Latest List For Flight In Volaticotheres

23121fa0c9eac909f5f79c1e1b376655d859eec44e3e0b1a2b6609a4229ccf37Ichthyoconodon jaworowskorum by Julio Lacerda. Its my banner, why do you ask?

I’ve been less vocal than usual on flying volaticotheres lately, but given a recent jelly flood (from moribund site, no less…) I feel like I need to reaffirm that yes, this hypothesis is still very much viable. Here’s a list of reasons as to why:

  • Carnivory. As stated before and again, gliding mammals are mostly herbivorous; this is kind of Meng 2017’s point, actually. Conversely, Volaticotherium antiquum has not only been compared to insectivorous bats (Meng 2006), but other taxa are large sized and are perfect bird-of-prey analogues in a Mesozoic devoid of other hawking predators.
  • Distribution. From the beginning of the Jurassic to at least the mid-Cretaceous; that’s twice the range of any gliding mammal group. Likewise, Ichthyoconodon jaworowskorum occurs at sea, Argentoconodon fariasorum in southernmost Gondwana and now Sangarotherium aquilonium in the Arctic, where no other mammal would have gone until the Late Cretaceous.
  • Limbs. See here.
  • Teeth. Yes; this is a new development, and the basis of my current research. Stay tuned!

There is a surprise coming soon enough. Stay tuned!

Jugulator amplissimus

Still true

Ichthyoconodon

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Another commission by Julio Lacerda, this time depicting Jugulator amplissimus as a majestic flying mammal. As you’ve known I long argued for powered flight in volaticothere mammals, to which I will up-date my list of reasons:

– Like most eutriconodonts volaticotheres are carnivorous (Kielan-Jaworowska et al 2005, Meng 2006, Grossnickle et al 2013, Sigogneau-Russell & Butler 2016). Gliding mammals are noted as primarily herbivorous, with gliding having evolved to cover large distances in the search for static food sources (Luo et al 2017, Jackson et al 2012). Volaticotherium antiquum in particular is compared to insectivorous bats (Meng 2006) and falls under the insectivore range in  study documenting Mesozoic mammal diets (Grossnickle 2013), while Argentoconodon fariasorum falls under the carnivore/omnivore range in the same study (Grossnickle 2016).

– They are cosmopolitan, with Argentoconodon fariasorum’s occurrence in the Toarcian of South America in particular being noted as rather unusual given its…

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On the earliest ratites

tumblr_inline_p7axqi9hKg1rx4yme_500Lithornis by Jack Wood. Tree-climber, soarer, sniffer, prober. Nothing like a modern ostrich, yet it is closer to it than the woodcocks, storks and vultures it seems to be copying.

So lately with the discovery that elephant birds were nocturnal, that the earliest Neornithes were passerine-like and the adage that the survivors of the KT event were tinamou-like species returning, I’ve had a lot to think about in regards to lithornithids and other ancient flying relatives of modern ratites. I think they are a severely understudied lot, and they demonstrate a massive problem with ornithology and paleontology: namely, they don’t see eye to eye, making all sorts of assumptions without considering one another.

Lithornithids and other early palaeognaths are generally not considered in regards to the evolution of modern birds as a whole, which is something I do indeed think needs to change quickly.

Where To Relate

fig-115Pseudocrypturus cercanaxius fossil.

Lithornithids are a rather controversial group, some studies classifying them as a grade of stem-ratites (i.e. Houde 1988) while others do recover them as a fully monophyletic group (i.e. Worthy 2016); complicating the matters further is that, without available genetic sampling, we’re looking at characters that can be very mutable and deceptive (i.e. Worthy 2016 recovers tinamous as basal palaeognaths when abundant genetic tests have shown them to be deeply nested among ratites, for example).

Furthermore, lithornithids have been simultaneously compared to flightless early Cenozoic ratites like Palaeotis (Mayr 2009), tinamous (Houde 1988, Worthy 2016) and kiwis (Mayr 2009; though see Worthy 2013 for a revision of supposed similarities and how to differentiate them), so assuming they are a monophyletic group their exact placement gets even muddier still.

Based on their relative early age, the various traits seen in the groups they are compared to and the fact that a laurasian origin for Palaeognathae as a whole can be inferred, I think lithornithids are most likely polyphyletic, representing various early palaeognath lineages. However, they can at least be distinguished from flying stem-kiwis (Worthy 2013) and aside from Paracathartes they are different enough from tinamous, so treated them as a grade is for the moment being appropriate. Even if members of the “lithornithid” assemblage weren’t closely related to each other, they for the most part occupied a fairly consistent set of ecological niches.

The sooner we figure out how to map them within Palaeognathae, and if at least some are direct antecedents to modern ratite clades, the better. As seen below, features like their lack of tails and reproductive style could be a good start.

Not Really Grounded

flightAn assemblage of avian flight apparatuses (Houde 1988). Notice how lithornithids are much more similar to vultures than to modern tinamous and fowl, but note also that this does not apply to Paracathartes, which is tinamou-like.

I admit I came across the idea that early Neornithes were tinamou-like way back, so I can see why some researchers might be tempted to characterise the avian survivors of the KT event as fowl-like ground dwellers. However, the fact of the matter is that the idea of early Neornithes being terrestrial and that being the key to their success seems to be washing away. For one thing, many Enantiornithes – which these studies often compare Neornithes too, painting them as the failed arboreal specialists who got their forests burnt down in the KT event – were also terrestrial and aquatic, and with the also mostly non-arboreal pterosaurs gone I think the issue has more to do with superprecocial, megapode-like species being targeted rather than those with parental care. At least a few studies agree that incubation is the more likely cause for Neornithe survival (Fernández 2013, Mayr 2018)

Rather, as noted before Felice 2018 seems to find the earliest neornith to have been passerine-like and potentially arboreal, and looking at lithornithids I am inclined to agree. It is rather well accepted now that lithornithids had adaptations towards perching, with large halluxes and curved claws (Houde 1988, Mayr 2009), and while at least some might have been ground nesters due to the abundance of lithornithid egg finds (Mayr 2009) recent studies vindicate the preference for forest environments in at least Lithornis (Torres 2018).

Thus, unless lithornithids began exploiting unique arboreal niches (which mind you is not entirely unreasonable given how birds as a whole underwent an explosive adaptive radiation in the early Cenozoic, though given the presence of possible Cretaceous lithornithids I’m inclined to disagree), I think the characterisation of tinamous as the “baseline bird” is a bit unwarranted. In fact, aside from Paracathartes, which was tinamou-like in several respects and was probably also a terrestrial burst flyer (albeit much larger than any modern tinamou, about the size of a wild turkey), I don’t think tinamous have any business in being compared to lithornithids at least in terms of lifestyle, and likely represent a rather specialised group by themselves.

Lithornithids aside from Paracathartes are generally accepted to have been decent to excellent flyers, being capable of soaring and even compared to birds like vultures and storks (Houde 1988, Mayr 2009). That ratites dispersed on air prior to becoming flightless is now well accepted thanks to various genetic tests as well as the discovery of the late surviving flying proto-kiwi Proapteryx, so this is rather well vindicated and leads credence to the idea that “better” flyers are more likely to produce flightless forms rather than “poor” flyers.

