Biostratigraphic and biogeographic implications of a hadrosaurid (Ornithopoda: Dinosauria) from the Upper Cretaceous Almond Formation of Wyoming,USA

The results of Barnum Brown's 1937 expedition to the Almond Formation of Wyoming consisted of two unidentified ceratopsian skulls and a partial hadrosaurid specimen (AMNH 3651). The hadrosaurid is here attributed to the Maastrichtian genus Saurolophus, verifying previous biostratigraphic correlations of this formation using ammonite zones. Fossiliferous lower Maastrichtian formations occurring latitudinally between those of Alberta, Canada, and southwestern Texas, USA, such as the Almond Formation, are essential for testing the effects and duration of apparent hadrosaurid faunal segregation earlier in the Campanian, and indirectly aiding in the placement of faunal boundaries that are currently unknown for the late Campanian. The discovery of Saurolophus in Wyoming, a close relative of the Campanian genus Prosaurolophus, affirms that the segregation of hadrosaurid faunas established in the late Campanian (～75 Ma) continued for at least 3 million years. Combining occurrences of Saurolophus from Mongolia and the Moreno Formation of California with those of Alberta, Canada, this genus appears to have had one of the largest geographic ranges of any equivalent clade of hadrosaurid dinosaur, although species level distributions are still uncertain.     

The youngest records of mosasaurid reptiles from the Upper Cretaceous of the South-Western Desert in Egypt

Mosasauroid squamates were abundant and had a worldwide distribution during the Late Cretaceous, but records from Sub-Saharan Africa are comparatively scanty and based mainly on fragmentary and isolated material. Here new mosasaur remains from the Maastrichtian (Upper Cretaceous) of Dakhla Oasis in the South-Western Desert of Egypt are recorded, including: a small, fragmentary right dentary of an indeterminate mosasaurine with a single tooth preserved in situ and an isolated tooth crown of the genus Globidens. This material stems from fossiliferous, calcareous sandstones with intercalated shales that form the lower portion of the Dakhla Formation, known to be an intertidal to subtidal deposit. Previously recorded mosasaur remains from the Eastern Desert in Egypt included Globidens phosphaticus, Platecarpus sp., and Igdamanosaurus aegyptiacus. In Africa, mosasaurs of the Maastrichtian age have been recorded from Morocco, Nigeria, Angola, the Democratic Republic of Congo, and Niger. The newly collected material from Dakhla Oasis currently constitutes the youngest record of mosasaurs in Egypt.     

New evidence for the possible occurrence of Tyrannosaurus in West Texas,and discussion of Maastrichtian tyrannosaurid dinosaurs from Big Bend National Park

Speculation regarding Tyrannosaurus in West Texas has been largely based upon a sub-adult tyrannosaurid maxilla from the Javelina Formation (Late Cretaceous–Maastrichtian) of Big Bend National Park. However, a very large anterior caudal vertebra, recently collected from the Javelina Formation, exhibits a morphology that can confidently be assigned to Tyrannosauridae and, because of its size, likely pertains to an adult Tyrannosaurus. The stratigraphic position of the specimen is closely bracketed by titanosaurid remains and further supports coexistence of these taxa. The stratigraphic position of the specimen possibly records one of the earliest occurrences of Tyrannosaurus. If so, Tyrannosaurus likely existed during roughly equivalent temporal intervals in disparate paleobiomes in both northern and southern late Maastrichtian faunal realms of North America.     

A primitive euhadrosaurian dinosaur from the uppermost Cretaceous of the Ager syncline (southern Pyrenees,Catalonia)

New hadrosaurid material is recorded from Fontllonga (Ager syncline, province of Lleida), in the Catalonian Pyrenees, comprising a dentary with part of the dental battery, recovered from Late Maastrichtian strata (Tremp Formation), close to the presumed Cretaceous/Tertiary boundary. This hadrosaurid is more derived than is Telmatosaurus transsylvanicus from the Haeg Basin (Romania), but lacks a number of features that diagnose the clade Hadrosaurinae plus Lambeosaurinae. It is attributed to the Euhadrosauria on the basis of the following synapomorphies: dentary teeth not recurved distally, narrow dentary crowns, and denticles not supported by subsidiary ridges. The phylogenetic relationships of the best-known iguanodontids and hadrosauroids are evaluated using mandibular and dentary tooth characters. On the basis of a cladistic analysis, the family of Hadrosauridae is diagnosed by more than 29 dentary tooth positions, parallel-sided vertical furrows formed by dentary alveoli, and absence of caudal secondary ridges. Within Hadrosauridae, successive sister-groups of Hadrosaurinae plus Lambeosaurinae are the Fontllonga taxon and Telmatosaurus. The clade Hadrosaurinae plus Lambeosaurinae is characterised mainly by a coronoid process inclined rostrally and by the absence of secondary ridges on dentary crowns. The Fontllonga find suggests the diversity of European hadrosaurids at the end of the Cretaceous to have been greater than previously thought. Moreover, the primitive character of European hadrosaurids as compared to western North American and Asian relatives may be explained in terms of geographical isolation during the Late Cretaceous.     

