A fossil discovery in New Zealand has revealed a new species of seal that once called Australasia home. Eomonachus belegaerensis is the first monk seal, living or extinct, ever found in the Southern Hemisphere. Its presence in our region turns the evolution of southern seals on its head. Curator Vertebrates Felix Marx talks about what this discovery means.
Monk seals are some of the world’s rarest and most endangered marine mammals. Unlike their largely polar relatives, they prefer the balmy waters of the Mediterranean, Hawaii and – until their extinction there in the 1950 – the Caribbean.
Fewer than 2,100 individuals remain today, and conservationists are now scrambling to save what’s left of Earth’s only tropical seals.
The origin of monk seals has long remained a mystery. Fossils are few and far between, and their closest relatives – elephant and Antarctic seals – live far away in the Southern Ocean.
Until now, what little evidence we had suggested that all of these seals (collectively known as ‘monachines’) evolved in the North Atlantic Ocean, despite their rather disparate geographical distribution today.
The unexpected discovery of a monk seal from New Zealand by a trans-Tasman team from Te Papa and Canterbury Museum, as well as Australia’s Monash University and Museums Victoria, now turns this idea on its head.
A monk seal from Aotearoa
The new monk seal is known from several beautifully-preserved fossils from the coast of Taranaki on New Zealand’s North Island, within the rohe of Ngāti Ruanui and Ngāruahine.
They were found by local collectors and are now preserved here at Te Papa and also Canterbury Museum. Their discovery was a triumph for citizen science, and shows what can be achieved when scientists and members of the public work together.
After identifying the new fossils as monk seals by studying their detailed shape, we eventually described them as a new species: Eomonachus belegaerensis, named after the great sea Belegaer to the west of J.R.R. Tolkien’s Middle Earth (aka the Tasman Sea).
The discovery of such a ‘northerner’ in our region was unexpected, to say the least. What were (sub)tropical monk seals doing in frigid New Zealand three million years ago?
Well, in the past, the waters around Taranaki were a lot warmer than they are today, perhaps by as much as 4 degrees. Perfect monk seal territory, it seems.
Seal evolution revisited
The discovery of Eomonachus in ancient Aotearoa casts light on more than just the evolution of monk seals.
Our observations now suggest that all monachines originated in the Southern Hemisphere. Elephant and Antarctic seals then simply stayed here, while monk seals (and one species of elephant seal) later migrated to the North Atlantic and North Pacific.
This is the exact opposite of what scientists had thought so far, and implies that monachines crisscrossed the equator up to eight times throughout their evolutionary history. This may not sound like much, but is actually quite remarkable, as the warm waters of the tropics are often thought to be difficult for marine mammals to cross.
Jumping between hemispheres means that ancient monachines likely had broad environmental tolerances (unlike their distant cousins, the fur seals and sea lions) that enabled them to spread around the world.
Climate change and seal extinction
So why aren’t monk seals living around New Zealand now? About 2.5 million years ago, many large animals in the ocean went extinct as global temperatures plummeted towards the ‘ice ages’. This event likely spelt the end for our monk seals, leaving their northern relatives as the last vestiges of a once widespread lineage.
Extinction is part of the circle of life, and has shaped ecosystems for as long as they have existed. Yet Eomonachus also serves as a cautionary tale: just like natural climate change drove monk seals to extinction in the south, human actions and global warming today threaten the survival of their northern relatives.
Many more stories of life and death, evolution and extinction, are hidden in Aotearoa’s past. New Zealand rocks hold an abundance of ancient marine life, some of its stretching back to the time of the dinosaurs: from giant marine reptiles, penguins, and turtles to sharks, whales, dolphins, and seals. Much of its remains to be explored – so far, we have barely scratched the surface.
Parts of this blog have been adapted from an article published in The Conversation 11 November, 2020 by:
Palaeontology PhD Candidate Monash University
Curator Vertebrates Te Papa Tongarewa
Senior Curator Vertebrate Palaeontology from Museums Victoria
Senior Lecturer, Department of Anatomy and Developmental Biology Monash University
Hi Felix,
Thank you for your comments. My responses below:
“For example, you seem to present vicariance as a the basic default process that needs to be disproved, while regarding widespread dispersal as “fantasifull”.