Lithornithids curiously lack tails much like modern tinamous, but given that Mesozoic birds like Enantiornithes also lacked tailed feathers this is not particularly shocking as the absence of a tail apparently does not constrict agile flyers. That said, given that modern ostriches do have tails, the absence of tails in lithornithids could be a massive stepping stone in figuring out how to locate them in the ratite tree of life: maybe the ancestors of ostriches diverged before tail loss, and the presence or absence of tails in forms like Palaeotis could reflect whereas they are ostriches or not. Likewise, lithornithids appear to have brooded much like modern ratites including tinamous, with males incubating the eggs and raising the young (Mayr 2009), while in ostriches there is still rudimentary monogamy (as in, there is a pair raising the young but the male already forms a harem).

That Lithornis ranks among paleognaths with a good sense of smell (Torres 2018) has been interpreted as suggestive of forest dwelling and perhaps nocturnal habits, a rather stark contrast to the study’s conclusion that diurnality was ancestral to ratites and then lost independently at least four times in flightless species. A few living flying birds do have large olfactory lobes such as cathartids (which as we’ve seen been extensively compared to lithornithids already), and at any rate lithornithids are noted to have large optic lobes as with all flying birds including even the “poor flying” tinamous. The implications for the ancestral paleognath lifestyle seem like an untapped well, and I do think that an ancestral, nocturnal origin for ratites is quite likely.

Probing Into The Past

rhynchokineticLithornithid rhynchokinetic range (Houde 1988). Like modern woodcocks, lithornithids could bend the tip of their upper jaws.

Lithornithids are typically interpreted as probing birds, due to their curved, relatively long bills and pores similar to those found in shorebirds and kiwis. Likewise, their upper jaws can bend, much like those of woodcocks (Houde 1988), and we now know Lithornis itself had a good sense of smell (Torres 2018). I have to admit I have some doubts due to the fact that lithornithids have proportionally shorter jaws than those of most probing birds, which to me implies a more generalistic lifestyle, but the evidence is pretty staggering and undeniable.

At least two early ratites, Palaeotis and Diogenornis, have also noted as having probing beaks and likely being carnivorous (Mayr 2009, Mayr 2017). Assuming they are ancestors to ostriches and rheas (or emus and cassowaries) respectively and not unique lineages on their own, this could have rather large ramifications in how we understand ratite evolution and indeed the evolution of flightless birds:

Typically, modern ratites, as well as other Cenozoic herbivorous birds like, gastornithids, dromornithids and geranoidids, are assumed to have increased in size in the Palaeocene to fill the niche left by herbivorous dinosaurs at a time mammals were still small, and therefore having only been capable of evolving due to the absence of mammalian competitors. Except:

  • Said early ratites were carnivorous/insectivorous, so they became herbivorous well after they became flightless and certainly well after mammalian herbivores expanded
  • At least one lineage of herbivorous flightless birds, the gruiform eogruiids, became flightless well after the rise of herbivorous mammals, the Eocene/Oligocene genus Eogrus still capable of flight in contrast to its vestigial winged, two-toed Miocene relatives (Mayr 2009). The same may also apply to the ratite elephant birds, which diverged from kiwis somewhere between the Eocene and the Miocene (Mitchell 2014), and perhaps also ostriches (which do not have unambiguous pre-Miocene fossil records, which might suggest a more recent flightlessness episode than previously thought) and the mysterious indian Hypselornis (which does retain a hallux)
  • Given the presence of fairly large immediately-post-KT mammal herbivores like Taeniolabis and Peligrotherium, while placentals might have taken their time other mammal groups did not waste any to grow larger.

Longrich 2018 lists lithornithids as among the large sized flying birds that appeared after the extinction of pterosaurs, so this could indeed imply that lithornithids expanded into ecological niches previously held by azhdarchids and other terrestrial inland flyers. Certainly, given that lithornithids were soarers and long legged we could expect them to have filled stork or crane-like niches.

Regardless of the particular nuances of lithornithid lifestyle, we know several species co-existed in the same regions (Nesbitt 2016), lending credence to the idea that this was a diverse bird lineage.

How long did they last?

frThe world’s youngest lithornithid skeleton (Mayr 2008). Though, of course, if Lithornithidae is paraphyletic or even polyphyletic then all modern ratites are lithornithids.

As mentioned before, Lithornithidae may be paraphyletic (a series of lineages leading to modern ratites) or even polyphyletic (scrambled within Palaeognathae), so by technicality the group is alive as the various flightless forms and small-hearted burst flyers we know and love. However, the grade of soaring, sniffing aeronauts is now quite extinct, and the skies now belong to a variety of neognath birds and bats both day and night.

Lithornithids may be represented in the Cretaceous, with possible paleognaths being known from the Campanian or Maastrichtian of New Jersey (Mayr 2009), making them among the earliest Neornithes known; several Cretaceous ornithurine birds like Limenavis have also been suggested to be palaeognaths, but have more recently been discovered to be more basal than the toothed Ichthyornis (Zhang 2013) and the fact that the palaeognath palate is the norm among various Mesozoic bird groups only makes it harder to distinguish concrete palaeognaths. Like other bird groups lithornithids certainly exploded in diversity in the Paleocene and early to mid Eocene, dominating North American and European avifaunas and probably spawning the various flightless ratites in Europe, Africa and South America (and other landmasses like Madagascar, Australia and New Zealand, which we currently have poor fossil records of in this time period).

However, by the mid-Eocene they begin to lose steam, and lithornithids as we know them disappear soon after, the last being a fossil from the Messel Pit that may or may not be aligned with Lithornis (Mayr 2009). Still, flying palaeognaths must have continued to exist for a while in at least the southern continents, given that tinamous still exist in South America and Proapteryx is known from the early-to-mid Miocene of New Zealand; as noted before, ostriches also do not have a pre-Miocene fossil record assuming european ratites are unrelated to them. Of course, whereas any of these can be considered lithornithid grade birds is debatable, and at any rate their ecological relevance in “traditional” lithornithid niches appears to have ended, existing at most as relics that either became terrestrial or died out soon after as the Miocene became drier.

Lithornithid extinction is sometimes blamed on the rise of charadriiform shorebirds, particularly scolopacids (Mayr 2009). Admitely, woodcocks at least occupy a similar ecological niche to that of lithornithids, also being probing, forest-dwelling and even having similar rhynchokinetic skills. However, I feel that this might be jumping the gun since it hasn’t been directly tested, and there are a few cons to this competitive exclusion hypothesis:

  • Lithornithids, while functionally very similar to scolopacids, still have a few fundamental differences, like their flight style (soaring vs duck-like flight), size range (excluding the turkey sized and like Paracathartes, larger Lithornis species reach wingspans above a meter), beak length (no known lithornithid has jaws as long as those of a woodcock) and arboreality. I seriously doubt that there was a near-perfect ecological overlap, especially one that resulted in the extinction of the larger species.
  • Different probing birds co-exist nowadays. For example, scolopacids co-exist with ibises in various ecosystems across the world, with other Charadriiformes such as ibisbills in at least specific localities and, most importantly, with kiwis in New Zealand (Coenocorypha snipes are closely convergent with woodcocks even, being called tutukiwi in Maori due to the similarity with the probing ratites). This extends to extinct palaeognaths as well; many scolopacids are known from the Saint Bathans fauna where Proapteryx occurs (Worthy 2013).
  • Scolopacids first appear in the Oligocene, at least fourteen million years after the last lithornithids (Paractitis bardi, from the Early Oligocene of Canada).

