% decline in bird species and the extinction of bat, frog, and numerous lizard species.

The oldest verified archeological site in NZ is Wairau Bar in the South Island, dating to 1285 to 1300. Amongst many artifacts are the bones of 8000-odd moas and 2000 moa eggs - suggestive of the ease with which they were killed.

The Polynesian rats apparently took less than 80 years to spread throughout the North and South Islands - about the same time as it took the European rat, Rattus norvegicus, once introduced.


This is not a tale of Maori destruction - rather, it's the familiar one of human destruction: "The first duty of colonisers is to survive. That requires rapid population growth sustained by consuming the richest resources" (Atholl Anderson, Australian National University).


The main reference for the article is: Wilmshurst, J. M., A. J. Anderson, T. F. G. Higham and T. H. Worthy. 2008. Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proceedings of the National Academy of Sciences 105:7676–7680.



*Albeit some theories (discussed earlier, here) posit complete submergence of the land mass for a period of time. I'm not convinced of this, particularly given the continued existence of the flightless ratite moa, discussed here.
**The recently discovered 'waddling mouse'/'sb mammal' -see here and here - notwithstanding. There doesn't seem to be any evidence that it impacted the significant and burgeoning terrestrial habiting bird population.

Multituberculates - early successful mammals

Multituberculates are an early order of mammals that was very successful - as mammals go - during the Mesozoic era (in the shadow of the dinosaurs), but became extinct in the Paleozoic, at a time when mammalian species were radiating to fill the gaps in environmental niches left vacant when the dinosaur-killing meteor hit at 65 million years ago.

Their molars each have rows of tubercles - cusps - hence the origin of the name. Teeth are frequently an important determinant of how and what any given species eats, and thus their longer-term survivability. Much analysis has been given over to this aspect of mammalian evolution. (Yet toothed names are sometimes given simply because teeth are all that are found of a given species).

Multituberculates (covering a variety of species and environmental niches) survived to the end of the Eocene (according to Kielan-Jarowowska), perhaps up to 34 milion years ago. Their niches were roughly those filled today by rodents, who are perhaps the most successful - and definitely the most species-numerous - modern mammals.

That might suggest that the Rodentia order actually crowded out the Multituberculata order, and was more fit in evolutionary terms. In fact, Kielan-Jarowowska reports a 1966 review (Van Valen and Sloan) that concluded their demise was due, successively, to condylarths (early placentals, now extinct), primates, then rodents.

As an older lineage, they were close to the monotremes (which are the most basal surviving lineage of mammals), but were slightly more derived ("modern"). Are Multituberculates egg-laying (oviparous) or live birth (viviparous)? Kielan-Jarowowska (from an older study) refers to them as viviparous with an "extremely small neonate". It lists this as a competitive inferiority, along with an abducted (or sprawled) limb angle. This latter would have made prolonged running more difficult than for its competitors.

Note that the New Zealand fossil, the SB mammal or "waddling mouse" had a 'somewhat abducted' posture. Yet that creature survived until 19 mya - isolated on an island chain that had no other mammalian competitors.



References
Kielan-Jaworowska Z, Cifelli R L, and Luo Z-X (2004): Mammals from the age of dinosaurs : origins, evolution, and structure. New York, Columbia University Press.
Worthy T, Tennyson A, Archer M et al (2006): Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific in PNAS vol 103 no 51.

Surviving early mammal lineages

Researching mammals seems to be a particularly popular occupation. Most of that research relates to extant species and their evolutionary history. So it's mostly eutherians (placentals), metatherians (marsupials), and monotremes. It's hard to find much detail about extinct lineages such as New Zealand's mysterious SB mammal fossil.

