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The toads series comes to SciAm: because Africa has toads too

The views expressed are those of the author and are not necessarily those of Scientific American.


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Don't worry about the small size of that cladogram; a larger version is included below. Western leopard toad photo by Serban Proches, Common European toad by Accipiter.

One of my long-term goals at Tet Zoo has been to complete my series of articles on the toads of the world… actually, this started out as a short-term goal, but it ended up taking rather longer than expected. This enormous, near-globally distributed anuran clade (properly termed Bufonidae) encompasses incredible variation and a huge amount of work has been published on the species involved, on their behaviour and biology, and on how they might be related to one another.

ResearchBlogging.org

As you’ll know if you’ve read any of the previous parts of the series (all of which – until now – have been published at Tet Zoo ver 2: links below), it seems that crown-toads originated in South America* during the Cretaceous or early Cenozoic. All ‘basal’ lineages within crown-Bufonidae consist of relatively small species that are poor at dispersing, produce small egg clutches and lack the large parotoid glands otherwise so typical of this clade. These ‘primitive’ toads have sometimes been grouped together in a non-monophyletic assemblage termed Atelopinae.

* It seems that the earliest toads inhabited western Gondwana prior to the splitting of South America from Antartica (an event that didn’t fully occur until the Oligocene). It might therefore be that the very first member of Bufonidae – even the first members of crown-Bufonidae – lived on Antartica. A ‘crown group’ is the group delimited by extant species; the clade that includes the crown as well as its close fossil relatives is the ‘total group’.

At some stage during the Eocene, what I’ve termed ‘ver 2’ bufonids evolved. In contrast to those earlier lineages, these are typically large to very large toads that produce large to stupendously large clutches, have exotrophous larvae (viz, larvae that feed from the environment, rather than rely on a maternally provided food source), and possess large to enormous parotoid glands. Combined, these features make ‘ver 2’ bufonids supremely successful colonisers of new areas. Van Bocxlaer et al. (2010) wrote of them as exhibiting a ‘range-expansion phenotype’ and described how the evolution of these new features “initiated the episode of global colonization and triggered the major radiation of toads” (p. 679).

The evolution of the ‘range-expansion phenotype’ in toads (the key evolutionary events occurred within the part of the tree marked by the grey box), as depicted by Van Bocxlaer et al. (2010).

In the previous parts of this series, I got through all the ‘atelopine’ toads and then looked at the South American and Caribbean members of the ‘ver 2’ clades Rhaebo and Peltophryne. These taxa seem to be outside the enormous clade that includes all the remaining toads. So, what next? Several phylogenetic studies have found an assemblage of African toads to occupy the part of the tree between Rhaebo and Peltophryne and the generally more familiar Old World and American toad lineages Bufo (sensu stricto!), Anaxyrus (Common American toad and kin), Incilius (Central American toads) and Rhinella (the Cane toad and kin). It therefore seems appropriate to consider these African groups next.

Turning part of the tree on its head

However… in the time that I’ve been writing this series of articles, several major analyses of toad phylogeny and biogeography have appeared. You might say that toad phylogeny became rightfully high profile in 2010, thanks in part to Ines Van Bocxlaer et al.’s excellent paper in Science* (Van Bocxlaer et al. 2010). While broadly agreeing on several major aspects of tree shape, the different studies on toad pnhylogeny differ radically in some important respects. This makes it difficult to make generalisations about the position of particular taxa within the tree.

* In saying this I certainly mean no disrespect to the many other authors who have published on toad phylogeny, sometimes in such august publications as Zoological Journal of the Linnean Society, Molecular Biology and Evolution, Zootaxa and Evolution.

Highly simplified toad cladogram (with some hypothesised biogeographic events marked at appropriate places), mostly based on Pramuk (2006) and Pramuk et al. (2008).

Several phylogenies find New World toad clades like Anaxyrus, Incilius and Rhinella to be nested within an Old World clade that includes Bufo (sensu stricto) as well as many African and Eurasian toad lineages. It’s generally agreed nowadays that toads originated in the Americas (see caveat above) and later spread into the Old World, but in the scenario resulting from this phylogeny [see above], one Old World lineage (leading to Anaxyrus, Incilius and Rhinella) later migrated back into the Americas, presumably during the Paleogene (Pramuk et al. 2008). In this model, Old World toads are paraphyletic with respect to a ‘young American’ clade.

