Friday, February 26, 2021

Tuatara, One of a Kind Survivor

The biological and paleontological history of New Zealand’s tuatara troubles me.  Might the fate of a few thousand tuatara speak to our own in this period of the sixth mass extinction?

I am easily distracted, a propensity that has been exacerbated by the pandemic.  Certainly giving in to a new, random impulse requires less energy than seeing something through to its conclusion.  This post was born of a distraction that proved so irresistible and interesting that it brooked few further distractions and, as a result, I may have come to a sort of exhausted closure.  All of this spins off a postage stamp.  As I have noted once or twice, I not only carry the “burden” of collecting fossils, I am also a stamp collector.  These interests sometimes intersect.  (See, for example, the post titled Where Worlds Meet or Perhaps Collide.)  They do at the outset of this post as well.

I have a fondness for the postage stamps of New Zealand, specifically those issued from the 19th century up till about 1950.  The commemorative stamps often draw on the uniqueness of New Zealand, portraying its history, cultures, flora, fauna, and geography in striking designs that are typically superbly engraved.  In a series of pictorial stamps (so-called Second Pictorial) that had its original release in 1935, the 8 pence stamp stands out.  It features the image of a tuatara, a reptile endemic only to New Zealand.

This engraved image was designed by New Zealander Leonard Cornwall Mitchell (1901-1971) who was responsible for some 90 stamps for the post office department over the course of 40 years.  (Design – Victorian Design to Modernism, 1980s to 1940s, Te Ara – The Encyclopedia of New Zealand; see also, Mitchell, Leonard Cornwall, 1901-1971, New Zealand’s National Library.)  (The story of this 8 pence stamp is for a different time and venue, though it resonates with the theme of this post.  I will simply note that the individual stamp shown above was printed in England during the Blitz, the German bombing campaign against England in 1940 – 1941.  As such, it is a survivor of the awful vicissitudes of war.)

The species of tuatara on this stamp is Sphenodon punctatus Gray, belonging to the taxonomic order Rhynchocephalia (also called Sphenodontia).  Its common name, tuatara, means “peaks on the back” in Māori.  The reptile, weighing as much as three pounds and being up to nearly three feet in length, is presently found in small numbers in the wild on only a handful of offshore New Zealand islands.  The tuatara was driven out of the New Zealand mainland by the arrival of humans, and the dogs and rats that accompanied them.  The animal is long lived (upwards of 80 years in the wild), matures late (at roughly 14 years of age) and reproduces exceedingly slowly (females lay small clutches of eggs only every 2 to 5 years).  It survives in relatively cold temperatures.  Long-lived, for sure.  The tuatara named Henry living in captivity, shown below in a picture from 2007, sired offspring in 2012, when he was 111 years old.

(This image, downloaded from Wikimedia Commons, is in the public domain and licensed under the Creative Commons Attribution 3.0 Unported license.)

Despite the superficial similarities, the tuatara is not a lizard, having evolved separately from the order Squamata (lizards and snakes), beginning roughly 250 million years ago.  An initial signal to scientists that this was not a lizard was its dual rows of upper teeth, seen in the interior skull view (upper right) in this illustration of a tuatara that appeared in The Zoology of the Voyage of H.M.S. Erebus & Terror 1839 – 1843 (Volume 2, Reptiles, Fishes, Crustacea, Insects, Mollusca, 1845).  (This was one of many publications that resulted from the scientific exploration lead by James Clark Ross in command of the H.M.S. Erebus and Terror.  This expedition explored the flora, fauna, and geography of the Antarctic region, including New Zealand.)

The specimen in this illustration is identified in the publication as Hatteria punctata and was found on New Zealand’s North Island.  This image was downloaded from the Museum of New Zealand/Te Papa Tongarewa.

The tuatara is a taxon in the amniote order Rhynchocephalia whose members are distinguished not only by the “enlarged palatine tooth row near parallel to the maxillary row” (i.e., that upper dual rows of teeth), but also by having teeth fused to the jaw (so-called acrodont teeth).  (Marc E.H. Jones, et al., A Sphenodontine (Rhynchocephalia) From the Miocene of New Zealand and Palaeobiogeography of the Tuatara (Sphenodon), Proceedings of the Royal Society B, January 20, 2009.)  It occupies a unique niche among the six “terminal taxa” of the amniotes shown in the tree below.  (Amniotes are those animals whose fetuses develop within a fetal tissue membrane called the amnion.)

Evolutionary biologist Marc E.H. Jones makes the tuatara’s unique status quite clear:

The animal group known as “amniote vertebrates” includes more than 30,000 species divided between six major radiations:  mammals (5,416 species), turtles (341), crocodylians (25), birds (at least 15, 845), lizards and snakes (10,078) and (tuatara).  (Not a Lizard, Nor a Dinosaur, Tuatara Is the Sole Survivor of a Once-Widespread Reptile Group, The Conversation, May 11, 2017.)