I'm currently browsing with curiosity a hefty tome called Mammals From The Age Of Dinosaurs. It's about as comprehensive as one can get, and is destined to be the reference book for early mammals. Written by three of the leading experts (and most widely quoted) in the field of mammal paleontology: Zofia Kielan-Jaworowska, Richard Cifelli, and Zhe-Xi Luo. It's dated 2004; to get anything more current, you'd have to be constantly scouring the journals (which is not a bad thing, as these are times of rapid change in knowledge and understanding in this field.)

The book contains many seminal reference points, including a full survey of distribution by location and period (to just past the Mesozoic boundary), and a fully detailed survey of each major lineage of the Mesozoic.The diagrams I find particularly useful are - two (alternative) cladograms of all major mammal taxa up to eutheria (pp521 & 522); - an overview of the changed view of the evolution of the major lineages (p5); - most importantly, a diagram of the temporal distribution (through the Mesozoic) and relationships of the main lineages (p3); - a clade table (listing) of all lineages down to family level (pp 14-15).

Of major interest is the comment (p13) that only four major lineages have a significant presence after the KT boundary (end of the Mesozoic, and the dinosaurs).

Four? To the extant lineages mentioned above, the book adds multituberculates (p15). In a footnote, they elaborate the list with the multituberculate suborder Cimolodonta (lasting to the Eocene), and one dryolestoid from the Paleocene of South America. However, that note is not complete, as there are scatterings of other multituberculate taxa that are mentioned as passing through to the Paleocene. These include Ptilodontidae, and Gondwanatheria. The latter are admitted as uncertain placement (Incertae sedis) - somewhere between monotremes and (metatherians plus eutherians) - but discussed with multituberculates.

Ptilodus, a Ptilodontid

[Update 27-Mar: Dryolestes is Trechnotherian - a clade (a superset of both eutherians and metatherians) which covers all mammals that give birth to live young. I'll now exclude these from the discussion, since I'm focusing on egg-laying mammals, which it looks like the SB mammal is.]

So far, then, we have three non-therian - egg-laying - groupings surviving into the Paleocene (which ended 55 million years ago): Monotremes, multituberculates and, arguably, Gondwanatherians. To this, we now add the even more enigmatic SB mammal, surviving all the way to the Miocene, 19mya.


So what does this say about the SB mammal?

On the one hand, Worthy et al place this mammal in an unresolved trichotomy with multituberculates (which it says are more basal) and the more derived clades that include (Tinodon + the viviparous therians). In effect, pretty close to multituberculates, but no match. On the basis of the femur fragment (specifically, the greater trochanter), it's more primitive than the latter - but that's predicated on the femur and jaw fragments matching. Parsimony suggested so, but it's not a guarantee.

On the other hand, the paucity - and piecemeal nature - of the book's references to non-therian KT survivors is a good reminder that we are dealing with a matching scarcity of pertinent fossils. What has been reported so far should not be taken as a complete and reliable guide to what did survive. New Zealand has, after all, sheltered such oddities as the lizard-like Tuatara and the Leopelmatid frog, no less surprising in their uniqueness.


Next up: more on multituberculates.




References
Kielan-Jaworowska Z, Cifelli R L, and Luo Z-X (2004): Mammals from the age of dinosaurs : origins, evolution, and structure. New York, Columbia University Press.
Worthy T, Tennyson A, Archer M et al (2006): Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific in PNAS vol 103 no 51.

Birds, ratites, moas, kiwis, predators

This is a story about native New Zealand birds and animals, one that gets more interesting as it goes.

First, I'll start by drilling down through the origin of birds.

Birds are members of the taxonomical class Aves, and are the only living descendants of dinosaurs.  The earliest recognised Ave is the archaeopteryx, dates from the Jurassic period, about 250 million years ago (mya). As a result of the dinosaur-killing KT meteor 65 mya, most near relatives are gone: apart from birds (as descendants), the closest remaining relatives to dinosaurs are the crocodilians. In fact, what we now know as birds are the Avian sub-class Neornithes, and are not direct descendants of the archaeopteryx.