The newer phylogenies recovered by Van Bocxlaer et al. (2009, 2010) switch this relationship, however, and find Anaxyrus, Incilius and Rhinella to be outside the clade that includes all Old World toads. In this scenario resulting from this phylogeny, the ancestors of Anaxyrus, Incilius and Rhinella never left the Americas, and Old World toads are monophyletic, not paraphyletic with respect to AnaxyrusIncilius and Rhinella.

Highly simplified competing toad cladogram (again, with some hypothesised biogeographic events marked at appropriate places), this time mostly based on Van Bocxlaer et al. (2010).

We’ll come back to this issue again later on, but for now I’m going to stick with the way in which the text was originally arranged. That is, we’ll get through the various African and many Eurasian lineages before getting to Rhinella and so on. Time, then, to look at the various African toads. As we’ll see, many of these are peculiar, poorly known little animals, often of uncertain phylogenetic placement and conservation status. If you’re blogging about them, there’s the added complication that they’re obscure enough to be rarely illustrated or photographed…

Time to deal with the Africans

At least some African toads seem to belong to an endemic African clade (as yet unnamed) (Frost et al. 2006, Van Bocxlaer et al. 2009, 2010). For convenience, I’ll refer to it from hereon as ‘the African clade’. Frost et al. (2006) and Pramuk et al. (2008) found ‘the African clade’ to be deeply nested within Bufonidae, close to Bufo sensu stricto and to the ‘young American’ clade, while Van Bocxlaer et al. (2009, 2010) found it to be the sister-group to all remaining Old World lineages.

Tadpole of the Chirinda forest toad Mertensophryne anotis. Yes, it has a freakin' ring on its head. From Channing (1978).

‘The African clade’ consists of several toad lineages traditionally included in Bufo but recently given their own names for the usual reason (this being that they don’t group close to Bufo sensu stricto in phylogenies). These are Amietophrynus Frost et al., 2006, Capensibufo Grandison, 1980, Vandijkophrynus Frost et al., 2006 and Mertensophryne Tihen, 1960 (and perhaps Schismaderma Smith, 1849 too). Some of them are very odd (check out the tadpole shown in the adjacent image).

We’ll be looking at them in the next article.

For previous articles in the Tet Zoo toads series see…

For previous articles on hyloid anurans see…

Refs – -

Channing, A. 1978. A new bufonid genus (Amphibia: Anura) from Rhodesia. Herpetologica 34, 394-397.

Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., De Sá, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green, D. M. & Wheeler, W. C. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297, 1-370.

Pramuk, J. (2006). Phylogeny of South American Bufo (Anura: Bufonidae) inferred from combined evidence Zoological Journal of the Linnean Society, 146 (3), 407-452 DOI: 10.1111/j.1096-3642.2006.00212.x

- ., Robertson, J. B., Sites, J. W. & Noonan, B. P. 2008. Around the world in 10 million years: biogeography of the nearly cosmopolitan true toads (Anura: Bufonidae). Global Ecology and Biogeography 17, 72-83.

Van Bocxlaer, I., Biju, S. D., Loader, S. P. & Bossuyt, F. 2009. Toad radiation reveals into-India dispersal as a source of endemism in the Wester Ghats-Sri Lanka biodiversity hotspot. BMC Evolutionary Biology 2009, 9:131 doi:10.1186/1471-2148-9-131

- ., Loader, S. P., Roelants, K., Biju, S. D., Menegon, M. & Bossuyt, F. 2010. Gradual adaptation toward a range-expansion phenotype initiated the global radiation of toads. Science 327, 679-662.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Heteromeles 8:01 pm 09/2/2011

    Great to see more posts. Are there any theories for the use of that ring on the Mertensophryne tadpole?

