The pinnae of Megazostrodon

I do a lot of consultancy work for children’s books, and one of the things I’m asked about most often is whether certain anatomical details should or should not be included in life restorations of fossil animals. Most artists, whether they’re good and informed ones or not, are now decking out their small theropods with feathers, for example. Even so, companies that recycle old artwork still try and sneak in scaly-skinned dromaeosaurs if you let them. Yesterday I was asked if Megazostrodon (a basal mammaliaform from Lower Jurassic Lesotho) should be depicted with ‘ear flaps’ (= pinnae). I immediately said yes: after all, all the life restorations I could think of depicted them. But then I became very, very scared.

It’s generally agreed that Megazostrodon is right down at the base of Mammaliaformes, and thus, under current convention, outside of the mammaliaform crown-group Mammalia. And the thing is, that (1) while we know that mammals have pinnae, that doesn’t mean that non-mammalian mammaliaforms did, and (2) it’s well established that basal mammaliaforms like Megazostrodon differed in bony ear anatomy from mammals. Mammals have an elongated cochleal canal and a promontorium on the petrosal, features involved in improving both high-frequency hearing and the acoustic isolation of the inner ear. But several studies have shown that basal mammaliaforms were intermediate in these regards, and different from mammals (Rosowski & Graybeal 1991, Graybeal et al. 1998, Luo et al. 1995). If Megazostrodon and related forms didn’t have the same sort of dependence on high-frequency hearing as modern mammals, would they have attempted to ‘maximise’ their hearing by evolving pinnae?

Ultra-rigorous uber-scientist that I am, the first thing I went and did was look at some life restorations. In May 2001 Zhe-Xi Luo and colleagues described Hadrocodium, a new basal mammaliaform from Lower Jurassic China. The fossil was interesting enough to make the cover of the issue of Science that it was described in: the cover features a paper-clip, and a skull and life restoration of Hadrocodium (Hadrocodium was tiny you see, with an estimated weight of 2 grams). And the good news? It’s depicted as having little pinnae. In phylogenies, Hadrocodium is recovered as two or three steps up from Megazostrodon, so as a piece of circumstantial evidence this is at least suggestive. On rather more technical details, Rosowski & Graybeal (1991) thought it at least possible that Morganucodon – a close relative of Megazostrodon (according to most phylogenies, never mind McKenna & Bell) – may have ‘had ears like those of modern small mammals that are selectively sensitive to high-frequency sounds’ (p. 131). It’s hard to imagine that this might be true, and that a pinna be absent. Furthermore, while the cochleal canal is shorter, and the promontorium is smaller, in basal mammaliaforms than is the case in mammals (Luo et al. 1995), Morganucodon in more mammal-like in these features than are more basal mammaliaforms (like Sinoconodon). Forms even closer to mammals, like Haldanodon and Hadrocodium, are more mammal-like in these ear features than Morganucodon. So what we’re seeing is a gradual approach to the mammalian condition. It’s all a bit arm-wavy, but I would guess that we’d see the gradual enlargement of pinnae occurring at the same time as these changes, given that they seem to be correlated.

Do we even know whether pinnae were present in the mammalian common ancestor, as I’ve been assuming. The basal split in the mammal tree, according to current morphological phylogenies anyway, is between Australosphenida and Theriiformes, and given that both clades have extant representatives, we can make inferences about their common ancestor. Well, we all know that members of Theriiformes have pinnae, but as for australosphenidans (most of which are scrappy fossils)… Every now and again you’ll hear people saying that monotremes don’t have them, and thus they can’t be assumed to have been present, ancestrally, in mammals (incidentally, I can’t find an assertion along these lines in the literature, but there’s probably one out there somewhere). But it’s not true. While the platypus lacks a pinna, this isn’t so of echidnas. On them, Nowak (1999) wrote ‘The pinna of the external ear is well developed, but is partly concealed in the pelage’ (p. 7). So it is ok to assume pinnae in the mammalian common ancestor after all.

Back to Megazostrodon: wouldn’t these animals look pretty dumb without pinnae anyway? Of course we don’t really know whether they had fur either (despite dubious suggestions of evidence for such structures in non-mammaliaform synapsids), but it’s reasonable to think that they did given that, phylogenetically, they’re very close to the mammalian common ancestor (and, again, on the basis of parsimony that animal would have been furry), and not all that different from basal mammals.

So, next time I say ‘yes’ on the phone I’ll just assume I’m right – checking the literature is so time-consuming.

More things arose today that I’ll talk about here at some stage. Whales with trunks, the swan-necked seals, and sex determination in dinosaurs. Now back to those Wealden sauropods… For the latest news on Tetrapod Zoology do go here.

Refs - -

Graybeal, A., Rosowski, J. J., Ketten, D. R. & Crompton, A. W. 1989. Inner-ear structure in Morganucodon, an early Jurassic mammal. Zoological Journal of the Linnean Society 96, 107-117.

Luo, Z., Crompton, A. W. & Lucas, S. G. 1995. Evolutionary origins of the mammalian promontorium and cochlea. Journal of Vertebrate Paleontology 15, 113-121.

- ., Crompton, A. W. & Sun, A.-L. 2001. A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics. Science 292, 1535-1540.

Nowak, R. M. 1999. Walker’s Mammals of the World (Sixth Edition), Volume 1. The Johns Hopkins University Press, Baltimore and London.

Rosowski, J. J. & Graybeal, A. 1991. What did Morganucodon hear? Zoological Journal of the Linnean Society 101, 131-168