Competitive exclusion from Charadriiformes, let alone scolopacids in particular, is further diffused with the fact that both lithornithids and these neognath shorebirds co-existed from the Paleocene onwards, for at least a span of 20 million years. Therefore, I think that it is more likely that scolopacids moved into niches vacated after the extinction of lithornithids, rather than the other way around. The same applies to other bird groups; for example, unambiguous storks only occur well after the extinction of lithornithids.

Rather, I think lithornithid extinction is more closely tied to the climatic upheavals of the Eocene. Being forest dwellers, these birds would have been susceptible to the ecological disruptions of the epoch as after the Azolla event we see a drop in global temperatures and increased aridity. Forest decline has been implicated as having caused a drop on ratite diversity in the Cenozoic of South America (Agnolin 2016) and most likely killed off European ratites as well (Mayr 2017); it can probably also be inferred as the reason Australia lacks flying kiwis, since other groups like mystacine bats became extinct as it became more arid in the Miocene (Hand 2008). Ancestrally forest dwelling (Agnolin 2016, Torres 2018), ratites were forced to adapt to increasingly more open environments across the Cenozoic, with only a handful of species surviving, sadly none of which being good flyers.

Of course, it is entirely possible that gaps in the fossil record are at fault here, and the presence of at least a few decent flying ratites in the mid-Cenozoic, as well as the mysterious Hypselornis in the Pliocene of India, might imply that lithornithids weren’t quite as done for as early as we think.

Final Thoughts

image_4002_1-Calciavis-grandeiCalciavis grandei and rail friend by Velizar Simeonovski.

Ratites are definitely rather charismatic birds, with moas and elephant birds being a staple of paleontological literature. But I think it is time we take notice of their earlier relatives, from the sniffing soarer Lithornis to the terrestrial hunter Palaeotis. From what we know thus far, we are only finding our way in a strange night, much like the kiwi in the forests of New Zealand.

References

Torres, C. R.; Clarke, J. A. (2018). “Nocturnal giants: evolution of the sensory ecology in elephant birds and other palaeognaths inferred from digital brain reconstructions”. Proceedings of the Royal Society B: Biological Sciences285 (1890): 20181540.

Ryan N. Felice; Anjali Goswami (2018). “Developmental origins of mosaic evolution in the avian cranium”. Proceedings of the National Academy of Sciences of the United States of America. 115 (3): 555–560. doi:10.1073/pnas.1716437115.

Field, Daniel J.; Bercovici, Antoine; Berv, Jacob S.; Dunn, Regan; Fastovsky, David E.; Lyson, Tyler R.; Vajda, Vivi; Gauthier, Jacques A. (2018). “Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction”. Current Biology28 (11): 1825–1831.e2.

Atterholt, J., Hutchison, J.H., O’Connor, J.K (2018) “The most complete enantiornithine from North America and a phylogenetic analysis of the Avisauridae” PeerJ November 13, 2018

Nesbitt, Sterling J.; Clarke, Julia A., The anatomy and taxonomy of the exquisitely preserved Green River Formation (early Eocene) lithornithids (Aves) and the relationships of Lithornithidae. (Bulletin of the American Museum of Natural History, no. 406), 2016-06-30

Houde, Peter W. (1988). “Paleognathous Birds from the Early Tertiary of the Northern Hemisphere”. Publications of the Nuttall Ornithological Club (Cambridge, MA) 22.

Worthy, T., Mitri, M., Handley, W., Lee, M., Anderson, A., Sand, C. 2016. Osteology supports a steam-galliform affinity for the giant extinct flightless birds Sylviornis neocaledoniae (Sylviornithidae, Galloanseres). PLOS ONE. doi: 10.1371/journal.pone.0150871

G. Mayr. 2009. Paleogene Fossil Birds. 1-262

Worthy, Trevor H.; et al. (2013). Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves (PDF). Paleornithological Research 2013, Proceedings of the 8th International Meeting of the Society of Avian Paleontology and Evolution.

Yonezawa, T.; Segawa, T.; Mori, H.; Campos, P. F.; Hongoh, Y.; Endo, H.; Akiyoshi, A.; Kohno, N.; Nishida, S.; Wu, J.; Jin, H.; Adachi, J.; Kishino, H.; Kurokawa, K.; Nogi, Y.; Tanabe, H.; Mukoyama, H.; Yoshida, K.; Rasoamiaramanana, A.; Yamagishi, S.; Hayashi, Y.; Yoshida, A.; Koike, H.; Akishinonomiya, F.; Willerslev, E.; Hasegawa, M. (2016-12-15). “Phylogenomics and Morphology of Extinct Paleognaths Reveal the Origin and Evolution of the Ratites”. Current Biology27 (1): 68–77. doi:10.1016/j.cub.2016.10.029. PMID 27989673.

Fernández, Mariela S.; García, Rodolfo A.; Fiorelli, Lucas; Scolaro, Alejandro; Salvador, Rodrigo B.; Cotaro, Carlos N.; Kaiser, Gary W.; Dyke, Gareth J. (2013). “A Large Accumulation of Avian Eggs from the Late Cretaceous of Patagonia (Argentina) Reveals a Novel Nesting Strategy in Mesozoic Birds”. PLoS ONE. 8 (4): e61030. doi:10.1371/journal.pone.0061030. PMC 3629076 Freely accessible. PMID 23613776.

D. C. Deeming; G. Mayr (2018). “Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their eggs”. Journal of Evolutionary Biology. in press. doi:10.1111/jeb.13256.

Mayr, G. 2017. Avian Evolution: The Fossil Record of Birds and its Paleobiological Significance. Topics in Paleobiology, Wiley Blackwell. West Sussex.

Mitchell, K. J.; Llamas, B.; Soubrier, J.; Rawlence, N. J.; Worthy, T. H.; Wood, J.; Lee, M. S. Y.; Cooper, A. (2014-05-23). “Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution”. Science344 (6186): 898–900.

Longrich, N.R., Martill, D.M., and Andres, B. (2018). Late Maastrichtian pterosaurs from North Africa and mass extinction of Pterosauria at the Cretaceous-Paleogene boundary. PLoS Biology, 16(3): e2001663. doi:10.1371/journal.pbio.2001663

O’Connor, J. K.; Zhang, Y.; Chiappe, L. M.; Meng, Q.; Quanguo, L.; Di, L. (2013). “A new enantiornithine from the Yixian Formation with the first recognized avian enamel specialization”. Journal of Vertebrate Paleontology33: 1

Worthy, Trevor H.; et al. (2013). Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves (PDF). Paleornithological Research 2013, Proceedings of the 8th International Meeting of the Society of Avian Paleontology and Evolution.