    Link to this
  2. 2. naishd 9:48 pm 09/2/2011

    This issue gets some coverage in the next article in the series. And it isn’t the only weird cranial structure seen in ‘African clade’ tadpoles…

    Darren

    Link to this
  3. 3. Ranjit Suresh 10:57 pm 09/2/2011

    If the Old World clade reinvaded the Americas (assuming that phylogeny is correct) would they have used vegetation maps and island hopping to reach South America? In the Paleogene the continents would have been much closer together. So, this would be a toad equivalent of what Platyrrhine monkeys did in coming to the New World.

    Link to this
  4. 4. BrianL 7:48 am 09/3/2011

    I’d say that rafting would be the only possible way for trans-Atlantic dispersal of toads during the Paleogene. Mind you, I think this particular route of dispersal may have been more often taken than normally acknowledged. While platyrrhines and hystricognaths are the popular example, it seems pretty likely that early, fourlimbed and semi-aquatic sirenians, the ancestors of the passeriform sapayoa, New World parrots, trogons, ratites and New World barbets (including toucans) and of course toads took the same route.
    The sapayoa might be a particularly interesting case, as it seems quite possible that eurylaimoids invaded Africa from South America, with the sapayoa being an eurylaimoid re-invading South America. In any case, it appears deeply nested within the clade.

    That newly described African ‘phororhacoid’ could be another example of trans-Atlantic dispersal, though in the reverse direction. Speaking of which, could this bird have anything to do with *Eremopezus*?

    Of course, it is worth mentioning that even recent species have managed to cross the Atlantic. *Sarkidiornis*, for example, occurs in both tropical Africa and South America. The dispersal of the cattle egret from the Old World to the New World has even been historically documented.

    Link to this
  5. 5. John Harshman 10:42 am 09/3/2011

    And while we’re at it, two species of Dendrocygna

    Link to this
  6. 6. vdinets 12:26 pm 09/3/2011

    It’s really difficult to imagine a phororhacoid crossing the Atlantic on a vegetation raft. I keep thinking that there should have been some other way. How certain can we be that there were no land bridges? Could Sierra Leone Rise be a former land mass that had sunk after moving away from the Mid-Atlantic Ridge? Rio Grande Rise and Walvis Ridge could also be parts of a former land bridge, now separated by the Mid-Atlantic Ridge.

    Link to this
  7. 7. naishd 1:53 pm 09/3/2011

    I was getting concerned that this obviously excellent and fascinating article wasn’t going to get any comments – glad I was wrong :)

    With reference to Ranjit’s comment about crossing the Atlantic (comment 3), that ‘young American’ toad clade is thought to have evolved in the Late Eocene or Early Oligocene. It’s therefore most likely that its ancestor crossed via one of the several terrestrial routes that existed at this time – the Beringian, De Deer or Thulean routes. Pramuk et al. (2008) considered this issue at length and thought that a Thulean route was most likely. So, no trans-oceanic rafting required. However, all of this is academic given that – as discussed in the article above – it now seems that the ‘young American clade’ isn’t a clade, nor did the ancestors of the lineages concerned ever leave the Americas.

    Trans-oceanic crossing sure is an interesting thing. Of course, the Atlantic wasn’t as wide in the early Cenozoic as it is today, but crossing it would still be difficult. As stated in some of the comments here, we know that certain animals simply must have crossed it. Manatees must have swam from the Americas to Africa, crocodiles from Africa to America, and thrushes crossed the Atlantic at least three times, for example. As for flightless forms that are unlikely to have swam for 100s of km, I can’t comment much on the phorusrhacoid as I haven’t yet read the paper. But one thing to remember is that – as I’m sure I’ve mentioned before – rafts are not ‘rafts’ as we typically imagine them (that is, a few logs lashed together). Rather, the rafts that have been seen carrying animals across large distances are really big, structurally complicated floating islands. The 1995 raft observed to carry Green iguanas to Gaudeloupe was (if I remember correctly) over 40 m long and partly formed from trees more than 10 m long.

    Darren

    Link to this
  8. 8. David Marjanović 3:18 pm 09/4/2011

    Mind you, I think this particular route of dispersal may have been more often taken than normally acknowledged. While platyrrhines and hystricognaths are the popular example, it seems pretty likely that early, fourlimbed and semi-aquatic sirenians, the ancestors of the passeriform sapayoa, New World parrots, trogons, ratites and New World barbets (including toucans) and of course toads took the same route.