Vertebrate tracks from the Paso Córdoba fossiliferous site (Anacleto and Allen formations,Upper Cretaceous), Northern Patagonia,Argentina: Preservational,environmental and palaeobiological implications

The Paso Córdoba fossiliferous site (Río Negro, Northern Patagonia) is one of the first Mesozoic fossiliferous localities studied in Argentina. There, turtle, crocodile and dinosaur remains as well as dinosaur and bird tracks have been recorded. Recently, a new locality with vertebrate tracks, the Cañadón del Desvío, has been discovered in Paso Córdoba. Six track-bearing layers were located in outcrops belonging to the Anacleto (lower to middle Campanian, Neuquén Group) and Allen (middle Campanian-lower Maastrichtian, Malargüe Group) formations. The Cañadón del Desvío locality reveals that vertebrate trace fossils are distributed in two distinct environments, floodplains of a meandering fluvial to shallow lacustrine system and a wet interdune deposit that is associated to an aeolian setting. Also, in the logged section several soft sediment deformation structures were found. In regard of this, a sedimentary facies analysis is provided in order to assess the palaeoenvironmental implications of this new record. The analysed tracks are preserved in cross-sections, on bedding-planes and as natural casts. When it is possible, the tracking surface, true tracks, undertracks and overtracks/natural casts have been identified and the track preservation and the formation history of the tracksite are discussed. Only two tracks preserve enough anatomical details to relate them with their trackmakers, in this case hadrosaurid dinosaurs. The stratigraphical, facial and palaeoenvironmental data of this study support the idea of a transitional passage between the Anacleto and Allen Formation in Paso Córdoba. The presence of hadrosaurid dinosaur tracks suggests that the upper part of the log, where this kind of tracks were found, likely belong to Allen Formation due to this dinosaurs appear in the Southern Hemisphere in this epoch. The sum of osteological and ichnological remains improve the Paso Córdoba palaeofaunistic knowledge. The presence of six different levels in which the trackmakers walked reflects the abundance of vertebrates in the transition between Anacleto and Allen formations.     

A juvenile Edmontosaurus from the late Maastrichtian (Cretaceous) of North America: Implications for ontogeny and phylogenetic inference in saurolophine dinosaurs

The anatomy of an articulated juvenile specimen of the saurolophine hadrosaurid dinosaur Edmontosaurus annectens, LACM 23504, is described in detail. This individual consists of a partial skull and nearly complete articulated postcranium, collected from upper Maastrichtian strata of the Hell Creek Formation of Montana, USA. This study aims to help understand the ontogenetic morphological changes occurring in the cranium and postcranium of saurolophine hadrosaurids using E. annectens as a case study. Emphasis is placed on those morphological attributes that are ontogenetically variable via comparative osteology between the juvenile and the available adult specimens. It is observed that much of the cranial ontogenetic variation relates to the elongation of the skull and mandible. In the postcranium, most of the ontogenetic variation concentrates in the pectoral and pelvic girdles and the stylopodia. Not all the identified patterns of ontogenetic variation may be generalized to all hadrosaurids. The impact of ontogenetic variation on phylogenetically informative characters of saurolophine hadrosaurids is evaluated. It is concluded that, at least for Edmontosaurus annectens (and perhaps other saurolophine hadrosaurids), most characters used in phylogenetic inference of these animals are not affected by ontogeny. Thus, juvenile specimens are still a source of substantial character data suitable for use in phylogenetic analyses of saurolophine relationships. Nevertheless, it is recommended that ontogenetically variable characters are left as missing data in a character–taxon matrix when only juvenile material is available for a given saurolophine taxon. Scoring those characters based solely on juveniles is likely to decrease the accuracy of the phylogenetic inference.     