The earliest division of modern birds is between the two superorders Palaeognathae - nearly all are ratites - and Neognathae - the rest. Ratites (properly, the order Struthioniformes) are effectively one of the oldest surviving avian lineages - and they're all flightless, descending from a flying ancestor but at some point no longer needing to fly.

Not all flightless birds are ratites. Many different bird families have evolved flightlessness on islands, where they have found themselves with no predators. Richard Dawkins notes that rails in particular "lend themselves to island-hopping followed by flightlessness".

Why lose flight? Colin Tudge: "flight ... exacts an enormous price. It requires commitment from the entire anatomy, and a huge input of energy".

Ratites are the most well-known of the flightless birds, encompassing the ostrich, emu, cassowary, rhea, kiwi, moa, and extinct elephant bird. Between them, they represent pretty well all the current pieces of the old southern land mass Gondwana. That suggests survivability of this grouping, even when exposed to predators. Some were pretty large, though, but perhaps ironically the largest ones, Madagascar's elephant bird and New Zealand's moa, survived only until the last millenium - finally a victim of the greatest predators, humans. The elephant bird was the heaviest bird ever, at up to 500kg and three metres tall.

cassowary

The ostrich is hard to place, but Dawkins reckons it was with the elephant bird when India and Madagascar became isolated from the rest of Gondwana then, like other species, took the fast "Indian ferry to Asia", later to take a slow way back to Africa.

Moas diverged from the rest only about 80 mya - when New Zealand split from Gondwana. A couple of notes about their traditional representation in New Zealand: first, they did not tend to hold their heads fully erect, as you see in the museums. Rather, the head was closely level with the body, for browsing. Second, one of their predators was something called Haast's Eagle, the largest eagle that ever lived. This extinct giant eagle is something I certainly never heard about growing up in New Zealand. Intriguing.

Anyway, given the robust ratite's story, you'd expect the kiwi to be closely related to the moa. Well, it's not. It's closer to the emu and cassowary, both from Australia and New Guinea. Dawkins suggests the kiwi arrived in New Zealand via New Caledonia.

kiwi

It's worth considering the effects of predation over time.  For the elephant bird and moa, size was a boon, but ultimately the source of their demise as they became relatively fearless. For the kiwi, nocturnalism may have been a recent survival mechanism; yet Wikipedia notes that in predator-free sanctuaries, the notoriously nocturnal kiwi is often seen in the daytime. For others, speed has been on their side.

I mention all this to help give context to the unusual 19myo mouse-size SB mammal fossil (discussed here and here) recently found in NZ. Predation has to be a significant issue in the story. How did it survive 60 million years in NZ? What size changes might have happened to avoid predation in a land of bird seeking small morsels? Why did they ultimately disappear? This species doesn't seem to have had any impact on the NZ story as it is known so far...



Reference
Dawkins, Richard (2004): An Ancestor's Tale. Phoenix, London.
Tudge, Colin (2000): The Variety of Life. Oxford University Press, Oxford.

Thingodonta: a salient lesson in analysis

So much to do, so little time. I would do more on non-paleontological subjects, but they keep getting crowded out...

New Zealand's SB mammal fossil has been interpreted as more archaic than therians (marsupials and 'placentals', referred here as metatherians and eutherians), although on a relatively similar par with monotremes, Australia's earliest known mammals and the world's most ancient extant lineage.


Mike Archer (et al)'s book Australia's Lost World includes a relevant anecdote about mammal classification.

The mammal is now called Yalkaparadonta, but started out as Thingodonta, because the teeth, uncovered first, were unlike any known clades of mammals.

A few months later, a jawbone turned up. It demonstrated three molars, where all plesiomorphic (both primitive and derived) metatherians had four [apart from some specialised groups] but eutherians properly had only three. Further, dental action was much more akin to that of eutherians, ie this would be the first early 'placental' in Australia.

In the third step, a skull turned up. Evidence now suggested it was a metatherian albeit quite primitive, and with a front section unlike any other.