    There is no evidence for paleognaths in Africa before the Miocene. Ostriches come from Eurasia.

    Link to this
  9. 9. vdinets 5:46 pm 09/4/2011

    Darren: what is “De Deer route”? Doesn’t come up in Google search at all.

    Link to this
  10. 10. naishd 8:03 pm 09/4/2011

    Oops. Evidently I have deer on the brain. It should be ‘De Geer’: the northern route between Europe and North America, at times available via Scandinavia and Greenland.

    Darren

    Link to this
  11. 11. BrianL 3:02 am 09/5/2011

    @ David Marjanovic:
    I don’t think there is any evidence of pre-Miocene ostriches in Eurasia either. *Palaeotis* is, as far as I know, not a stem-struthionid but outside the ‘ostrich-rhea’-clade, whatever that might mean. *Struthio* simply does not have a pre-Miocene fossil record and when it does appear it is widely distributed across the Old World.

    Given the generally good fossil record in Paleogene of Eurasia, I think the sudden appearance of the ostriches in the Miocene is suggestive of a dispersal event from Africa.

    Eurasia also offers ostrich analogues in the eogruids, which to me suggests there were no real ostriches these birds had to compete with.

    The Fayum does not include fossils of ratites, that’s true, but being a largely tropical wetland-environment I wouldn’t classify it as prime stem-ostrich habitat anyway.

    Link to this
  12. 12. David Marjanović 3:38 am 09/5/2011

    *Palaeotis* is, as far as I know, not a stem-struthionid but outside the ‘ostrich-rhea’-clade, whatever that might mean.

    There is no ostrich-rhea clade. The ostriches are the sister-group to all other paleognaths together.

    There has never been a phylogenetic analysis with Palaeotis, Remiornis, Eleutherornis or any “lithornithid” other than Lithornis in it. Before that sad state of affairs has been fixed, I don’t think we can say much about where they belong — but at least Eurasia has Eo- and Paleocene flightless paleognaths, while Africa has, so far, nothing.

    Eurasia also offers ostrich analogues in the eogruids, which to me suggests there were no real ostriches these birds had to compete with.

    It suggests to me that the ostriches were cassowary-like forest birds till the eogruids died out. Indeed, Palaeotis from Messel can’t help having been a forest bird.

    The Fayum does not include fossils of ratites

    …but it does contain the mysterious neognath Eremopezus.

    Link to this
  13. 13. David Marjanović 3:40 am 09/5/2011

    Correction: there have been phylogenetic analyses with Palaeotis in it, but they assumed ratite monophyly, contained far too few characters, and didn’t contain any other fossils at all, except sometimes Lithornis as the outgroup.

    Link to this
  14. 14. naishd 10:14 am 09/5/2011

    Thanks for comments above. Unfortunately we don’t know enough about the early evolution of ostriches to say anything about their history with confidence… and phylogenies have put them in just about every conceivable position within Ratitae: in one of the most recent efforts, Bourdon et al. (2009) found ostriches to be sister to rheas + casuariiforms, with kiwi + moa forming the sister-taxon to (ostriches + (rheas + casuariiforms)). An unknown African ancestry has been argued for by some, while an also unknown Indian ancestry – and out-of-India dispersal – has been supported by others.

    Ref – -

    Bourdon, E., de Ricqlès, A. & Cubo, J. 2009. A new transantarctic relationship: morphological evidence for a Rheidae-Dromaiidae-Casuariidae clade (Aves, Palaeognathae, Ratitae). Zoological Journal of the Linnean Society 156, 641-663.

    Darren

    Link to this
  15. 15. BrianL 12:28 pm 09/5/2011

    Basically, both David Marjanovic and I are basing our thoughts on the absence of fossil ostriches from the Paleogene. I think the fossil record of Africa will in the future reveal ostriches, which I just like David expect to have been cassowary-like forest dwellers.