Petreşti-Arini – An important but ephemeral Upper Cretaceous continental vertebrate site in the southwestern Transylvanian Basin,Romania

The Transylvanian region of Romania preserves some of the most unusual and iconic dinosaurs in the global fossil record, including dwarfed herbivores and aberrant carnivores that lived during the very latest Cretaceous (Maastrichtian) in an ancient island ecosystem (the Haţeg Island). A series of artificial outcrops recently exposed during a hydroelectric project, the Petreşti-Arini section near Sebeş in the Transylvanian Basin, records a 400+ meter sequence documenting the transition from fully marine to terrestrial environments during the Campanian–Maastrichtian. Calcareous nannofossil biostratigraphy indicates that the lower marine beds in this section, part of the uppermost Bozeş Formation, can be assigned to the CC22 biozone, corresponding to the lower–mid upper Campanian. These beds smoothly transition, via a brackish-water unit, into the fully continental Maastrichtian Sebeş Formation. Dinosaur and pterosaur fossils from the uppermost Bozeş Formation can be assigned a late Campanian age making them the oldest well-dated terrestrial fossils from the Haţeg Island, and indicating that the classic Haţeg dinosaur fauna was becoming established by this time, coincident with the first emergence of widespread land areas. Vertebrate fossils occur throughout the overlying Sebeş Formation at the site and are dominated by the small-bodied herbivorous dinosaur Zalmoxes. The dominance of Zalmoxes, and the absence of many taxa commonly seen elsewhere in Maastrichtian sites in Romania, suggests the possibility that either the Petreşti-Arini section preserves a somewhat unusual near-shore environment, or the earliest Haţeg Island dinosaur communities were structured differently from the more diverse communities later in the Maastrichtian. Alternatively, due to the limited sample size available from the studied succession, it is also conceivable that sampling biases give an incomplete portrayal of the Petreşti-Arini local fauna. Support for any one of these alternative hypotheses requires further data from Petreşti-Arini as well as from the larger Transylvania area.     

Maastrichtian carbon cycle changes and planktonic foraminiferal bioevents at Gebel Matulla,west-central Sinai,Egypt

Comparison between the planktonic foraminiferal bioevents from different palaeolatitudes suggests that the biostratigraphic criteria used to identify the Maastrichtian stage boundaries are problematic. A new high-resolution calibration of planktonic foraminiferal biostratigraphic, carbon-isotope, and sequence-stratigraphic criteria has been recorded for the first time from the Maastrichtian Sudr Formation at Gebel Matulla, west-central Sinai. The sedimentary successions allow the identification of prominent long-term carbon isotope events in the Maastrichtian, namely the negative excursion of the Campanian–Maastrichtian Boundary Event (CMBE), the positive excursion of the mid-Maastrichtian Event (MME), and the decline towards the Cretaceous-Palaeogene transition (KPgE). Termination of these well known δ¹³C events is associated with unconformities, created by eustatic sea-level changes, although the long duration argues for superimposed local tectonic control.     

Planktonic foraminifera and calcareous nannofossils record in the upper Campanian-Maastrichtian pelagic deposits of the Malatya Basin in the Hekimhan area (NW Malatya,eastern Anatolia)

The uppermost Cretaceous (upper Campanian-Maastrichtian) pelagic successions from the Malatya Basin (NW Malatya, eastern Anatolia) were studied by 688 samples, which were collected from five stratigraphic sections in the Hekimhan area. The pelagic deposits conformably overlie rudist bearing shallow-water limestones and are overlain conformably by Maastrichtian dolomites and unconformably by Paleocene-Eocene deposits, respectively.The pelagic successions in the Hekimhan area comprise the Kösehasan Formation at the base and the Zorbehan Formation at the top and reach up to 1100 m in thickness. The Kösehasan Formation rests over the neritic rudist-bearing limestones of the Güzelyurt Formation along a sharp contact and consists mainly of flysch-type sandstone-mudstone alternation with complete and partial Bouma sequences. The carbonate content of abundant planktonic foraminifera and nannoplankton-bearing 980-m-thick succession increases upwards and the formation passes gradually to the clayey limestones and marlstones of the Zorbehan Formation to the top. Occurrences of nannoplankton Lithraphidites quadratus Bramlette and Martini and Micula praemurus (Bukry) in the first beds of the Kösehasan Formation indicate that the age of the Kösehasan Formation and overlying Zorbehan Formation is of late Maasthrichtian. Another late Maastrichtian taxa Cribrosphaerella daniae Perch-Nielsen and Arkhangelskiella maastrichtiana Burnett are observed from the lowermost part of the succession. Maastrichtian planktonic foraminifera such as Contusotruncana walfischensis (Todd) and Globotruncanita pettersi (Gandolfi) were recorded through the successions. Although planktonic foraminifera are diverse and abundant particularly in the Kösehesan Formation, index late Maasthrichtian species were not encountered. Campanian and Santonian-Campanian planktonic foraminifera, e.g. Radotruncana calcarata (Cushman) and Globotruncanita elevata (Brotzen), obtained particularly from the lower part of the succession and calcareous nannofossils such as Broinsonia parca parca Bukry, Reinhardtites anthophorus (Deflanre) and Eiffellithus eximius (Stover) are interpreted as reworked from older strata. Trace fossils are common throughout the succession.Rareness of planktonic foraminifera and nannoplankton in the uppermost part of the succession (Zorbehan Formation) indicates maximum shallowing of the latest Maastrichtian sea in this part of the basin. Rare echinoids, bivalves and ammonites are observed in that part of the sequence.The obtained data indicate that sediment accumulation rate of the pelagic deposits is rather high and about 27.5 cm/ky for this part of the basin. Changes in thickness of the formations along short distances in the five stratigraphic sections analysed in this study should be related to the diachroneity of the depositional and erosional events.     