I only viewed the chance dispersal mechanism for allopatry as fantasiful because there was no direct evidence. Allopatry requires isolation and by attributing allopatry to movement one ends up imagining rare or chance events to explain both isolation and movement. Vicariance is not a default, but it is consistent with allopatry without seeing up contradictions between isolation and accessibility of localities. Regarding your individual objections:
i) The volcanic islands issue, such as Hawaii, have nothing to do with vicaraint origins of widespread taxa. While each Hawaiian island is ephemeral and of course they were colonized, just as new habitats are colonized from populations from older habitats on mainland areas. The Hawaiian islands are surrounded by a multitude of former islands (now guyots) and also the region was crossed by major large igneous provinces that included subaerial landscapes. Thus there is no need to imagined current Hawaiian biota having accidentally dispersed from far away.
(ii) questions of how large ancestral ranges can plausibly be achieved;
Plausibility has nothing to do with anything. There are species today that have global or near global distributions
(iii) In your scenario, monk seals would have had to gradually extend their range across the very barrier (the tropics) that then later led to vicariance.
The geography of the Pacific in the past was very different to the present, and depending on the age of the ancestral range, the climatic as well as ancestral ecology may have been very different as well. And there is no evidence at all to restrict the origin of Monk seals to the Miocene.
“Plus, there is – at least currently – no fossil evidence at all to suggest the existence of a wide-spread ancestor across the Pacific and the North Atlantic.”
There is no fossil evidence to the contrary either. Fossils rarely provide a complete evolutionary record – something that even Darwin recognized.
“I am keen to see new evidence for the prevalence of vicariance, whether in relation to monk seals or indeed any other biogeographical question.”
That’s great! I did not realize that you were not familiar with the major biogeographic works of recent times on vicariance. I suggest you read the following as a good starting point.
Heads, M. 2012. Molecular Panbiogeography of the Tropics
Heads, M. 2014. Biogeography of Australiasia
Heads, M. Biogeography and Evolution in New Zealand
“I look forward to seeing your data and analyses published. Until then, I think this discussion has achieved as much as it can.”
Well, as above, there is no shortage of that. And there are plenty more in the current biogeographic literature which you can access via Research Gate for example. I think your work will greatly benefit from a general familiarity with biogeography in general and also its spatial correlation with tectonics.
Good reading to you,
John
Felix, I notice you have not responded to my point that BioGeoBEARS is not empirical evidence and further suffers from a programmatic flaw in its failure to recognize vicariance possibilities for basal grade area relationships. I think it is incumbent on you to make a response. We have a president over here who believes that people should believe the election was rigged simply because he said so. Quite a few federal judges have pointed out that saying so is not the same as evidence. So it is with your study. You have presented no evidence that your analysis is based on anything empirical at all. I do hope that TePapa sets higher standards for it researchers and you will present here to your public (who pay taxes that support the museum) the basis for your claim for empirical evidence to support your contention about the origins of these seals. Science has to be more than just saying so, otherwise it is not really science. I am sure you want to avoid that.
Kind regards, John
Hi John,
as I pointed out in my previous reply, BioGeoBEARS is a framework within which different processes/ models (including vicariance) are fitted to empirical data, namely, the morphological and molecular observations that give rise to our phylogeny and the geographical distributions of the species under study. Models are not empirical data, and nor do they claim to be. They are simply a way to explain available observations in the best possible manner, and liable to change as more data are uncovered.
Models make assumptions that ought to be queried. This is true of BioGeoBEARS, but also your own ideas. For example, you seem to present vicariance as a the basic default process that needs to be disproved, while regarding widespread dispersal as “fantasifull”. Vicariance is plausible and likely widespread, but treating it as almost a basic assumption ignores (i) scenarios like volcanic islands, as outlined in my previous reply; (ii) questions of how large ancestral ranges can plausibly be achieved; and (iii) the somewhat volatile nature of certain (especially marine) barriers themselves. In your scenario, monk seals would have had to gradually extend their range across the very barrier (the tropics) that then later led to vicariance. I am unsure what mechanism you envisage for this, unless you assume the tropics were either non-existent during the Miocene or somehow had no effect then. Plus, there is – at least currently – no fossil evidence at all to suggest the existence of a wide-spread ancestor across the Pacific and the North Atlantic.
You are right that statements are not evidence. This is why discussions like this are generally based on data and formal analyses, which can subsequently be tested and challenged with new observations. I am keen to see new evidence for the prevalence of vicariance, whether in relation to monk seals or indeed any other biogeographical question. If you think it exists, I would invite you to collect and analyse appropriate data, and then submit your results to the scrutiny of formal peer review – as we did. Blog posts and comments are powerful tools for communication, but without underlying data they are – as you say – ultimately just “saying so”.
I look forward to seeing your data and analyses published. Until then, I think this discussion has achieved as much as it can.