    I do not know if *Palaeotis* being outside the ‘ostrich-rhea’-clade is as much nonsense as David would imply: Given that ostriches might well be sister to all other living ratites, a position of *Paleotis* outside this clade would be significant, as it would make any special relationship between *P.* and ostriches very unlikely, in which case it is gone as a possible proto-ostrich.

    As David rightly points out, *Eleutherornis*, *Remiornis* and lithornithids may or may not be significant to the question, but so far they only prove palaeognaths were present in Europe. Given that at the time palaeognaths most likely had a worldwide distribution (being definately present in South America, Europe, North America, Antarctica (and likely Australia and New Zealand/Zealandia)), their presence in Europe may not be relevant to ostrich origins.
    In fact, I suspect they might be a red herring as David himself seems to imply that ostrich ancestors may have to be looked for in Asia rather than what was the European archipelago. Given that no Paleogene fossil paleognaths are known from Asia at all, suggesting that they hid in Asian forests while eogruids took the ostrich niche, only to displace the latter later seems not particularly parsimonious to me.

    The signifance of *Eremopezus* here is not evident to me. Clearly, large and likely flightless birds were present in the Fayum but *Eremopezus* presumably was a mobile-toed denizen of vegetation-choked wetlands. This makes it a very unlikely competitor/analogue of any known ratite. Surely its existence would not imply the absence of Paleogene African ratites! Also, following David’s own logic the implications of saying ‘but *Eremopezus* was present!* would work the same way for eogruids, yet he proposes that the presence of eogruids in Asia is not at all a problem for his hypothesis of contemporaneous ostriches. I don’t think it neccessarily is either, but the argument amounts to the same as me saying ‘Ostrich absence in the Fayum is not at all a problem as it wouldn’t be ostrich habitat.’

    I would also like to point out that while the fossil record of mammals in the Fayum is good, the (at least described) fossil record of birds there is not all that great and heavily biased towards aquatic types. We know Fayum ospreys, shoebills, herons, storks, jacanas, palaelodids, cormorants and rails (and perhaps *Eremopezus*) but only very few taxa not clearly associated with water. Just very limited remains of touracos and accipitrids, if I’m not mistaken.
    This does not suggest a habitat very conductive to preserving the remains of terrestrial, forest-dwelling and possibly still fairly small palaeognaths. That these birds may still have been small, perhaps still flighted is significant here as the mammalian fossil record of the Fayum is also biased towards larger species and mammals are, generally, much better studied than birds are. Add to that the overal fossilisation bias against small birds and the absence of our proto-ostrich may not be all that significant. Regardless of size, I would not expect the Fayum environment to have been very suitable habitat for these birds anyway.

    That *Struthio* itself is likely ecologically unusual compared to its ancestors is likely. Its sudden appearance in the Old World Miocene is strongly suggestive of its open-habitat, cursorial habits to be adaptations to the spread of grasslands and that being a breakthrough achievement. However, I think the presence of very ostrich-like eogruids in open habitats in Eurasia during the Oligocene and Miocene simply makes it unlikely that ostriches would ‘suddenly’ evolve so strongly convergently with them on the eogruids’ own turf, especially given that they didn’t do so before. The open habitats were already present in Asia during the Eocene and Oligocene. Why didn’t ostriches take advantage of that at a time when contemporary eogruids were not yet at the top of their game so to speak and presumably easier to compete with?

    This alone makes it likely to me that ostriches evolved parallel to eogruids in a region where the latter did not exist. An African origin for ostriches would fit both this and the Miocene ‘take-over’ of *Struthio*. Perhaps, take-over may have to be taken more or less literally here: Eogruids may have been outcompeted by ostriches, I suppose. The large clutch size of ostriches compared to the small clutch size of modern Grues may play a role here.

    Regardless of this argument, I enjoy talking about the succes of Miocene ostriches. I feel, for being large, basal flightless birds succesfully existing in predator-infested Old World communities dominated by large mammals, the succes and uniqueness of *Struthio* is terribly unappreciated.

    Link to this
  16. 16. David Marjanović 6:31 am 09/9/2011

    I’ve read Bourdon et al. (2009). I don’t particularly like it. Correlated characters, no ordered characters, few characters, tinamous explicitly in the outgroup, no fossils in the ingroup.

