| dc.description.abstracteng | Fossil remains of archosaurs, the dominant clade of non-marine tetrapods during the Mesozoic, are known from the Berriasian to Lower Valanginian Bückeberg Formation (the “German Wealden”) of northwestern Germany since the first half of the 19th century. Since the 1880s the formation became renown for the abundant occurrence of fossil vertebrate tracks, most of which have been left by dinosaurs. Despite being located within a crucial but poorly known interval of archosaur evolution, most of the material (especially the skeletal elements) has rarely been examined in the past.
The Bückeberg Formation predominantly consists of siliclastics deposited in the Lower Saxony Basin, an epicontinental basin ranging from the Netherlands in the west eastwards across northern into eastern Germany. The general palaeogeographical and palaeoenvironmental setting was that of a large freshwater lake which at least received inflow from rivers along its southern margin entering the basin in extensive deltaic systems. Episodically this lake communicated with the open Boreal Sea in the west, leading to the inflow of saline waters, which was, however, restricted to the western part of the basin during deposition of the lower part of the formation.
The present thesis provides a catalogue of archosaur material from the Bückeberg Formation which includes crocodilians (Pholidosauridae, Goniopholididae, Atoposauridae, Hylaeochampsidae), dinosaurs (Ankylosauria, Ornithopoda, Theropoda, Sauropoda) and pterosaurs. The specimens include skeletal material as well as abundant tracks.
Dinosaur tracksites of the Bückeberg Formation, some of which have produced material of impressive quality, are stratigraphically concentrated in the Obernkirchen Sandstone, a thin subunit within this formation. The Obernkirchen Sandstone represents mainly a sandy barrier to back-barrier and lagoonal setting within a limnic deltaic facies complex, which was deposited during the late Berriasian (Cypridea alta formosa ostracod subzone) in the southeast of the Lower Saxony Basin. A few tracksites occur more proximally in coeval fluvial deposits. Dinosaur footprint assemblages were produced by ornithopods, theropods, sauropods, ankylosaurs, and small, bipedal ornithischians (dryosaurids?). Other vertebrate tracks are those of turtles and, possibly, crocodilians. Due to the decrease in sandstone quarrying in recent decades, many old tracksites are inaccessible today. Additionally, historical descriptions of the tracks are of highly variable quality and often published in remote and today nearly unobtainable sources. A catalogue of 16 tracksites has been compiled from the literature and some new observations. Of these 16 tracksites, only five are still accessible and currently under study. Descriptions of each locality are provided, with a comprehensive compilation of existing data on lithofacies, stratigraphy, palaeogeography and palaeoecology of the Obernkirchen Sandstone and equivalent strata. A short review of the track-bearing lithofacies assemblage indicates that the outcrop areas have distinctly different facies and environments, and, therefore, track-bearing horizons can only be correlated stratigraphically between adjacent outcrops. For this reason, the identification of a megatracksite in the Obernkirchen Sandstone is currently regarded as premature and uncertain.
From the Bückeberg Formation, the first ichnotaxon ever attributed to a thyreophoran dinosaur, Metatetrapous valdensis, has been described in 1922/23. However, the subsequent loss of the
original material made its identification, or the existence of this ichnotaxon at all, questionable for many subsequent authors. This situation was aggravated by the fact that there are only very brief original descriptions accompanied by a single drawing. A reconsideration of the original description recognises M. valdensis as a valid ichnotaxon, which, albeit showing great resemblence in pes morphology to similar ichnotaxa, stands out from them by a tetradactyle manus. It not only still holds its original systematic attribution, but has also sparked early hypotheses on the phylogeny of dinosaurs already in 1922, possibly for the first time based upon tracks. Two surviving natural hypichnial casts of ankylosaurian pes imprints from the same stratum cannot be straightforwardly identified with the type material due to a lack of documentation. However, comprehensive circumstantial evidence, including complete accordance in size and morphology among others, strongly supports such an association. The tracks confirm the presence of ankylosaurs in this lacustrine-deltaic setting as a very rare element of the local dinosaur fauna.
Ankylosaurs are also represented by skeletal material. A fragmentary cervico-pectoral lateral spine and partial humerus of an ankylosaur from the Early Cretaceous (early Valanginian) of Gronau in Westphalia (northwestern Germany) are described. The spine shows closest morphological similarities to the characteristic pectoral spines of Hylaeosaurus armatus from the late Valanginian of England. An extensive comparison of distal humeri among thyreophoran dinosaurs support systematic differences in the morphology of the distal condyli between Ankylosauria and Stego-sauria and a referral of the Gronau specimen to the former. The humerus fragment indicates a rather small individual, probably in the size range of H. armatus, and both specimens are deter-mined herein as ?Hylaeosaurus sp. A short overview of other purported ankylosaur material from the Berriasian-Valanginian of northwest Germany shows that otherwise only the tracks can be attributed to this clade with confidence at present.