Kind regards,
Felix
Felix – thank you for the copy of your paper, but you have not demonstrated that your BioGeoBEARS program represents empirical evidence given the fact that it is based on purely theoretical notions of area relationships that cannot distinguish a vicariance origin for a basal grade. I challenge you to demonstrate otherwise – that the recipe is empirical as opposed to a fabrication. Also, I challenge you to justify treating molecular divergence ages as actual or maximal when they are calibrated by fossils that can only provide minimum ages. I would note that for the Monachini you have a phylogeny and distribution entirely consistent with vicariance with a widespread Pacific – Tethys ancestor where the basal divergence separated the Hawaiian clade, then the NZ-Australia clade, and finally they Mediterranean-Caribbean clade. That is clear from the empirical distributional and phylogenetic data. The pattern is entirely consistent with many other plant and animal taxa. No scientific need to create fantasifull trans-global migrations for this group or you have to show that it really is based on an empirical reality rather than a theoretical supposition. Look forward to seeing that defense.
BioGeoBEARS makes the assumption that a paraphyletic basal grade occupies a centre of origin, but this is unwarranted. The problem is discussed in my books ‘Molecular biogeography of Australasia’ (Cambridge UP) and ‘Biogeography and evolution in New Zealand’ (Taylor & Francis).
Hi John and Michael,
Thanks again for your comments. I have sent a copy of the paper to both of you.
It’s true that BioGeoBEARS relies on models, but it ultimately still draws on empirical data in the form of (i) morphological and molecular observations which are used to create phylogenetic framework for the biogeographical analysis; and (ii) the geographical distributions of the species/ fossils under study. Also, BioGeoBEARS does not follow a single, fixed ‘recipe’. Rather, it is a framework implementing a broad range of models, which include vicariance as a possible process and are evaluated against the empirical data using standard model selection procedures (e.g. the Akaike Information Criterion). You are right, of course, that all models make assumptions, which ought to be queried. Pitting models against each other is a step in this direction, although it is admittedly not absolute – i.e. model selection only works on the models actually tested.
All of this means that your comment really comes down to a more philosophical point, as mentioned in the books you cite: should dispersal feature in biogeographical models in the first place? I am more of a marine mammologist than a biogeographer, but it seems to me that, at least in some scenarios, the process is the best and – perhaps – the only explanation. Volcanic islands like Hawaii, which rise above the surface and afterwards only persist for a few million years are a good example. These islands house no life when they first form, hence the ancestors of any endemics later found on them presumably dispersed there. You could perhaps make a case that the ancestral species arrived as part of a ‘normal’ range extension, and that the conditions between the islands and the nearest mainland (wind patterns, etc.) somehow drastically changed post-colonisation so that the island population became isolated. Perhaps, but this starts to paint a rather complex scenario that is as assumption-laden (or perhaps more so) than dispersal, especially where the distances involved are huge. Generally, dispersal is a plausible process, and – in my view – ought to be tested, rather than just excluded a priori.
As far as marine mammals are concerned, it’s maybe also worth considering the nature of the main barrier discussed here, i.e. the tropical waters near the equator. Unlike an isthmus or a mountain chain, which are clear physical obstacles and persist for millions of years, the tropical barrier is ‘soft’ and relatively fickle. Among marine mammals, there are plenty of antitropical species pairs and populations (e.g. right, humpback, fin, and minke; several beaked whales; several dolphins; elephant seals) showing that the tropics can isolate populations, yet are sometimes also porous enough to allow some level of interhemispheric exchange. Dispersal in this context may be ‘opportunistic’, rather than a ‘chance’ event.
Hi Felix. I would point out that BioGeoBEARS is not empirical evidence. It is basically a recipe that works of certain assumptions of area relationships and employs techniques that can create dispersal as an artifact for situations for where vicariance could be the explanation, such as where there is a basal grade of area relationships (e.g. areas A (A (A (A, B))). So, regardless of using a recipe such as BioGeoBEARS I have yet to see any basis for this qualifying as empirical evidence.
I do not have access to your paper, but you can send a copy to me at calabar.john@gmail.com
I would suggest that there is no empirical evidence at all to support the claim that monachines originated in the southern hemisphere and then moved to the north. Even if the new fossil is the basal sister group to the northern seals that could just indicated an original vicariance of a widespread ancestor. I would be interested to see if the authors can substantiate their claim based on empirical evidence as opposed to the usual practice of invoking imaginary centers of origin.
Hi John, many thanks for your comment. The southern origin of monachines is supported by a formal biogeographic analysis (BioGeoBEARS, implemented in R; best model identified via AICc) based on our total-evidence phylogeny. So yes, there is empirical evidence. For details, please have a look at the paper.