EUDIMORPHODON AND THE EARLY HISTORY OF PTEROSAURS
DAVID M. UNWIN
Institut für Paläontologie, Museum für Naturkunde, Zentralinstitut der Humboldt-Universität zu Berlin, D-10115 Berlin, GERMANY.
Eudimorphodon is certainly the most well known and arguably the most important Triassic pterosaur (Wellnhofer 1991). Principally distinguished by its multi-cusped teeth, this long-tailed apparently piscivorous form reached at least one metre in wingspan (Wild 1978, 1994, Dalla Vecchia 1995). Initially thought to be one of the most primitive pterosaurs (Wellnhofer 1978), an idea consistent with its Late Triassic age, Wild (1978) allied Eudimorphodon with Campylognathoides and argued that this lineage had branched off later than others such as the Dimorphodontidae, a hypothesis that has been supported by several cladistic studies (Unwin 1992, 1995, 2003, Kellner 1996, Peters 1997), but not yet substantiated in detail. Results of a new phylogenetic analysis, focused on basal taxa ('rhamphorhynchoids') and presented here, reveal considerable support for 'Wild's hypothesis' and throw new light on the early history of pterosaurs.
The fossil record of Eudimorphodon
First reported by Zambelli (1973), the principal description of the type species, E. ranzii, was published by Wild (1978) on the basis of several specimens from the Upper Triassic (Norian) Calcare di Zorzino of Cene, near Bergamo, Italy. An isolated sternum of E. ranzii from the same stratigraphic unit at Endenna was noted by Renesto (1993) and another small, apparently juvenile individual of this species, with evidence of wing membranes, was recovered from the Argilliti di Riva di Solto (Norian), at Ponte Giurno, north of Bergamo (Wild 1994). If attributable to E. ranzii, a large, isolated, wing phalanx 4 from the Dolomia di Forni (Norian) of Friuli, Italy (Dalla Vecchia 2000) indicates an individual of approximately 1.75 m in wingspan and larger than any other Triassic pterosaur presently known.
A second species, E. rosenfeldi, so far known from a single incomplete skeleton (wingspan ~ 0.7 m), was recovered from the Dolomia di Forni of Udine province, Italy (Dalla Vecchia 1995), and an articulated series of 24 caudal vertebrae associated with part of a wing-finger phalanx 3 and both terminal wing-finger phalanges from the same region and rock sequence (Dalla Vecchia 2002) might also belong to Eudimorphodon. A third species, E. cromptonellus, based on a relatively small (wingspan ~ 0.24 m) and seemingly very young individual from the Fleming Fjord Formation (?Norian-Rhaetian) of Greenland was described by Jenkins et al. (2001). Isolated teeth attributed to Eudimorphodon have been reported from Europe and the USA (see Dalla Vecchia 2003) although these identifications are uncertain (Dalla Vecchia 1995). An incomplete skeleton of Eudimorphodon sp. from the Seefeld Beds (Norian) of the Tyrol, Austria will be described by Wellnhofer (2003) and Austriadactylus cristatus, shown below to be a close relative of Eudimorphodon and also known only from a single incomplete skeleton, has been reported from the same region and stratigraphic sequence (Dalla Vecchia et al. 2002). In summary, Eudimorphodon is the commonest pterosaur from Norian deposits, and may also occur in the Rhaetian. It is certainly known from Europe and Greenland and possibly also from North America.
The phylogenetic relationships of Eudimorphodon reanalysed
The relationships of Eudimorphodon to other pterosaurs were investigated using a data matrix (Table 1) consisting of 15 genera and an outgroup, and 53 characters: 27 cranial and 26 postcranial. Twenty five characters, slightly modified in several cases, were derived from an older compilation (Unwin 2003), 28 are new (Table 2). Rooting of the tree using an outgroup did not present a problem because although the relationship of pterosaurs to other diapsids is unclear (Brochu 2002), pterosaur skeletal anatomy is so derived that the plesiomorphic state is usually the same irrespective of which outgroup is chosen: Ornithodira, Archosauria, or Prolacertiformes.
