所有现存鳄鱼类动物，包括crocodiles、alligators和gavialoids，都是半水生的。但它们据信是从被称为Sphenosuchians的陆地祖先进化来的。来自中国西北戈壁沙漠的一个新发现的Sphenosuchian化石，促使研究人员重新思考鳄鱼类动物进化过程中可能经历的一些事件。新发现的化石是彻底陆生的，然而却与现存鳄鱼类动物密切相关。看起来，现存鳄鱼类动物坚硬的头骨（非常适合在水中捕获猎物）最初是由其体形小、比较细弱的陆地形式用来钳住猎物、并将猎物钳紧的。这种头骨的有用之处只是后来才在水环境中得到验证。
A Middle Jurassic 'sphenosuchian' from China and the origin of the crocodylian skull
The skull of living crocodylians is highly solidified and the jaw closing muscles are enlarged1, allowing for prey capture by prolonged crushing between the jaws. Living species are all semi-aquatic, with sprawling limbs and a broad body that moves mainly from side-to-side2; however, fossils indicate that they evolved from terrestrial forms. The most cursorial of these fossils3-6 are small, gracile forms often grouped together as the Sphenosuchia, with fully erect, slender limbs; their relationships, however, are poorly understood5, 7-10. A new crocodylomorph from deposits in northwestern China of the poorly known Middle Jurassic epoch possesses a skull with several adaptations typical of living crocodylians. Postcranially it is similar to sphenosuchians but with even greater adaptations for cursoriality in the forelimb. Here we show, through phylogenetic analysis, that it is the closest relative of the large group Crocodyliformes, including living crocodylians. Thus, important features of the modern crocodylian skull evolved during a phase when the postcranial skeleton was evolving towards greater cursoriality, rather than towards their current semi-aquatic habitus.
Figure 1 Results of a maximum parsimony phylogenetic analysis. Strict consensus of 81 trees; length = 109, consistency index (CI) = 0.569, retention index (RI) = 0.659. Junggarsuchus is the closest relative of the Crocodyliformes and the Sphenosuchia are paraphyletic. See Supplementary Information for details of the analysis.
Figure 2 Junggarsuchus sloani holotype skull. a, b, Left lateral view. c, d, Dorsal view. e, f, Occipital view. a, angular; aof, antorbital fenestra; bo, basoccipital; bs, basisphenoid; cf, carotid foramen; d, dentary; eo, exoccipital; f, frontal; j, jugal; la, lacrimal; mf, external mandibular fenestra; mx, maxilla; n, nasal; o, orbit; p, parietal; pa, palpebral; pf, prefrontal; pl, palatine; pm, premaxilla; po, postorbital; pt, pterygoid; q, quadrate; qf, quadrate fenestra; qj, quadratojugal; rp, retroarticular process; sa, surangular; saf, surangular foramen; sc, sagittal crest; soc, supraoccipital; sq, squamosal; stf, superior temporal fenestra. Scale bar, 5 cm.
Figure 3 Postcranial skeleton of Junggarsuchus sloani. a, Skeleton in left lateral view. b, Left humerus with hemispherical head in proximal view. c, Posteroventrally facing glenoid fossa of left scapula in distal view. d, Left carpus and manus in posteroventral view (arrow points to reduced fifth metacarpal). e, f, For comparison, the living Alligator mississippiensis left humerus and shoulder in lateral view (e) showing the hemi-sellar glenoid fossa, and left hand in dorsal view (f) showing the asymmetry of the digits and carpals, the rounded ulna–carpal contact (arrow) and the single distal carpal (stippled area is cartilaginous). dc, distal carpal; g, glenoid fossa; h, humerus; hh, head of humerus; hyp, hypapophyses; ra, radius; rl, radiale; sc, scapula; ue, ulnare; ul, ulna; I–V, digits of hand. f was modified from ref. 16. Scale bars, 4 cm (a) and 1 cm (b–d).
