[D] Asylosaurus yalensis [sG] [T]
Triassic Late Rhaetian
Fissure Fill in Quarry Steps Quarry off Upper Belgrave Road in Clifton, Bristol, England
Genus - Typespecies
Etymology: Greek asylos = unharmed, safe from violence or asylon = refuge, sanctuary + sauros = lizard ; yalensis = of Yale College (now University), where O. C. Marsh took specimen so it was unharmed in air raids on BCM in November, 1940.
Holotype: YPM 2195, an associated partial skeleton consisting of dorsal vertebrae, ribs and gastralia, pectoral girdle, right and proximal left humeri, articulated left forearm (radius very incomplete) and manus also YPM 2195a-g, disarticulated bones on block tentatively referred to holotype individual (or species for femora), viz., vertebrae (cervical ~4 or 5, caudals ~10 and ~15), proximal medial part of right ulna, distal left ischium, 2 distal femora : larger right and smaller left, distal right tibia.
Hypodigm: proximal left humerus (BMNH R1542), manual phalanx 1 of digit I (YPM 56745), ischia (YPM 56725, 56726, 56739).
Diagnosis : autapomorphies : deltopectoral crest symmetrical in lateral view with the gently rounded apex at 25 % (measured perpendicular to long axis of humerus) of humeral length, manus with lateral reduction so phalangeal formula 2-3-4-2-1, distally the thickest parts of the ischia are separated by a cleft that is Λ-shaped in dorsal view ; plesiomorphies : low deltopectoral crest (also in Pantydraco caducus but apex asymmetrical at 40 % of humeral length) with large medial tubercle adjacent to head (also in most basal sauropodomorphs, small in P. caducus), distal tarsal I transversely narrower than metatarsal I (occurs in no other basal sauropodomorph) and transversely narrow metacarpals I (and phalanx 1, also in Efraasia minor) and II-IV (also in E. minor and Anchisaurus polyzelus).
Asylosaurus yalensis n. gen. et sp. Based on the delicate form and structure of the distal expansion, the scapula of YPM 2195 was referred by Huene (1914a) to Thecodontosaurus antiquus, the gracile form. He discussed and figured the associated forearm and manus but, rather surprisingly, the complete humerus was not even mentioned. The rounded apex of the deltopectoral crest of this humerus is proximally placed, with the apex at about 25 % (measured perpendicular to long axis of humerus) of humeral length, as against 40 % in Pantydraco caducus.
In both the crest is anteroposteriorly low in lateral view but in YPM 2195 it forms a symmetrical triangle, with the edges on either side of the gently rounded apex being equal in length. In Pantydraco caducus (crest low) and in other basal sauropodomorphs (crest high), including the gracile and robust morphs from Clifton, the deltopectoral crest is sub-quadrangular or hatchet shaped with an angular apex (at 40 % and +50 % of humeral length) followed by a short and steeply inclined distal edge.
A derived character for YPM 2195 (and the other Clifton humeri) compared to Pantydraco caducus is the strongly projecting medial tubercle of the proximal humerus (posteromedial corner of bone, shown too small for YPM 2195) compared to the plesiomorphic small tubercle in Pantydraco caducus (Yates, 2003a ; Galton et al., 2007). At node 3 (Saturnalia) the deltopectoral crest terminates at or below the mid-length of the humerus [Reversed at node 6 (Anchisauridae) and “Gyposaurus” sinensis, Galton & Upchurch, 2004].
In the three most complete gracile Clifton humeri the apex of the deltopectoral crest is at about 40 %, as in Pantydraco caducus, but the crest extends further anteriorly. In the two robust Clifton humeri, the anterior extensions are even more pronounced and the complete crest, and even the apex, clearly extended beyond 50 % of humeral length. Based on the form of the deltopectoral crest (anteroposteriorly low, symmetrical outline, proximal position of rounded apex at 25 % of humeral length) and the large size of the medial tubercle adjacent to the head, the humerus shows that YPM 2195 represents a new taxon, Asylosaurus yalensis n. gen. et sp., distinct from Pantydraco caducus (with low but asymmetrical crest with apex at 40 %, small medial tubercle).
The deltopectoral crest is hatchet shaped, projecting well anteriorly with the apex at 40 % or +50 % of length, in most other basal Sauropodomorphs. The transverse width of distal carpal I is less than the width of metacarpal I, and there are no ossified proximal carpals, plesiomorphic characters for basal Sauropodomorpha. As noted by Galton (1973), metacarpal I (and its proximal phalanx) are transversely slender, a plesiomorphic character for basal sauropodomorphs that is also present in Efraasia, and it is not short and robust, a derived character of node 4 (Galton & Upchurch, 2004). Metacarpals II-IV are also slender, the plesiomorphic condition as in Efraasia and Anchisaurus, rather than broad as in other basal sauropodomorphs.
Phalanx 1 of digit I has a proximal “heel” and the long-axes of its proximal and distal articular surface are twisted at about 45° to each other, synapomorphies of basal sauropodomorphs. It is also plesiomorphic for basal sauropodomorphs in being narrow in dorsal view, so its transverse width matches that of metacarpal I as in Efraasia, rather than being broad, a derived character of node 4.
The structure of the manus of Asylosaurus is that of a very basal Sauropodomorpha. Unfortunately, the phalanges of digits IV and V are rarely completely preserved in the manus of basal sauropodomorphs (see Galton & Cluver, 1976). The phalangeal formula for Asylosaurus yalensis at 2-3-4-2-?1 is derived in showing lateral reduction compared to the condition in Efraasia, Massospondylus and Lufengosaurus at 2-3-4-3-2 and especially Plateosaurus at 2-3-4-3-3 or even 2-3-4-4-2 (Galton & Cluver, 1976.
For the phalangeal formula, Benton & Storrs (in Benton et al., 2000, p. 107, character 30) gave a plesiomorphic state “based on outgroup comparisons with a wider group of dinosaurs and dinosauromorphs”, but further on noted that “Scleromochlus and Herrerasaurus are included as postulated outgroups of the crown-group clade” for the plesiomorphic state of 2-3-4-1/2-0/1 (0) as against 2- 2-2-2-1 (1), the derived state (Benton et al., 2000, p. 107). However, it is coded as character state uncertain for Scleromochlus (Benton, 1999) and it ignores the lateral reduction that occurred within basal sauropodomorphs (Asylosaurus, Massospondylus, Lufengosaurus), comparable but less extreme to that which occurred in bipedal theropods (Rauhut, 2003, p. 97), as distinct from the distal reduction that affected digits II-V in quadrupedal sauropods (2-2-2-2-1).