    I do not know if *Palaeotis* being outside the ‘ostrich-rhea’-clade is as much nonsense as David would imply

    Oh, sorry. It’s not nonsense. Nobody knows, because there is no good phylogenetic analysis of its position.

    the argument amounts to the same as me saying ‘Ostrich absence in the Fayum is not at all a problem as it wouldn’t be ostrich habitat.’

    Point taken.

    I would also like to point out that while the fossil record of mammals in the Fayum is good, the (at least described) fossil record of birds there is not all that great and heavily biased towards aquatic types. We know Fayum ospreys, shoebills, herons, storks, jacanas, palaelodids, cormorants and rails (and perhaps *Eremopezus*) but only very few taxa not clearly associated with water. Just very limited remains of touracos and accipitrids, if I’m not mistaken.

    I see. I didn’t know that (though I should have expected it).

    The open habitats were already present in Asia during the Eocene and Oligocene. Why didn’t ostriches take advantage of that at a time when contemporary eogruids were not yet at the top of their game so to speak and presumably easier to compete with?

    Because the eogruids got into that niche first?

    the presence of very ostrich-like eogruids in open habitats in Eurasia during the Oligocene and Miocene

    Is there temporal overlap between eogruids and ostriches?

    And the African record – it’s not just Fayum; ostriches have not been reported from any Paleogene African site, not even those that have yielded antelope-sized and -shaped hyraxes.

    Link to this
  17. 17. David Marjanović 6:32 am 09/9/2011

    …What is nonsense is to suppose that Palaeotis is the sister-group of an ostriches + rheas clade that doesn’t contain any other paleognaths. Such a clade pretty clearly doesn’t exist; ostriches and rheas aren’t sister-groups.

    Link to this
  18. 18. BrianL 11:52 am 09/9/2011

    @ David Marjanovic:

    The fact that eogruids got into that niche first, is a good reason to me why ostriches would not have evolved in the same Asian open habitats that were eogruid habitat. Not in the Paleogene, and not in the Neogene.

    Eogruids and ostriches did overlap in the latest Miocene/ Pliocene. I’m not saying that eogruids were outcompeted by ostriches, but I think it’s an interesting idea.

    Of course it would be nonsense to suppose there is an ostrich + rhea clade, exclusive of other paleognaths. What I meant was that if ostriches are sister to all other (living) paleognaths, finding *Palaeotis* outside the most inclusive clade that includes both ostriches and rheas would imply a position outside crown-group Paleognathae. In this scenario, *Palaeotis* would be a stem-group ratite/paleognath and therefore irrelevant to the origin of ostriches.

    I know the African record amounts to more than the Fayum, but the Fayum is the most complete or at least best-studied Paleogene fossil site in Africa we have. Considering that its birds are not all that well-studied, this has serious implications for the avian fossil record of other sites. How many Paleogene African avians outside the Fayum do we know of? Make it better:How many terrestrial Paleogene African avians outside the Fayum do we know of? It’s not as if the record is impressive. These sites are still being explored as we speak and are likely to reveal more of their secrets in the future.

    As it happens, the recent discovery of *Lavocatavis* shows that there might be a few surprises awaiting paleontologists researching Paleogene Africa. I’d consider a stem-struthionid to be a much less surprising find than an African phorusrhacid/phorusrhacoid.

    It might also be worth noting that said fossil sites are strongly biased towards northern Africa. If stem-struthionids lived further south, it’s hardly surprising we haven’t found them yet. Africa is and was a big place after all.

    Basically, we can only hope for better and more fossils and discoveries further south. An African Paleogene lagerstätte would be awesome.

    Link to this
  19. 19. David Marjanović 4:40 am 09/12/2011

    The cursor inside the comment window disappeared, and it was completely impossible to make it reappear by clicking anywhere into the comment window. I had to hit Ctrl+A, Ctrl+X, and refresh the page to be able to continue writing. The software here has serious problems.

    Eogruids and ostriches did overlap in the latest Miocene/ Pliocene.

    Interesting!

    An African Paleogene lagerstätte would be awesome.

    As would a Paleocene one from pretty much anywhere!

    Link to this

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