Agnolin, Federico L., Unexpected diversity of ratites (Aves, Palaeognathae) in the early Cenozoic of South America: palaeobiogeographical implications Article in Alcheringa An Australasian Journal of Palaeontology July 2016 DOI: 10.1080/03115518.2016.1184898

Suzanne J Hand, Vera Weisbecker, Robin MD Beck, Michael Archer, Henk Godthelp, Alan JD Tennyson and Trevor H Worthy, Bats that walk: a new evolutionary hypothesis for the terrestrial behaviour of New Zealand’s endemic mystacinids, BMC Evolutionary Biology20099:169 DOI: 10.1186/1471-2148-9-169© Hand et al; licensee BioMed Central Ltd. 2009 Received: 28 November 2008

Could ‘Archaic’ Mammals Get Trunks?

image_2256_2e-Vintana-sertichiVintana sertichi cast and skull by Story Brook University. While it doesn’t seem to have a trunk per se, it does already have an enlarged nasal cavity with deep muscle attachment sites. Note also massive xenarthran-like lateral flanges.

As question I sometimes ask myself when designing fictional mammal species is what can and can not have trunks. A trunk as typically defined with a complex fusion of the upper lip and nostrils, resulting in a muscular appendage. Trunks evolved multiple times in placental mammals, but once you get outside of Placentalia they become frustratingly rare.

Among non-placental eutherians zalambdalestids are usually described as having a proboscis like modern sengi and shrews (Archibald 2011, for example), but if this can be described as a trunk sensu stricto is hard to say, as the upper-lip is usually not involved, muscle systems outside of the nasal area are involved and this proboscis is not as flexible as proper trunks, being more used as a sensory organ. Palorchestes and related marsupials have tapir-like nasal anatomy and so far I haven’t found any studies dismissing this, meaning that Palorchestidae was indeed the sole clade of non-placental mammals to have unambiguous trunks, though some bandicoots, shrew-opossums and possibly a few extinct taxa all have shrew-like muscular proboscises.

It’s possible that brandoniid meridiolestidans had sengi-like snouts (Rougier 2008) and Necrolestes has been suggested to have appendages similar to the star-nosed mole. Dryolestoids and other cladotherians in general had fairly therian-like facial muscle anatomy to hypothetically pull out trunks given the opportunity.

But the further back you go the more uncertain things get. Neither eutriconodonts, symmetrodonts not multituberculates have trunks, not even shrew-like nasal proboscises; all tend to have proportionally small nostrils and almost no instances of expanded nasal cavities. While we do know these animals suckled milk and had muscular lips (Kielan-Jaworowska 2004), the exact nature of their facial anatomy and how it compares to modern mammals leaves a lot to be desired.

The further back we go, things continue to become more bizarre, as we do not know precisely when muscular lips evolved in synapsids or which shape they took. Certainly, the development of a fleshy beak in monotremes suggest that early mammals could experiment in ways therians cannot, and vice versa. However, one particular group of non-mammalian synapsids does offer some very interesting answers: haramiyidans.

As opposed to their historical classification as multituberculate relatives, haramiyidans have consistently been recovered as non-mammalian mammaliaforms as of late (Luo 2015, Meng 2017), with Huttenlocker 2018 being of particular interest in both the discovery of the skull of Cifelliodon wahkarmoosuch as well as the placement of gondwanatheres as haramiyidans rather than as mammals. All these taxa have done several odd things with their snouts: in Cifelliodon wahkarmoosuch, the snout is elongated and flat, much as in sirenians, while in gondwanathere skulls we see rather large, expanded nostrils (Vintana sertichi) and massive flanges similar to those seen in ground sloths and glyptodonts (Vintana sertichi, Groeberia minoprioi). None of this indicates a trunk in these taxa, but it certainly suggests that these animals experimented with a variety of labial tissues and have anatomy that could lead to the evolution of trunks or similar proboscises.

So, trunked haramiyidans, and gondwanatheres in particular, are perfect for any speculative evolution projects you have in mind.

Because haramiyidans share the same palinal mastication mechanism as multituberculates, why they have extensively experimented with their lips while multies did not remains a mystery for now. As noted in the description of Vintana sertichi, its is possible that gondwanatheres at least had some subtle nuances in their chewing mechanism multituberculates did not have (Krause 2014).

So there you have it. Surprise me with your trunked abominations.

References

J. David Archibald, Extinction and Radiation: How the Fall of Dinosaurs Led to the Rise of Mammals, JHU Press, 15/03/2011

Guillermo W. Rougier, Laura Chornogubsky, Silvio Casadio, Natalia Paéz Arangoa, Andres Giallombardo, Mammals from the Allen Formation, Late Cretaceous, Argentina, Cretaceous Research Volume 30, Issue 1, February 2009, Pages 223–238, doi.org/10.1016/j.cretres.2008.07.006

Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 14, 531.

Luo, Zhe-Xi; Gates, Stephen M.; Jenkins Jr., Farish A.; Amaral, William W.; Shubin, Neil H. (16 November 2015). “Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution”. PNAS: 201519387. doi:10.1073/pnas.1519387112. PMC 4697399

Qing-Jin Meng; David M. Grossnickle; Di Liu; Yu-Guang Zhang; April I. Neander; Qiang Ji; Zhe-Xi Luo (2017). “New gliding mammaliaforms from the Jurassic”. Nature. in press. doi:10.1038/nature23476.

Huttenlocker AD, Grossnickle DM, Kirkland JI, Schultz JA, Luo Z-X. 2018. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature Letters

Krause, David W.; Hoffmann, Simone; Wible, John R.; Kirk, E. Christopher; Schultz, Julia A.; von Koenigswald, Wighart; Groenke, Joseph R.; Rossie, James B. (2014-11-05). O’Connor, Patrick M., Seiffert, Erik R., Dumont, Elizabeth R., Holloway, Waymon L., Rogers, Raymond R., Rahantarisoa, Lydia J., Kemp, Addison D., Andriamialison, Haingoson. “First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism”. Nature. Nature Publishing Group, a division of Macmillan Publishers Limited. 515: 512–517. Bibcode:2014Natur.515..512K. doi:10.1038/nature13922. ISSN 1476-4687. PMID 25383528.

Eulogy to a Snail

Once upon a time a snail came with the Autumn rains.

From where it came, no one knows, and for how long it stayed is known only to the eyes of light in the ceiling. Still, it must have come from somewhere pleasant, where it grasped and felt wet, marshy soil and not a dry, cold altar, where the wind blowed and the air wasn’t dead and hot, where soothing darkness rather than harsh light surrounded it.

And it left this place, for some reason or another. It happened upon a strange, bright, harsh world, and it curled within. For a moment it was back home: darkness surrounded it, wind enveloped it, water flowed on the landscape of its senses.

And then a boot crushed it.

Its body returned to its home, but now without a life to feel comfort and satisfaction in this reunion. Some of the waters stayed, green and thick, enveloped in a lightless white. Its shell, now fragments, were picked, caressed by unfathomably alien wrinkles veiling another type of whiteness.

Light, paper, bone. It cannot be said that death came unannounced, its heraldic banner so vividly displayed. Yet, it was a pointless affair nonetheless.

Guilt works in such strange ways.