Classification had switched back and forward; fortunately publication hadn't happened until the fuller story had emerged.

This creature isn't even listed on Paleos, although there's an entry at the Australian Museum site Australia's Lost Kingdoms and Wikipedia has some good (sourced but broken-linked) discussion on the classification difficulties. That discussion ultimately placed it in its own order, with some dissent from one Frederick Szalay who placed it as a (primitive) diprotodontan metatherian.

Some distinctiveness, some alignment with metatherians and with eutherian insectivores, with a good dose of convergent evolution thrown in.

Ah, convergence. Must be the bane of a taxonomist's life. But the lesson is that classification can be disrupted by hitherto unseen characteristics. For example, in the SB mammal, the femur suggests an abducted, or sprawling gait, although not as much so as monotremes. Yet that abduction must help place it, as it is placed, close to monotremes.

One must easily accept that the paper on the mammal brings together all current knowledge - at the time of publication. Yet it's understandable if there's much keenness to uncover more evidence, as a femur and two jaw fragments tell a tantalisingly curtailed story.

Reference

Archer M, Hand S J, Godthelp H (1991): Australia's Lost World: Riversleigh, World Heritage Site. Reed, Sydney.

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.

Mammals 3: mammaliformes

Again: What is a mammal?

Of course, it's hard to draw the line. Any line is arbitrary, and doesn't reflect the incremental nature of evolution.

Taxonomists must be particularly grateful that the fossil record is gappy. The result is more harmonious: the appearance that ancestor and descendant species are somehow separate, whereas each evolutionary lineage is best represented as a continuum.

There are a number of characteristics that go towards making an animal distinctly mammal. The "canonical" signs include:
- the movement of the articular and quadrate bones from the jaw to the ear (as anvil and hammer)
- hair
- secondary palate (which enables mammals to eat and breathe at the same time).

Colin Tudge reckons homiothermy is a defining characteristic. This allows mammals to simply burn heat to keep warm, and is seen as a particular evolutionary advantage.

In a previous post, I mentioned the definition that mammals are the common ancestors (and all descendents thereof) of monotremes, marsupials, and placentals.

Other relateds are thereby referred to as mammaliformes. However, this is quite arbitrary, as the line is drawn at those currently living. Egg-laying monotremes are thus admitted in somewhat anomalously. They are non-therian mammals.


The split between mammals and reptiles occurs from the amnoite level. Reptiles (and birds and dinosaurs) are sauropsids. The mammal line of descent from amniotes runs through synapsids (from the Carboniferous period) to therapsids (mid-Permian) to cynodonts (late Permian). Synapsids through to cynodonts have been at various times in the past referred to as "mammal-like reptiles".

From there, we have the mammaliformes. The oft-cited Paleos is a fairly authoritative web site, referenced in Wikipedia (amongst many others) and with copious primary references (Paleos itself is being slowly expanded as a wiki here). The journey from cynodonts to mammals is detailed here.

Only trouble is, it's not quite in harmony with the paper on New Zealand's SB mammal (aka "waddling mouse"). The paper describes them as "more derived than morganuconodonts, and more primitive than multituberculates". However, the cladogram in the paper differs from that in Paleos. Both quote a nearly-identical set of sources, one of the main of which is a Chinese paleonotologist called Zhe-Xi Lou, who has done extensive work on early mammalian taxonomy.

More to come.

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.

The SB mammal: New Zealand's non-therian mammal

The traditional view of New Zealand fauna is that its wide range of unique bird life flourished due to the absence of mammals to prey on or compete against them. That is, until the arrival of humans (who also brought rats and dogs) around a thousand years ago.

In fact, there are at least three species of native mammals - but they are excluded in the above narrative - bats, seals and sealions.

taking a long step back, New Zealand was once part of the great southern land, Gondwana, finally separating around 85 million years ago (see Wikipedia and University of Waikato) - the former indicates New Zealand separated specifically from the Antarctic land mass before Australia did; however, it's not clear whether Australia was at the time counted as Antarctica. It seems to me most likely that Australia was its last port of call, since the coastlines match so neatly.


