Darwinopterus and mosaic, modular evolution
It's yet another transitional fossil! Are you tired of them yet?
Darwinopterus modularis is a very pretty fossil of a Jurassic pterosaur, which also reveals some interesting modes of evolution; modes that I daresay are indicative of significant processes in development, although this work is not a developmental study (I wish…having some pterosaur embryos would be exciting). Here it is, one gorgeous animal.
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Figure 2. Holotype ZMNH M8782 (a,b,e) and referred specimen YH-2000 ( f ) of D. modularis gen. et sp. nov.: (a) cranium and mandibles in the right lateral view, cervicals 1-4 in the dorsal view, scale bar 5cm; (b) details of the dentition in the anterior tip of the rostrum, scale bar 2cm; (c) restoration of the skull, scale bar 5cm; (d) restoration of the right pes in the anterior view, scale bar 2 cm; (e) details of the seventh to ninth caudal vertebrae and bony rods that enclose them, scale bar 0.5 cm; ( f ) complete skeleton seen in the ventral aspect, except for skull which is in the right lateral view, scale bar 5 cm. Abbreviations: a, articular; cr, cranial crest; d, dentary; f, frontal; j, jugal; l, lacrimal; ldt, lateral distal tarsal; m, maxilla; mdt, medial distal tarsal; met, metatarsal; n, nasal; naof, nasoantorbital fenestra; p, parietal; pd, pedal digit; pf, prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sq, squamosal; ti, tibia.
One important general fact you need to understand to grasp the significance of this specimen: Mesozoic flying reptiles are not all alike! There are two broad groups that can be distinguished by some consistent morphological characters.
The pterosaurs are the older of the two groups, appearing in the late Triassic. They tend to have relatively short skulls with several distinct openings, long cervical (neck) ribs, a short metacarpus (like the palm or sole of the foot), a long tail (with some exceptions), and an expanded flight membrane suspended between the hind limbs, called the cruropatagium. They tend to be small to medium-sized.
The pterodactyls are a more derived group that appear in the late Jurassic. Their skulls are long and low, and have a single large opening in front of the eyes, instead of two. Those neck ribs are gone or reduced, they have a long metacarpus and short tails, and they've greatly reduced the cruropatagium. Some of the pterodactyls grew to a huge size.
Here's a snapshot of their distribution in time and phylogenetic relationships. The pterosaurs are in red, and the pterodactyls are in blue.
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Time-calibrated phylogeny showing the temporal range of the main pterosaur clades; basal clades in red, pterodactyloids in blue; known ranges of clades indicated by solid bar, inferred 'ghost' range by coloured line; footprint symbols indicate approximate age of principal pterosaur track sites based on Lockley et al. (2008); stratigraphic units and age in millions of years based on Gradstein et al. (2005). 1, Preondactylus; 2, Dimorphodontidae; 3, Anurognathidae; 4, Campylognathoididae; 5, Scaphognathinae; 6, Rham- phorhynchinae; 7, Darwinopterus; 8, Boreopterus; 9, Istiodactylidae; 10, Ornithocheiridae; 11, Pteranodon; 12, Nyctosauridae; 13, Pterodactylus; 14, Cycnorhamphus; 15, Ctenochasmatinae; 16, Gnathosaurinae; 17, Germanodactylus; 18, Dsungaripteridae; 19, Lonchodectes; 20, Tapejaridae; 21, Chaoyangopteridae; 22, Thalassodromidae; 23, Azhdarchidae. Abbreviations: M, Mono- fenestrata; P, Pterodactyloidea; T, Pterosauria; ca, caudal vertebral series; cv, cervical vertebral series; mc, metacarpus; na, nasoantorbital fenestra; r, rib; sk, skull; v, fifth pedal digit.
Darwinopterus is in there, too—it's the small purple box numbered "7". You can see from this diagram that it is a pterosaur in a very interesting position, just off the branch that gave rise to the pterodactyls. How it got there is interesting, too: it's basically a pterosaur body with the head of a pterodactyl. Literally. The authors of this work carried out multiple phylogenetic analyses, and if they left the head out of the data, the computer would spit out the conclusion that this was a pterosaur; if they left the body out and just analyzed the skull, the computer would declare it a pterodactyl.
What does this tell us about evolution in general? That it can be modular. The transitional form between two species isn't necessarily a simple intermediate between the two in all characters, but may be a mosaic: the anatomy may be a mix of pieces that resemble one species more than the other. In this case, what happened in the evolution of the pterodactyls was that first a pterodactyl-like skull evolved in a pterosaur lineage, and that was successful; later, the proto-pterodactyls added the post-cranial specializations. Not everything happened all at once, but stepwise.
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Schematic restorations of a basal pterosaur (above), Darwinopterus (middle) and a pterodactyloid (below) standardized to the length of the DSV, the arrow indicates direction of evolutionary transformations; modules: skull (red), neck (yellow), body and limbs (monochrome), tail (blue); I, transition phase one; II, transition phase two.
This should be a familiar concept. In pterodactyls, skulls evolved a specialized morphology first, and the body was shaped by evolutionary processes later. We can see a similar principle in operation in the hominid lineage, too, but switched around. We evolved bipedalism first, in species like Ardipithecus and Australopithecus, and the specializations of our skull (to contain that big brain of which we are so proud) came along later.
As I mentioned at the beginning, this is an example of development and evolution in congruence. We do find modularity in developmental process — we have genetic circuits that are expressed in tissue- and region-specific ways in development. We can talk about patterns of gene expression that follow independent programs to build regions of the body, under the control of regional patterning genes like the Hox complex. In that sense, what we see in Darwinopterus is completely unsurprising.
What is interesting, though, is that these modules, which we're used to seeing within the finer-grained process of development, also retain enough coherence and autonomy to be visible at the level of macroevolutionary change. It caters to my biases that we shouldn't just pretend that all the details of development are plastic enough to be averaged out, or that the underlying ontogenetic processes will be overwhelmed by the exigencies of environmental factors, like selection. Development matters — it shapes the direction evolution can take.
Lü J, Unwin DM, Jin X, Liu Y, Ji Q (2009) Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proc. R. Soc. B published online 14 October 2009 doi: 10.1098/rspb.2009.1603
I should have mentioned that Darren Naish has a very thorough write-up on Darwinopterus!