A brief run down of Meridiolestida

Meridiolestida.pngA run down of several meridiolestidans by Christopher Rigobello. Note: artist insisted on external scrotum for Leonardus, though I have my personal doubts.

Though today most mammal faunas are composed of placentals and marsupials, a variety of other groups thrived across the Late Cretaceous and even until fairly recently in the Late Cenozoic.

One of the most successful of these groups were the meridiolestidans, a group of dryolestoid mammals that became the dominant mammals of Late Cretaceous South America, ranging from small insectivores to some of the largest Mesozoic mammals. Even after the Cretaceous/Paleocene mass extinction, they managed to continue to thrive for a while longer, with the very youngest species a mere 10-17 million years ago. Some even suggested a modern presence in marsupial moles, though as we will see this is sadly likely not the case.

In any case, they are certainly an interesting bunch, with several meanderings and successes up to their extinction.

Origins at my doorstep(!)

HenkelotheriumHenkelotherium by ‘Fuzhong!’. Although not a meridiolestidan, this is just one of many dryolestoids native to the Jurassic and Early Cretaceous of the Iberian Peninsula, indicating a local radiation there.

As noted before, meridiolestidans are part of a lineage of mammals known as Dryolestoidea. Some studies have recovered Dryolestoidea as paraphyletic and with meridiolestidans slightly closer to therians than classical dryolestoids like Dryolestes itself (Averianov et al 2013), but most studies seem to recover Dryolestoidea as monophyletic and with Meridiolestidans either nested within the northern forms (Rougier et al 2011, Rougier et al 2012, Chornogubsky 2011, Rougier 2018) or as a sister-taxa outgroup (Thompson et al 2014). If the former, the north american and european genus Laolestes appears to be their closest relative (Rougier 2018).

Dryolestoids are overall similar to therian mammals and relatively closely related to them as far as most phylogenetic studies are concerned, being usually the most basal members of Cladotheria. They nonetheless differ in details of their tooth anatomy like for instance prismatic enamel, “eupantotherian” style molars* and double canines roots as well as their braincase; meridiolestidans themselves differ from other dryolestoids in lacking a parastylar hook on their molars, freeing their teeth for better grinding over piercing, most notably allowing transverse (i.e. side-to-side; imagine for instance horses or sheep) chewing. Non-meridiolestidan dryolestoids have slightly more ‘archaic’ ear and jaw bones than therian mammals, such as a well developed coronoid and splenial and the presence of a Meckelian groove, suggesting a connection of the ear bones to the jaw, but these features are independently lost in many dryolestoids, with mesungulatids and Dryolestes itself even losing the Meckelian groove, so in essence having a “modern” ear.

It’s been suggested that dryolestoids, like most cladotherians, already had erect limbs (Kielan-Jaworowska et al 2004), though personally I think we do need to rethink on how we demonstrate the limb gait in non-therian mammals, given the studies on other seemingly sprawling taxa like phytosaurs and Dimetrodon. The few full skeletons we have generally demonstrate that they lacked venomous tarsal spurs as earlier mammals such as multituberculates, symmetrodonts and gobiconodontids had, and their relatively “modern” ankle bones seem to confirm them as spur-less as far as I am concerned. Like most non-placental mammals, they have epipubic bones, suggesting that they gave birth to fetus-like undeveloped young.

They could have differed in a variety of soft tissue ways; for instance modern marsupial moles, which I regardless of its affinities see as a good functional model for at least a few species, have a true cloaca rather than separated genital and excretory holes, lack a prostate, have backwards-facing pouches and have a weird brain where the olfactory lobes are entirely in front of the cerebellum and a small neopallium. That said, as noted above some dryolestoids convergently evolved a few “modern” traits several times, and therians themselves changed their reproductive anatomy several times, with testicles dropping independently in marsupials and various boreoeutherian groups and a reversion to a true cloaca in tenrecs and marsupial moles, so it’s possible, if in my opinion not particularly likely, that some dryolestoids converged with therian mammals in these regards.

So overall, dryolestoids were similar to therians, if somewhat ‘quirky’.

Dryolestoids first appear in the mid-Jurassic, with amphitheriids being the oldest and most basal representatives. Dryolestoids quickly dispersed across the northern continents, but their Jurassic-Early Cretaceous diversity is noted as being focused on Western Europe, with the Iberian Peninsula in particular being a diversity hotspot (Chimento et al 2016). Sure enough, the portuguese Jurassic mammal fauna is uniquely dominated by dryolestoids compared to other faunas such as the American Morrison Formation’s, and laurasian dryolestoids remain in the Early Cretaceous of Spain longer than in other regions.

The story of meridiolestidans likely begins here, therefore. The earliest meridiolestidans are Donodon and Thereuodon (Chornogubsky 2011), which occur in the Berriasian of North Africa. Other contemporary mammals here, typically considered symmetrodonts or early therians, may also actually be dryolestoids (Chimento et al 2016), suggesting that Meridiolestida began as an African dryolestoid radiation. Their ancestors arrived northward from a temporary connection with the Iberian Peninsula, there being also several other laurasian taxa in North Africa to back up this claim, such as gobiconodontid mammals or choristodere reptiles (Allain et al 2014).

Once isolated in Africa, the earliest meridiolestidans most likely dispersed ecologically. Donodon itself is only known from a single tooth, but it is so similar to that of Late Cretaceous mesungulatids (Bonaparte 2002) that they might indicate a similar herbivorous lifestyle; at its putative rabbit-like size, one can easily call it the hyrax of Cretaceous Morocco. This is already a far cry from the insectivore/omnivore range occupied by most laurasian dryolestoids at the time, and yet another magnificent instance of Mesozoic mammals being more diverse than typically assumed.

The ultimate fate of the African and Iberian meridiolestidans is unknown. There is sadly no mammal fossil record from the Middle and Late Cretaceous of these regions aside from a single gondwanathere jaw (and if gondwanatheres aren’t true mammals then none at all), not even in sites like the famous Kem Kem Beds. Perhaps they became extinct in the Mid-Cretaceous, alongside so many other mammal groups, or perhaps they did endure until the KT event and we simply don’t know about it. The idea of hyrax-like mammals living alongside Spinosaurus is amusing to me at least.

Still, they certainly are gone from Cenozoic deposits in Africa and in Europe, where placentals and metatherians (and, for the briefest of whiles, multituberculates) are the dominant mammals to the exclusion of all others.

*To quote Grossnickle et al 2013. In truth I don’t think I’ve ever seen “eupantotherian” used to describe a molar type anywhere else, as “eupantotherian” anything has fallen into disuse in modern literature.

Refuge in Argentina (before it was disturbing)

Photo-and-line-drawing-of-Austrotriconodon-mckennai-lower-right-premolariformAustrotriconodon mckennai tooth.

Nonetheless, we do know one place where meridiolestidans survived further: South America.

As South America was connected to Africa, its extremely easy to suspect that meridiolestidans moved to there in the Early Cretaceous. Even after the two continents split apart, however, many mammal taxa moved from Africa to South America, perhaps by rafting, such as New World monkeys and rodents; I suspect whichever method of dispersal is ultimately irrelevant, since the result is the same.