An Upper Cretaceous (Campanian-Maastrichtian) actinopterygian fish assemblage from the marginal marine Adaffa Formation of Saudi Arabia

Actinopterygian remains have been recovered from Upper Cretaceous (lower Campanian to lower Maastrichtian) marginal marine deposits of the Adaffa Formation in northwestern Saudi Arabia. The fossils comprise gars (Lepisosteidae), pachycormids (cf. Protosphyraena sp.), indeterminate pycnodontiforms, enchodontid teleosts (cf. Enchodus sp.) and other indeterminate Teleostei. This assemblage is significant because it includes a novel occurrence for the Middle East (Pachycormidae) together with taxa (Lepisosteidae, Pycnodontiformes, Enchodontidae) that have been previously recorded from Late Cretaceous faunas elsewhere in the Mediterranean Tethyan region.     

First Brazilian carcharodontosaurid and other new theropod dinosaur fossils from the Campanian–Maastrichtian Presidente Prudente Formation,São Paulo State,southeastern Brazil

We report new theropod dinosaur material from the Presidente Prudente Formation (Campanian–Maastrichtian), Bauru Group, in southwestern São Paulo state. The material comprises a fragment of right maxilla of Carcharodontosauridae, an anterior portion of a left ilium of Abelisauroidea and a proximal portion of a right fibula of a coelurosaurian. Previous theropod records from the Bauru Basin comprise Abelisauridae and Tetanurae and in São Paulo state these have been represented by only a right premaxilla of an abelisaurid and isolated abelisaurid and carcharodontosaurid teeth. The new material reported here represents the first theropod remains from the Presidente Prudente Formation, and includes the first abelisauroid and coelurosaurian postcranial remains from the Bauru Basin in São Paulo state and the first latest Cretaceous carcharodontosaurid known from non-dental remains anywhere.     

Bird and dinosaur footprints in the Woodbine Formation (Cenomanian), Texas

The ichnofauna of the Woodbine Formation (Cenomanian), Denton County, Texas includes bird and dinosaur tracks. A new bird trackway,Magnoavipes loweiichnogen. et ichnosp. nov., appears to represent the largest bird tracks known from the Mesozoic. A theropod trackway,Fuscinapedis woodbinensisichnogen. et ichnosp. nov., is characterized by long digits of uniform width and pointed small claws. Six hadrosaurid trackways,Caririchnium protohadrosaurichnosichnosp. nov., are the oldest hadrosaurid tracks associated with skeletal elements. They include one isolated small footprint, a medium sized quadrupedal, and five large bipedal hadrosaurid trackways.     

Palynology of Upper Cretaceous (uppermost Campanian–Maastrichtian) deposits in the South Yellow Sea Basin, offshore Korea