The taxon-character matrix (Table 1) was analysed using PAUP 3.1.1 (Swofford, 1993), with the 'branch and bound' search option, addition sequence 'furthest' and both 'Acctran' and 'Deltran' settings. Multiple-state characters were always treated as unordered. Characters that exhibited more than one state for a particular terminal taxon (shown as 0/1 in the matrix) were treated as polymorphic. Continuously variable characters were broken into discrete states on the basis of breaks between distribution peaks.
The analysis resulted in 13 most parsimonious trees, each 84 steps in length and varying only in the relationships to each other of the taxa within Anurognathidae. The strict concensus tree, together with Bootstrap values (1000 replicates) and results of a decay analysis, is shown in Fig. 2. Tree topology is congruent with the results obtained by Unwin (1995, 2003).
Eudimorphodon is paired with Austriadactylus (presence of multicusped teeth) a relationship tentatively proposed by Dalla Vecchia et al. (2002), and these two, together with Campylognathoides, form the Campylognathoididae, diagnosed by a row of cranial and postcranial apomorphies and one of the better supported clades found in this analysis. This conclusion is consistent with other cladistic studies (Unwin 1995, Kellner 1996) and confirms Wild's original idea (1978) of a relationship between Eudimorphodon and Campylognathoides.
Significantly, this analysis also demonstrates strong support for a derived position of Eudimorphodon compared to other basal pterosaurs (Fig. 2). Eudimorphodon is a member of the Lonchognatha (Unwin 2003), a relatively derived clade principally distinguished by a long low skull, and also exhibits characters related to the elongation of the forelimb and modification of the pelvis that diagnose a more inclusive clade, the Caelidracones (Unwin 2003). By contrast, Preondactylus and Dimorphodontidae show the plesiomorphic condition for these characters confirming their basal position within Pterosauria. A surprising discovery, that the tail of Eudimorphodon lacks a sheath of elongate bony rods (Dalla Vecchia 2002), present in many other basal pterosaurs, is most parsimoniously optimised as retention of the plesiomorphic condition, but the alternative, secondary loss of this structure in the lineage leading to Eudimorphodon+Austriadactylus, is only slightly less parsimonious.
Eudimorphodon and the early history of pterosaurs
The early history of pterosaurs (Fig. 3) was reconstructed by combining data on the stratigraphic distribution of Triassic and Jurassic taxa (current up to July 2003) with the phylogenetic hypothesis presented above. Two significant aspects, both hinging on Eudimorphodon, are mentioned here. First, as noted previously (Wellnhofer 1978, Wild 1978, Dalla Vecchia et al. 2002, Unwin 2003), the primary radiation of pterosaurs was well underway by the early Late Triassic and almost all the principal basal clades were established by this time. Eudimorphodon provides critical support for this notion because, although a relatively derived form, it is practically coeval with the earliest known pterosaurs and indicates that Rhamphorhynchidae + Pterodactyloidea (the sister group to Campylognathoididae) and clades basal to the Lonchognatha (i.e. Anurognathidae, Dimorphodontidae, Preondactylus) must have existed at this time, even though there is, as yet, no fossil evidence of the Anurognathidae or the lineage leading to the Rhamphorhynchidae + Pterodactyloidea in the Triassic (Fig. 3).
Second, the range of taxonomic diversity evident in the Late Triassic is similar to that in the Early and Middle Jurassic (although this was greatly exceeded in later intervals following the radiation of pterodactyloids), but its composition underwent some important changes (Unwin 2003). The Late Triassic assemblage appears to have been dominated by campylognathoidids (represented by Austriadactylus and the seemingly ubiquitous Eudimorphodon) and this lineage, together with more basal groups such as dimorphodontids, persisted into the Early Jurassic. These clades appear to have become extinct by the end of this interval, however, and later Jurassic assemblages consist entirely of rhamphorhynchids and anurognathids, joined in the Late Jurassic by pterodactyloids.
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