What is known is that, by the Cenomanian, meridiolestidans became the dominant mammals of the continent (Pascual 2001, Rougier 2008, Rougier 2009, Rougier et al 2011, Rougier 2012, Forasiepi 2012, Averianov 2013, Chimento et al 2016, Grossnickle et al 2016, Rougier 2018, among most other sources here), ranging in a variety of niches from small insectivores to large herbivores. Other synapsid groups are present in the Late Cretaceous as well, just as gondwanatheres, multituberculates, monotremes and even some therians (Chimento 2016, Castro 2018), but none even remotely approach both the quantity of fossils as well as morphological diversity. Some supposed representatives of other mammal groups, such as the putative eutriconodont Austrotriconodon, have turned out to be meridiolestidans as well.

The diversity of meridiolestidans in South America in some ways mirrors the success of their therian relatives and multituberculates in the northern continents (Grossnickle 2013). The mid-Cretaceous was a period of faunal turnovers that turned out poorly for most mammal groups, with forms like symmetrodonts and eutriconodonts dying off around this time, presumably due to their more specialised carnivorous diets. By contrast, omnivorous groups prospered, quickly expanding to occupy empty niches.

The pre-Late Cretaceous mammal fossil record of South America is extremely poor, but we know that eutriconodonts (Argentoconodon, Condorodon), australosphenidans (Henosferidae, possibly Vincelestes) and multituberculates were present at least (Chimento 216). Meridiolestidans likely moved into the vacant niches after their competitors died off, assuming they did not outcompete them directly as marsupials are sometimes said to have done to Australia’s indigenous groups.

The inner structure of the south american Meridiolestida is usually fairly consistent: a most “basal” group composed of the Cronopio + Leonardus + Necrolestes clade (for the sake of simplicity we’ll call it Necrolestidae), followed by Austrotriconodontidae, Brandoniidae and the clade Mesungulatoidea, which sensu stricto includes Mesungulatidae and a Reigitherium + Peligrotherium clade (here called Reigitheriidae for the sake of simplicity); sometimes, Mesungulatoidea can be expanded to include Austrotriconodontidae and Brandoniidae, though the general relationship between these clades remains the same.

Minor points of controversy include the genus Groebertherium, which is a Late Cretaceous dryolestoid genus sometimes recovered as a meridiolestidan but sometimes also recovered as a non-meridiolestidan dryolestoid (Rougier 2012, Rougier 2018), how Donodon and Thereuodon fit into Meridiolestida (so far, comparisons with Mesungulatidae have been most prevalent, but it is usually classified as outside the south american group) and just how extensive Brandoniidae actually is. Otherwise, however, we have a fairly consistent portrayal of this clade across the various studies, which is quite charming.

 

Cretaceous saberteeth or Cretaceous pigs?

tumblr_inline_p61hn2lE6f1rfitvv_400

Cronopio dentiacutus skull reconstruction.

By far the most famous dryolestoid is none other than Cronopio dentiacutus, the “real Scrat” due to an almost prophetic decision by Michael J. Wilson’s daughter to fuse a squirrel and a sabertooth, resulting in the mildly amusing face of the Ice Age movies. It isn’t the only Mesozoic mammal to bear long canines – another dryolestoid, the portuguese Drescheratherium acutum, also bears elongated upper canines, though they are slightly less elongated and stouter, while some deltatheroideans like Lotheridium mengi are true sabertooths, not only possessing deeper fangs but also being unambiguous predators. Still, it can’t be denied that good ol’ Cronopio has its charm as the face of the fanged fuzzballs.

Like Scrat, Cronopio dentiacutus is a fairly small mammal, at around 20 centimeters long. However, it most certainly wasn’t an acorn addict, if for no other reason that there weren’t any acorns in the patagonian Cretaceous. What it actually ate is something of a controversy: on the one hand, its teeth appear more suited to a generalist omnivorous diet (Rougier 2011; see also Chornogubsky 2011 for the closely related Leonardus, which was capable of ungulate-like transverse chewing), but on the other it ranked among insectivore species in a study documenting mammal jaw shapes (Grossnickle 2013). Given the fossorial characteristics of its relative Necrolestes, it seems likely that Cronopio probably dug for food with its snout, making it fairly akin to a miniature pig. Indeed, from an at least superficial standpoint Cronopio‘s skull strongly resembles that of a suine.

Cronopio‘s fabled fangs are rather delicate and strongly recurved, implying that it did not use them to pierce prey. Rather, they are most likely sexual ornaments like the tusks of many living mammals, though I hesitate to say that they were used in interspecific fights like they are in pigs, chevrotains and many other species. Maybe they could also have been used in foraging to some extent, digging out soft substrate.

Closely related (though depending on the studies it can be closer to the other necrolestid, Necrolestes) is Leonardus cuspidatus. Unlike Cronopio it appears to lack the formidable fangs, but otherwise seems rather similar in terms of overall anatomy and likely lifestyle; it did have three molar roots, a trait otherwise seen only in the mesungulatid Coloniatherium cilinski, but the implications in terms of diet are unclear. Assuming Leonardus isn’t specifically closer to Cronopio than to Necrolestes, this mini-pig lifestyle appears to have been the norm to necrolestids, which proceeded to shift into the mole-like lifestyle we know and love sometime after or even within the KT event.

Meat’s back on the menu, boys

Photo-and-line-drawing-of-Austrotriconodon-mckennai-lower-left-premolariform

Another Austrotriconodon mckennai tooth.

Austrotriconodon is a genus composed of two species, A. mckennai and A. sepulvedai, the former larger than the latter, dating to the Campanian or Maastrichtian in the Los Alamitos formation of Patagonia. Bearing triconodont teeth, it has long been assumed that these animals were relictual eutriconodonts, but it since has been revealed that they were actually meridiolestidans, their triconodont molars having evolved secondarily from an cladotherian shape (Gaetano 2013). Sure enough, you’ll easier notice more similarities to therian carnassials than to eutriconodont molars and premolars.

This dental adaptation appears to have been in response to increased carnivorous habits. Like both eutriconodont teeth and therian carnassials, Austrotriconodon‘s teeth appear to be suited for shearing, allowing a butchering action. Being weasel-to-cat sized animals, it makes sense that they were predators of small tetrapods like other mammals, amphibians, lizards, small dinosaurs, small notosuchians and even sphenodontians (Apesteguia 2003), and thus we can probably see them as the gondwanan counterpart to the contemporary deltatheroideans and stagodontids of the northern continents.

What makes austrotricononodontids particularly interesting is how they’re relatively closely related to herbivorous mesungulatoids. Either they diverged just before the development of herbivory in Meridiolestida, or they might have actually be descended of herbivorous ancestors. With the herbivorous african meridiolestidans mixed in, this becomes a question rather worth making; similar positing that insectivorous afrotheres may all descend from hyrax-like ancestors crop frequently in literature as well (Gheerbrant 2014).

Brandon’s things

CapturarBrandonia intermedia  tooth.