In 2006, a mouse-sized mammalian fossil 16 to 19 million years old was discovered in New Zealand's St Bathans fossil bed, in the south of the South Island. This has been written up in PNAS (Proceedings of the National Academy of Sciences [of the USA], available here.  Interestingly, in the paper it is said to be "nontherian". That is, it split from the mammal lineage before marsupials: therians are defined as non-egglaying mammals: thus this one (the 'SB mammal') would be defined as an egglayer.


Colin Tudge's 2000 book The Variety of Life details the metatherians (marsupials) and eutherians (so-called placentals), then mentions eight groups of nontherians, of which only the monotremes (Australia's platypus and echidna) survive; of the others, only multituberculates and morganucodonts are mentioned. Wikipedia has an entry on Prototherians (aka non-therians), but Tudge doesn't recognise this as a clade, only as a small-p colloquial term for a non-therian mammal. Wikipedia adds to the group Triconodonta and Docodonta; as you can see, the classifications are mainly based on tooth characteristics. Why? Tudge: "Most of the extinct lineages are known only from a few teeth and jaws" - prescient, as this is also the case for the SB mammal, with the addition of a bit of hip. (Braincase is also mentioned as a differentiating characteristic.) I've seen no word on the placement of this creature within non-therians (Wikipedia's entry says the SB mammal is not a monotreme) - probably not an easy task, and the discovery's only about a year old.
I remain curious, too, about the other three of Tudge's non-therian clades; however, Tudge constantly reminds us that at this level of taxonomy, judgment calls are inevitably involved.

Eutherians ('placentals') and marsupials split 100 to 125 million years ago; this specimen dates from 16 to 19 million years (see the entry in New Scientist), suggesting this creature survived for at least 100 million years. Notably, this is the first land mammal fossil found in New Zealand.

The ubiquitous Mike Archer, current UNSW Science Dean, was involved in this study. He suggests it goes against the conventional wisdom that New Zealand temporarily disappeared below the water from 25 to 30 million years ago [due to some combination of rising oceans and/or erosion] after separation from Gondwana, and re-emerged due to its location on the boundary of two techtonic plates pushing against each other. Archer reckons it was too small to succeed at rafting [so it must have remained indigenous since before the split; others disagree.
So, did New Zealand stay above water, or did the SB mammal raft?
I'm currently reading Richard Dawkins' The Ancestor's Tale (2004, Phoenix, London), and he is a great believer in the ability of rafting to explain movements. In a nutshell, rafting is the drift of land-based animals on floating vegetation masses, typically due to the effects of storms. Dawkins (p150) details a large-scale eyewitness event (iguanas in the Carribean) and makes the case that, although this may call for highly improbable events, over a long enough period of time these can come to pass.

However, New Zealand's submergence (as well as erosive forces - see the U Waikato link) does explain the lack of landbased fossils.
What then, of the Tuatara? When I was growing up, the Tuatara was represented on the five cent piece, and typified as a rare lizard - preserved due to isolation on several island just off New Zealand's mainlands. They are actually uniquely-surviving lizard-like Sphenodontians, creatures that flourished 200 million years ago. Further, they show unique cold-climate adaptions over the original warm-climate sphenodontians. That's not unexpected in the time that they've been isolated, but it suggests they were in New Zealand when it was in a warmer climate. It seems to me less likely than that they rafted as recently as 25 million years ago, subsequently disappearing in their home location.
Neither explanation is fully satisfactory. On the one hand, New Zealand entirely lost its land-based fauna. On the other hand, two uniquely survived - at least until recently, and certainly after New Zealand's purported submergence.

It remains a puzzle for the moment.

Further discussion of this unusual find includes a number of reference links.