Sphenodon punctatus is not only the sole species of tuatara in existence, it is the single extant representative of an entire taxonomic order, the Rhynchocephalia.  This uniqueness means that the tuatara is

an extremely important component of extant biodiversity.  The species has played a key role in phylogenetic analyses investigating the origins of turtles and estimating the divergence dates of major amniote clades.  (Marc E.H. Jones and Alison Cree, Tuatara, Current Biology, Volume 22, Number 23, December 4, 2012.)

But the extreme paucity of species in the order Rhynchocephalia belies the diverse fossil record in the Mesozoic Era for this order, encompassing more than 40 different species.  The Paleobiology Database records the earliest known fossil of this order at some 247 million years ago (in the Triassic Period), and lists different fossils through to the end of the Cretaceous Period.  Following the Cretaceous, rhynchocephalian representatives are scarce.  Overall, these fossils appear in 67 collections and were found in many locations, including Triassic formations in the United Kingdom and the United States; Jurassic Period formations in Germany and France; and Cretaceous Period formations in Argentina.  After that, the database cites only finds in New Zealand – 2 in the Miocene Epoch and 13 in the Quaternary Period (2.58 to 0 years ago).

This record includes taxa with a variety of morphological configurations, including a Jurassic genus described by Jones and Cree as “small and gracile,” and another genus as “long-bodied and aquatic, plus a Cretaceous genus from Argentina that was “a large, heavily built herbivore.”

Why does only a single species survive from this long line of Rhynchocephalia?

Jones and Cree suggest some possible explanations.  Perhaps members of this taxon were unable to compete with lizards and mammals.  Perhaps climate change worked to shrink this order to a single species.  I would observe that, clearly, the tuatara is ill-equipped to withstand many challenges to its environment, be those related to the climate or to predators, because its reproduction rate is so dramatically slow it cannot easily recover from losses.  Its inability to withstand rising temperatures also puts it at risk.  Perhaps the other Rhynchocephalia taxa that have gone extinct were similar in these attributes.  Further, given that the fossil record of this order nearly disappears after the Cretaceous, one must also consider a role for the extinction event at the end of that period in setting the order on its downward trajectory.

The geologic history of New Zealand adds a further element of mystery to the history of this survivor.  New Zealand is known to be part of Zealandia, “a large submerged continental crustal fragment that rifted from Gondwanaland in Late Cretaceous times.”  (Steven A. Trewick, Hello New Zealand, Journal of Biogeography, Volume 34, 2007.)  The tuatara is considered by some scientists to be part of an ancient flora and fauna surviving from Gondwanaland and one whose distinctiveness is the result of prolonged isolation.  Evolutionary ecologist Steven Trewick and his colleagues observe that this supposition conflates the geological connection to Gondwanaland with a Gondwanalandian origin of the New Zealand flora and fauna.  An alternative scenario would see all or nearly all of the terrestrial portions of Zealandia that split from Gondwanaland having been completely submerged by the end of the Oligocene Epoch, thereby requiring that the present New Zealand flora and fauna be of a relatively recent origin.  They do note,

At present, there is insufficient geological evidence to compellingly demonstrate permanent land or total immersion.  Therefore both perspectives must be considered as real possibilities.

But they suggest that the evidence in favor of the latter hypothesis (submersion and recent colonization of New Zealand) is strong, conforming with molecular data showing such a recent colonization and with the absence of different, specific taxa.

With a more recent emergence of New Zealand, the absence of mammals and snakes is easier to understand as they tend to be the last to colonize over significant water gaps.

Yet, the tuatara is at the heart of a key challenge to this argument:  If Zealandia was completely submerged, how did the tuatara and a few other specific taxa come to be in New Zealand when they had already gone extinct in the rest of the world?

Trewick and his colleagues suggest that the submersion of Zealandia might not have been complete, and these taxa might have survived from Gondwanaland; “perhaps these taxa are indeed descendants of the lucky few survivors from Zealandia that persisted through a period of small, ephemeral islands.”

That’s a troubling scenario:  a handful of Sphenodon punctatus retreating before rising sea levels, living on the meager exposed remnants of Zealandia.  Surviving there until tectonic action brings New Zealand into being at some point in the last 25 million years.  Populating that newly emergent mainland, only to be driven to extinction there by colonizing humans, and, once again, managing to survive only in meager (and threatened) numbers on small offshore islands.  A sad, disquieting story to say the least.

Despite its nearly depleted numbers, I find it hard to consider the tuatara an evolutionary failure, given a track record stretching back to the time of the first dinosaurs.  Still, it’s a cautionary tale, conveying hard truth about the dire challenges faced by species, no matter how resourceful and tenacious they might be.

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