Ornithopods are represented by extremely abundant tracks, representing various morphotypes of iguanodontians with functionally tridactyl pedes. A more basal type with impressions of a hallux was reported in older studies but could not yet been verified by material. The same is true for didactyl footprints described earlier. However, the morphotypes most probably represent several orthotaxa. This is supported by the rare osteological record which supports the presence of several taxa, representing different phylogenetic grades. Rare, smaller tracks may represent dryosaurids.
Recent studies have found the enigmatic small ornithischian Stenopelix valdensis to be a basal ceratopsian. This view is adopted here, supporting palaeogeographical ties of the Bückeberg Formation fauna with Asia during the Berriasian.
According to their track record, theropods were highly diverse and abundant in the Bückeberg Formation. The diversity of track morphotypes suggests the presence of various clades, however only one of them, Troodontidae, can be determined with some confidence. In contrast, the osteological record is sparse. It includes the holotype tooth of Altispinax dunkeri, which is of uncertain systematic position but shows potentially diagnostic characters.
Sauropods are rare. They are represented only in a single tracksite (though by a large number of tracks). Their skeletal record is restricted to several caudal vertebrae and a tooth from the Early Valanginian upper Bückeberg Formation, which show titanosauriform affinities.
Crocodilians are abundant and represented by well-preserved material. In a case study, the
periorbital morphology of goniopholidids is discussed, exploring the diversity of patterns and the relevance of the data for phylogenetic studies. The revision of material focusses on Goniopholis spp. and closely related taxa from England, Germany, and Belgium, providing a comparative description of their interorbital morphology. Traditional interpretation of the interorbital elements in species of Goniopholis (G. simus, G. baryglyphaeus), where the frontal is interpreted as excluded from the orbit by a prefrontal-postorbital contact in the skull roof, is contested and clarified through the analysis of new specimens, including a morphometric analysis. In Goniopholis, failure to identify the palpe-bral and its subtle contact with the prefrontal has led to misinterpretation of elements and structures near the orbit, and the differential preservation/loss of palpebrals explains variability of the orbit in shape and orientation. In all European goniopholidids the frontal reaches the primary orbital border and there is no prefrontal-postorbital contact on the dorsal surface of the skull. Extensive contact of the palpebral with the primary orbital border creates a secondary (functional) orbital border, from which the frontal is excluded in most taxa. The condition is not exclusive of European goniopholidids and is paralleled by protosuchids, peirosaurids, and baurusuchids. At least four main morphological patterns are recognised, revealing a high diversity of European goniopholidids.
Other crocodilian material includes abundant and well-preserved material of the genus Pholidosaurus and rare material of atoposaurids and hylaeochampsids.
Pterosaurs are very rare. The skeletal record is confined to a few jaw and wing-phalanx fragments. The ichnological record includes a single specimen (manus impression) of Purbeckopus cf. penta-dactylus, which indicates the presence of a very large pterosaur (wing-span c. 6 m). When compared to the English type material of P. pentadactylus, the specimen shows some differences which are mostly related to different properties of the substrate on which they have been left. These include the presence of an impression of the metacarpo-phalangeal joint of the wing finger, normally not present in tracks. Another remarkable feature are the rather short and rounded claws which seem more suitable for walking than for grasping or climbing.
The occurrences of Purbeckopus in England and Germany are related to different environments: the English locality was situated close to a brackish lagoon while the German site was placed in a limnic-deltaic system at the margin of a large freshwater lake. The ichnotaxon may be considered to have an ichnostratigraphical potential.
A review of 6 other Berriasian-Valanginian archosaur faunas from Europe (Denmark, England, France, Spain, Romania), integrating their stratigraphic, palaeogeographical, and palaeo-climatological context, and their comparison to the Bückeberg Formation fauna reveal patterns of diversity and vicariance during this important phase of archosaur history. Similarly the evolution of faunal patterns across the Jurassic/Cretaceous boundary in Europe is investigated. A critical discussion of taphonomical constraints across various local faunas and palaeoenvironments show, that the differences in diversity and diversity distribution (dominance of taxa) is strongly dependant from taphonomical filtering and the applied sampling method. Contrary to previous assumptions, when integrating a conservative estimation of dinosaur diversity based upon the track record from the earliest Cretaceous of Spain and Germany, only a slight decrease of overall dinosaur diversity at the
beginning of the Cretaceous is considered, though a shift of dominance in the record of some groups is recognised. There is a lack of evidence for a massive, general “pauperisation” of the dinosaur fauna across the Jurassic/Cretaceous boundary in Europe, and these phenomena must be evaluated at a regional scale.
A novel approach is proposed for the analysis of habitat dominance based upon the abundance of tracks. Instead of a direct and indiscriminatory comparison of absolute track (footprint) numbers for each clade, a locality-based abundance (LBA) index is proposed. This index expresses the abundance of a clade as the ratio between the number of localities, in which a certain clade is present, to the total number of localities, assigned to the stratigraphic and regional context (SRC) that is investigated. The SRC has to be chosen to represent intervals that are characterized by stratigraphic, palaeoecological and taxonomical continuity, i.e. that it is inferred to represent a coherent ichnofauna. The LBA index is proposed to overcome adverse statistical effects by setting discrete extreme peaks of abundance into relation to a wider SRC (the ichnological equivalent of the “lagerstaetten effect”, previously defined for fossil vertebrate diversity analyses by Benson et al. 2010).