As mentioned before, there is some controversy in regards to how many species and genera Brandoniidae actually contains. For instance, some studies recover Barberenia and other genera as synonymous with Brandonia itself (Chornogubsky 2011, Forasiepi 2012), though given the clade’s success from the Cenomanian to Maastrichtian its diversity surely needs to be addressed further than this. Compared to other meridiolestidan clades, Brandoniidae is sadly rather underrepresented in literature.

From what can be gleaned, brandoniids were insectivores akin to modern elephant shrews (Rougier 2008), and indeed they appear to be mouse sized mammals. While their teeth are suited for transverse mastication, they have sharper cusps than most other meridiolestidans, contextualising it as grinding insect shells rather than plants. Still, as with austrotriconodontids an herbivorous ancestry can’t be denied fully.

Middle Ungulates

bd13bcef2c6598fa2533e0e73f6fb569Coloniatherium cilinski by Ceri Thomas.

Mesungulatoids are by far the most abundant meridiolestidans – and indeed, dryolestoids – in terms of remains, thanks both to their herbivorous habits and large size, being some of the largest Mesozoic mammals at 8 kg or more. Frustratingly we don’t have a complete skeleton yet, but jaws, skulls, ear bones and even hand and foot elements paint a more or less clear picture of how these animals looked in life.

As the name implies, mesungulatoids strongly converged with ungulates in terms of molar anatomy, bearing blocky, ridged molars not unlike those of a horse or an elephant. They were in fact first assumed to be Cretaceous ungulates, perhaps distant ancestors of those pesky mysterious meridiungulates, until dryolestoid hallmarks in their teeth were noticed (Pascual 2001). This offers a pretty interesting case of convergent evolution, and makes their sad demise in the Cenozoic all the more sadder to me, since there surely must be an alternate timeline where Darwin’s Toxodon was even weirder.

Mesungulatid molars and premolars typically number evenly, three of both in each jaw side., but some species were reducing their back teeth (Rougier 2009) so the big solitary blocks we see in elephant mouths could be an eventually had they survived. Their jaws are thick and offer a more or less blunt but vaguely dog-like profile, something comparable to hyrax snouts. Hyraxes are indeed a good ecological and morphological match among modern mammals, though unlike them mesungulatoids were not good climbers, at least the few species we have digit and ankle elements from (Rougier 2009). They were most likely fossorial, making them functionally close to wombats and marmots in these regards.

As noted before, mesungulatids lack the Meckelian groove, having thus a completely modern ear. Perhaps this was in part driven by their ungulate-like chewing mechanism; all meridiolestidans aside from austrotriconodontids have teeth adapted for transverse chewing, but mesungulatids were the most adapted for this due to their extensive herbivorous diet. Transverse chewing in general is assumed be one of the drivers in the detachment of ear bones from jaw bones in mammals (Luo 2018), so this does have precedent at least. Both Peligrotherium and Reigitherium appear to not have been as specialised to transverse mastication, however; while they could perform it, it seems they still mostly relied on vanilla chewing (Pascual 2001, Rougier 2009, Rougier 2018).

Mesungulatoids are among the most common vertebrates in some sites, with Coloniatherium cilinski actually outnumbering local dinosaur fossils (Rougier 2009, Rougier 2018). This firmly attests their hold on mid-sized herbivorous niches in their ecosystems, and combined with other herbivorous tetrapods such as gondwanatheres, sphenodontians and notosuchians it hints at a south american herbivorous guild dominated primarily by non-dinosaurian species (sauropods and large ornithopods aside).

Curiously, there are few gondwanatheres in the south american Late Cretaceous, while Madagascar and India, which still haven’t yielded dryolestoid fossils, show a much higher diversity as well as herbivorous zhelestid therians and the haramiyidan Avashishta bacharamensis.

The number of Cretaceous mesungulatoids is rather consistent. Reigitheriidae includes a single species, Reigitherium bunodontum, while Mesungulatidae has Mesungulatum houssayi, Mesungulatum lamarquensis, Quirogatherium/Mesungulatum major, Parungulatum/Mesungulatum rectangularisColoniatherium cilinski and an unnamed taxon (Forasiepi 2012); the main controversy here is whereas Quirogatherium and Parungulatum are distinct enough from Mesungulatum. Mesungulatum and Coloniatherium are the best represented, with various skull, jaw and limb elements, while other taxa are represented by teeth only, aside from the unnamed taxon which is represented by an edentulous jaw (teeth lost post-mortem).

Another controversy is whereas Groebertherium stipanicici is a mesungulatoid or not; as noted before, some studies recover it as such. However, most analyses recover it as a non-meridiolestidan dryolestoid and indeed it more closely resembles Dryolestes itself, rendering it a remarkable critter on its own.

 

After the dinosaurs

Capturar2Necrolestes mirabilis by Julio Lacerda.

With their Late Cretaceous success firmly set up, you might be disappointed to learn that, even immediately after the KT event, meridiolestidans are almost entirely gone from South America.

As other mammal groups were strongly affected by the KT event, it comes as no surprise that these critters were sadly mostly incapable of surviving the end of times. This no doubt helped paved the way for placentals and marsupials in South America, which quickly took over the vacant niches; in fact, direct comparisons between mesungulatoids and herbivorous south american marsupials and ungulates have been made (Rougier 2018).

In spite of this, however, at least two lineages survived: Reigitheriidae and Necrolestidae. These two lineages occupied radically different niches and are only represented by a few species, suggesting that they did not play a prominent role in South America’s Cenozoic ecosystems. Still, at least the latter survived as recently as 17 million years ago; a true ghost lineage, with a gap of at least 49 million years between Cretaceous necrolestids and Cenozoic ones.

Who knows, maybe there are more Cenozoic meridiolestidans awaiting to be discovered.

Among reigitheriids, we have Peligrotherium tropicalis, from the Palaeocene of Patagonia; its name alludes to Punta Peligro, where its holotype was discovered. It is known from a single dentary and associated molar and premolar teeth, more similar to those of Reigitherium bunodontum than those of mesungulatids. This suggests that, while reigitheriids were specialised herbivores, they clearly weren’t as much as their mesungulatid cousins, being just omnivorous enough to pass through the mass extinction event.

What does set Peligrotherium truly apart is its size; referred to as “dog-sized” (Rougier 2009), it is around seven times larger than the largest mesungulatids, rendering it a serious competitor to the multituberculate Taeniolabis taoensis as the largest non-therian mammal of all time at up to 100 kg (assuming typical mesungulatoid proportions it probably had a larger skull). Like many other mammals of the epoch, meridiolestidans did became giants soon after the extinction of the dinosaurs, and like the wave of multituberculate and placental giants in the northern continents they did not survive the chaotic world of the Cenozoic.

Just as interesting as Peligrotherium‘s large size is its coexistence with various early meridiungulates such as the didolodontids Raulvaccia peligrensis, Escribania chubutensis and Escribania talonicuspis and the litoptern Requisia vidmari. They were smaller than Peligrotherium, and thus the possibility that reigitheriids were competitively excluded by invading northern ungulates becomes a possibility, as this mirrors the usual pattern of specialised members of a clade being the last one standing while the competing, successful clade is represented by more generalistic species. Until further fossils are discovered, however, there is no way of knowing for sure, and it may just as likely be that reigitheriids simply did not survive the climatic changes brought by the PETM.

