Gliding haramiyidans and further arguments for flying volaticotheres
So recently we had some most wonderful discoveries: fossil specimens of gliding haramiyidans.
Haramiyidans were a lineage of mammaliaform cynodonts, once considered sister-taxa to the iconic multituberculates but now considered outside of the mammal crown-group (Chang 2015, Luo et al 2015, Luo et al 2017). In and of themselves they set a number of records, among other things being among the first mammaliaform herbivores, among the earliest examples for preserved fur in the synapsid fossil record and being an extremely long lived lineage, having endured from the Triassic to the Maastrichtian(!)(Anantharaman 2006).
With the discovery of not one, but two brand new taxa with preserved patagia, Vilevolodon diplomylos and Maiopatagium furculiferum as well as the discovery of patagia on the already known taxon Xianshou songae, haramiyidans also set the record for the first non-mammalian synapsids with gliding representatives. All the years of speculating on aerial cynodonts have thus come to be, if maybe not as fearsome as a pugbat.
These gliding haramiyidans all come from the same place, the Chinese Tiaojishan Formation. Hailing from the Middle to Late Jurassic, this formation also held a variety of other aerial vertebrates, including pterosaurs, scansoriopterygid dinosaurs and, of course, Volaticotherium antiquum, a true mammal and previously the only evidence of aerial locomotion in pre-Cenozoic synapsids, alongside the closely related Argentoconodon fariasorum. The environment in this formation appears to have been a subtropical or temperate open forest, which would have been particularly favorable to gliding and flying vertebrates, as modern Asian rainforests (Na 2015).
Unlike Volaticotherium, which has a poorly preserved hand (Meng 2006), the gliding haramiyidans all have well preserved digits, which leaves no ambiguity about these being gliders and not powered flyers. The digit proportions are fairly similar to those of modern colugos, showcasing these animals to have been exclusively arboreal and likely frequently hanging upside down in a sloth-like manner (Luo 2017).
Also like modern colugos, as well as the vast majority of gliding mammals (Jackson 2012, Luo 2017), these haramiyidans were herbivores. In particular, the larger Maiopatagium was probably a frugivore or folivore, while the smaller and more complex-toothed Vilevolodon and Xianshou were probably specialized towards more granivorous diets (Luo 2017).
This degree of diversity of arboreal gliding mammals is rather astonishing given that flowering plants had yet to become dominant and the Tiaojishan Formation woods were in fact mostly composed by conifers and ginkgos. This shows that pre-angiosperm forest ecologies were in fact capable of supporting herbivore guilds like those of modern rainforests, and that these types of plants weren’t less inviting to herbivorous vertebrates as previously assumed. Given that both conifers and ginkgos have forms with fruit-like seeds and Maiopatagium was probably a frugivore, some of these Mesozoic forest ecologies could probably indeed parallel modern rainforests in rather similar ways.
As the paper indicates, few gliding mammals are anything other than predominant herbivores, which neatly segues into…
What about volaticotheres?
As I have previously discussed, I think volaticotheres were true, powered flyers, not just gliders. The discovery of these new gliding haramiyidans has vindicated this opinion: it is pointed out outright that most gliding mammals are primarily herbivorous (Luo 2017), while it remains agreed that volaticotheres, like most eutriconodonts, were primarily carnivores/insectivores (Meng 2006, Butler 2016). Likewise, these gliding haramiyidans do not display the same odd femur speciations as volaticotheres, which constrained terrestrial movement (Meng 2006, Gaetano 2011).
Additionally, some of my other arguments in this matter also stand solid in a general comparison between volaticotheres and haramiyidans. Volaticotheres were a cosmopolitan group in the Early and Middle Jurassic, while haramiyidans were constrained to the northern continents until the Late Jurassic, when Allostaffia shows up in the Tendaguru Formation. Hahnodon teeth show signs of aquatic transportation degrading (Sigogneau-Russell 1991), as opposed to Ichthyoconodon‘s now infamous case.
There are a few gaps in understanding in regards to the flying volaticothere hypothesis, most notably Volaticotherium‘s forelimbs being slightly shorter than the hindlimbs (Meng 2006), but overall the information acquired with the studies on these gliding haramiyidan taxa vindicate my points. These follow typical gliding mammal conventions and are acknowledged as such by the researchers (Luo 2017), while volaticotheres remain an anomaly more easily explained by being true powered flyers.
For now, I can only hope more complete volaticothere fossils will show up, or that at least more comparisons between available volaticothere material and these haramiyidans will be made.
Zhe-Xi Luo; Qing-Jin Meng; David M. Grossnickle; Di Liu; April I. Neander; Yu-Guang Zhang; Qiang Ji (2017). “New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem”. Nature. in press. doi:10.1038/nature23483.
Chang, Kenneth (16 November 2015). “Jawbone in Rock May Clear Up a Mammal Family Mystery”. New York Times. Retrieved 17 November 2015.
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. Retrieved 17 November 2015.
Anantharaman, S.; Wilson, G. P.; Das Sarma, D. C.; Clemens, W. A. (2006). “A possible Late Cretaceous “haramiyidan” from India”. Journal of Vertebrate Paleontology. 26 (2): 488–490. doi:10.1671/0272–4634(2006)26[488:aplchf]2.0.co;2.
Na, Y.; Manchester, S.R.; Sun, C.; Zhang, S. (2015). “The Middle Jurassic palynology of the Daohugou area, Inner Mongolia, China, and its implications for palaeobiology and palaeogeography”. Palynology. 39 (2): 270. doi:10.1080/01916122.2014.961664.
Meng, J.; Hu, Y.-M.; Wang, Y.-Q.; Wang, X.-L.; Li, C.-K. (2007). “Corrigendum: A Mesozoic gliding mammal from northeastern China”. Nature 446 (7131): 102. Bibcode: 2007Natur.446Q.102M. doi:10.1038/nature05639.
Jackson, Stephen Matthew and Schouten, Peter. Gliding Mammals of the World, Csiro Publishing, 2012
Gaetano, L.C.; Rougier, G.W. (2011). “New materials of Argentoconodon fariasorum (Mammaliaformes, Triconodontidae) from the Jurassic of Argentina and its bearing on triconodont phylogeny”. Journal of Vertebrate Paleontology. 31 (4): 829–843. doi:10.1080/02724634.2011.589877.
Percy M. Butler; Denise Sigogneau-Russell (2016). “Diversity of triconodonts in the Middle Jurassic of Great Britain” (PDF). Palaeontologia Polonica 67: 35–65. doi:10.4202/pp.2016.67_035.
Sigogneau-Russell (1991), “First evidence of Multituberculata (Mammalia) in the Mesozoic of Africa”. Neues Jahrb Geol Paläontol, Monatshefte, p. 119–125.