Applying the LBA to the Obernkirchen Sandstone and Iberian Range (northeast Spain) Early Cretaceous dinosaur track assemblages reveal that in northern Germany ornithopods were dominant by during the late Berriasian at latest, while in Spain sauropods dominated herbivorous dinosaur communities into the Hauterivian, despite lower absolute numbers of sauropod tracks. Only in the Aptian ornithopods took over dominance in northeastern Iberia.
It emerges that an inclusion of most of the European Berriasian through Albian faunas in a “Greater Wealden” Fauna, as proposed earlier, proves problematic. At least two major and one minor faunal changes can be recognised between the Kimmeridgian and the Albian. These were however not strictly isochronous across Europe; e.g. as a typical Late Jurassic fauna probably survived in Iberia at least into the earliest Berriasian, while in central Europe a faunal hiatus is probably located at the Tithonian/Berriasian boundary.
The pre-Hauterivian faunas are characterized by a prevalence of inherited Late Jurassic ele-ments. Immigration of Laurasian clades may have occurred during this time (ceratopsians, euhelopodid sauropods, ?microraptors) but these are mostly uncertain in exact dating and extent. In fact, in terms of higher-level clades, the pre-Hauterivian faunas blend very well with the Late Jurassic North American-Iberian-African faunas (the Laurafrican Fauna). The pattern of changes in biotope dominance between sauropods and ornithopods appear to be complex – a trend for an ornithopod faunal dominance is most evident in southern England and Germany, areas which were most severely affected by the Tithonian-Berriasian aridity crisis and may have lost temporarily their large herbivore faunas completely. In these regions, large-bodied ornithopods may have spread into trophic niches which were previously occupied by mid-level canopy browsing sauropods, especially diplodocids, in more humid lowlands after climate recovery. In other areas, e.g. in Spain and probably also in Denmark, sauropods remained to form a quantitatively important, if not exclusive faunal constituent deeply into the Early Cretaceous. In Spain relative sauropod dominance can be traced up to the Hauterivian and may have been supported by relatively dry conditions. Reasons for the apparently early Berriasian extinctions of diplodocids and turiasaurs are unclear as an immediate relationship of their total demise with the Tithonian-Berriasian aridity crisis cannot be found. However, their absence may be one of the most distinctive criteria to distinguish the genuine earliest Cretaceous archosaur faunas from the typical Late Jurassic faunas, and the latter can therefore be considered to have persisted into the early Berriasian in Iberia.
Basal Early Cretaceous and late Early Cretaceous faunas show major differences in composition which reflect the demise of several groups from Europe before the Barremian. According to these differences it seems suitable to characterise the European Berriasian-Hauterivian archosaur faunas as a distinct Purbeck-Hastings Fauna in contrast to the late Hauterivian-Albian Wealden Fauna. The former term alludes to the close relationship of Berriasian-early Valanginian (typical “Purbeckian”) to mid-Valanginian-Hauterivian faunas, which are best represented in the Hastings Group (late Berriasian-Hauterivian) of England. A typical “Hastingian” element is the appearance of euhelopodid sauropods in this interval, which seems to show a stronger affinity to more humid environments shared by ornithopods. European-Asian interchange within the Purbeck-Hastings Fauna is evident by several clades, however, exact dating and directions of this interchange is mostly unclear. Assessing the relationships to North America is hampered by a stratigraphic hiatus between the Tithonian and the Barremian.
The Wealden Fauna shows some relationships to Asia and northern Africa, but is less comparable to North American faunas of the respective age. The changes in diversity and com-position to the Purbeck-Hastings Fauna are marked, however, unfortunately the history of these changes is complicate to reconstruct in detail, especially for the large-bodied theropods due to the “theropod gap”. Immigration from Africa may have played a role but can only be assessed safely for a few clades with some certainty due to an unclear palaeobiogeographical history of most groups. The most important immigrant clade may be represented by rebbachisaurid sauropods which flourished especially in southwestern Europe – co-occurring with rapidly evolving ornithopods – while they continued to be rare towards the north.
Other groups of archosaurs are less informative than dinosaurs, though goniopholidid crocodilians show some evolution during this time, and limnic pholidosaurids may be restricted to Purbeckian-Hastingian faunas. The record of pterosaurs is too scarce for an assessment, though filter-feeding ctenochasmatids may have had an abundance maximum in late Laurafrican and early Purbeck-Hastingian faunas due to ecological circumstances.
An indisputable African influence on the European archosaur faunas after the Late Jurassic and before the late Hauterivian cannot be proven and the recently proposed concept of a basal Cretaceous “Eurogondwanan Fauna” cannot be substantiated on the basis of available data. | de |