Flash forward 40 million years later, and two species of necrolestids, Necrolestes mirabilis and Necrolestes patagonensis, show up in Patagonia’s Miocene fossil sites, the final chapter in Meridiolestida’s story. Necrolestes patagonensis is one of the best understood dryolestoids, with most of its skeleton being known from various specimens. Necrolestes mirabilis is currently only known from a single lower jaw and associated molar teeth, depicting a slightly larger animal than its more well known cousin.

These findings depict Necrolestes as a mole-like animal, bearing massive limbs and small eyes. Its snout is long and upturned, the upperside probably ridged with a keratin shield much as in fairy armadillos and in marsupial moles as it was used to shovel earth, something further indicated by the ossification of its nasal cartilage, making it one of the very few mammals to have reverted to separate nostril holes as in earlier synapsids. The peculiar nasal cavity might have also suggest a complex organ like the tendrils of the star-nosed mole. As fitting its insectivorous diet, Necrolestes has large canines, more robust than those of its distant Cretaceous relatives, as well as zalambdodont molars, an unique feature among dryolestoids.

Necrolestes was not the first dryolestoid to be found – Dryolestes itself was described at least two decades earlier – but it was for the longest time the only dryolestoid with complete postcranial remains. As such, for most of the last two centuries it was rendered a taxonomical nightmare, with many sources putatively rendering it as a very atypical metatherian or non-placental eutherian (like most non-placental mammals, it has epipubic bones, and particularly large ones at that much like monotremes). It was only in 2012 when comparison with other dryolestoids allowed researchers to find its place among the meridiolestidan assemblage, finding many dental, ear bone, braincase and limb similarities (Rougier 2012); since then, posterior phylogenetic studies have consistently recovered it as one, with one in particular even demonstrating how its dental features evolved from the meridiolestidan norm (Rougier 2017).

Necrolestes found itself on a very cozy niche; due to the lack of available fossils, its unclear when the transition to a mole-like lifestyle took place, but the sheer rarity of necrolestid fossils in the Cenozoic seems to point out to soon after the KT event, likely outcompeted in other niches by therian mammals or perhaps even monotremes like Monotrematum sudamericanum. It co-existed with the relictual gondwanathere Patagonia peregrina, suggesting that non-therian synapsids, while rare, still endured for a long time during the Cenozoic in South America. As there are at least two contemporary Necrolestes species and no other burrowing carnivores and insectivores, it seems that necrolestids held their subterranean niches quite effectively.

Eventually, however, its good luck run out. The Late Miocene sees several cooling spells in South America, resulting in the decline of many groups from sebecids to terror birds. It’s possible that Necrolestes became extinct soon after its debut in the fossil record, not even seeing the arrival of new mammal groups from North America.

At the very least, Meridiolestida went out with a bang. Or did it?

Obligatory marsupial mole discussion

Southern-marsupial-mole-eating-a-geckoMarsupial moles are considered peaceful animals by the Pitjantjatjara people. Geckoes politely disagree.

No discussion on meridiolestidans, or indeed dryolestoids as a whole, is complete without the highly controversial 2014 paper that found these odd pouched morlocks to not be marsupials after all, but actually surviving meridiolestidans closely related to Necrolestes, Leonardus and Cronopio. Thus, not only does the dryolestoid story not have an end yet, but there are also two lineages of non-therian mammals living in Australia.

This hypothesis certainly has its appeal, to put it lightly, and it even parallels the story of Necrolestes (as well as the gondwanathere Patagonia peregrina) as going from an oddball shoved into Marsupialia because there is nowhere else to put it to a fairly normal member of a older mammal lineage.

Sadly, however, genetic studies are very much against this interpretation, having recovered marsupial moles as true marsupials closely related to dasyuromorphs and bandicoots. Not only that, but other studies have effectively demonstrated how the supposed similarities between marsupial moles and necrolestids are purely convergent; in particular, necrolestid zalambdodont teeth evolved from a typical dryolestoid mold, while the marsupial mole’s evolved from a therian one (Rougier 2017). Furthermore, the extinct marsupial Naraboryctes offers a clear bridge between a more normal marsupial anatomy and the bizarre mess that modern marsupial moles have now.

Plus, while marsupial mole anatomy does strongly resemble that of necrolestids, I do have to question their short canines and vestigial epipubic bones, the polar opposite of the situation seen in Necrolestes. This just further screams “different ancestry” to me.

That said, marsupial moles overall do offer a good model for not only necrolestid anatomy and lifestyle, but likely also for the soft tissue anatomy of dryolestoids as a whole. That said, just as one shouldn’t mistake the marsupial mole’s “primitive” anatomy as ancestral, neither should one assume that some dryolestoids didn’t converge with therian mammals, particularly at more active niches like in austrotriconodontids and brandoniids.

Final Thoughts

Sem TítuloI’m not the only one who noticed this, right?

Hopefully I have given you insight towards this majestic but underrated mammal clade.

References

Alexander O. Averianov, Thomas Martin and Alexey V. Lopatin (2013). “A new phylogeny for basal Trechnotheria and Cladotheria and affinities of South American endemic Late Cretaceous mammals”. Naturwissenschaften. 100 (4): 311–326. doi:10.1007/s00114-013-1028-3.

Guillermo W. Rougier, John R. Wible, Robin M. D. Beck and Sebastian Apesteguía (2012). “The Miocene mammal Necrolestes demonstrates the survival of a Mesozoic nontherian lineage into the late Cenozoic of South America”. Proceedings of the National Academy of Sciences of the United States of America109 (49): 20053–20058. doi:10.1073/pnas.1212997109. PMC 3523863.

Laura Chornogubsky, New remains of the dryolestoid mammal Leonardus cuspidatus from the Los Alamitos Formation (Late Cretaceous, Argentina), Article in Paläontologische Zeitschrift 85(3):343-350 · September 2011 DOI: 10.1007/s12542-010-0095-4

Richard Stephen Thompson, Rachel O’Meara, Were There Miocene Meridiolestidans? Assessing the Phylogenetic Placement of Necrolestes patagonensis and the Presence of a 40 Million Year Meridiolestidan Ghost Lineage, Article in Journal of Mammalian Evolution · September 2014 DOI: 10.1007/s10914-013-9252-3

Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 14, 531.

Nicholas Chimento, Frederico Agnolin, Agustin Martinelli, Mesozoic Mammals from South America: Implications for understanding early mammalian faunas from Gondwana, May 2016

Hamid Haddoumi, Ronan Allain, Said Meslouh, Grégoire Metais, Michel Monbaron, Denise Pons, Jean-Claude Rage, Romain Vullo, Samir Zouhri, Emmanuel Gheerbrant, Guelb el Ahmar (Bathonian, Anoual Syncline, eastern Morocco): First continental flora and fauna including mammals from the Middle Jurassic of Africa, doi:10.1016/j.gr.2014.12.004

Bonaparte, J.F. 2002. New Dryolestida (Theria) from the Late Cretaceous of Los Alamitos, Argentina, and paleogeographical comments. Neues Jahrbuch fu ̈r Geologie und Pala ̈ontologie, Abhandlungen224(3): 339–371.

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