Forty-seven samples from Upper Cretaceous sections penetrated by the Kachi-1 and Inga-1 wells in the South Yellow Sea Basin have been analysed for their spore and pollen content. Thirty-five species of 18 spore genera and 54 species of 28 pollen genera are documented. One new monotypic genus, Diporocolpopollenites, and its type species, D. kachiensis sp. nov., are erected, and Dilwynites Harris, 1965, and its type species, D. granulatus Harris, 1965, are emended. There are also three new combinations: Ephedripites eocaenicus (Selling, 1944), E. praeclarus (Chlonova, 1961), and Retitricolpites anguloluminosus (Anderson, 1960). Two palynological zones are erected: anAquilapollenites attenuatus Assemblage Zone, which encompasses deposits that are considered to be latest Campanian–Early Maastrichtian in age, and an Aquilapollenites eurypteronus Assemblage Zone for sections that have been dated as Late Maastrichtian. The assemblages are typical of the Yenisey-Amur Subprovince of the Aquilapollenites (floral) Province. Lowland floodplain to shallow, commonly mesotrophic, lacustrine environments of deposition are indicated. The climate was probably wet subtropical, with rainfall being somewhat higher during the Late Maastrichtian than through the latest Campanian–Early Maastrichtian.     

Upper Maastrichtian shallow marine environments and neoselachian assemblages in North Iberian palaeomargin (Castilian Ramp,Spain)

Upper Maastrichtian deposits formed in a nearshore subtidal environment within the Valdenoceda Formation (Castilian Ramp, North Iberian margin) are described together with two recently found selachian assemblages. Rare earth element concentrations (REE) have been used to assess the degree of taphonomic mixing and reworking, observing that it is minor or non-existent, and differences in degree of preservation and ecologic mixing can be explained by biostratinomic processes. The patterns of REE also helped to obtain a better understanding of the depositional environment, including the diagenetic history from burial to final degree of bone preservation.The fossil assemblages here described are close to that of the late Maastrichtian of Albaina (in the enclave of Condado de Treviño, Burgos), both in the Basque-Cantabrian Region, but their age may be slightly older (early late Maastrichtian). In total, the new assemblages consist of 17 taxa, assigned to 11 genera of shallow-water dwellers combined with individuals from the outer shelf. They represent cosmopolitan taxa (Squalicorax pristodontus, Serratolamna serrata and Rhombodus binkhorsti) together with local species (Rhinobatos echavei, Rhinobatos ibericus). Although there are not significant differences between Albaina and Quintanilla la Ojada faunas, the new assemblages add interesting taphonomic and geochemical information to the few existing uppermost Cretaceous deposits with fossil sharks in southwestern Europe.     

Small sized charophyte gyrogonites in the Maastrichtian of Coll de Nargó, Eastern Pyrenees: An adaptation to temporary floodplain ponds

Charophytes bearing small sized fructifications dominated in fluviatile floodplain facies (red beds) from the Maastrichtian of Coll de Nargó and neighbouring basins in the southern Pyrenees (Catalonia, Spain). These charophytes mainly belong to the genus Microchara and are often disregarded in biostratigraphic studies, which focus their attention instead on facies from permanent lakes that are richer in species which usually bear fructifications with a larger size range. However, small sized gyrogonites are also significant for biostratigraphic purposes and even include some of the index species for Maastrichtian biozones in Europe. Indeed, the charophyte assemblages from the Maastrichtian of Coll de Nargó belong to the Microchara punctata biozone, recently calibrated to the middle-upper Maastrichtian.Floodplain ponds from the Maastrichtian red beds of Coll de Nargó (Lower Red Unit) were extremely shallow, received considerable terrigenous influx and were frequently exposed, probably resulting in turbid, warm waters with high light availability. These conditions could explain the almost exclusive occurrence of charophytes with small fructifications in the Lower Red Unit. The available data, mainly based on oospores from extant species, indicate that the small size observed in gyrogonites from temporary ponds may represent an adaptation to environmental stress. Fossil species with small gyrogonites of Microchara cristata, M. punctata and Microchara nana would thus develop massively in stressed shallow ponds on fluvial floodplains. To contrast these hypotheses, we compared our results to those of four well-known case studies with similar sedimentological contexts, ranging from the Lower Cretaceous to the upper Eocene–lower Oligocene. Gyrogonite size patterns were similar in all cases, possibly suggesting that characeans display a long history of adaptation to shallow, temporary and turbid floodplain ponds by means of producing a high number of small gyrogonites, probably representing short life cycles and opportunistic strategies.     

Relative sea-level, tectono-eustasy, geoidal-eustasy and geodynamics during the Cretaceous

A priori, the recorded relative sea-level changes during the Cretaceous must be the combined effect of tectono-eustasy, geoidal-eustasy and various crustal level changes. To this we must add the human factor of differences and errors in interpretations.A posteriori, it is claimed that geoidal-eustasy dominated during the Hauterivian, Barremian, Turonian, Santonian and Maastrichtian, that tectono-eustasy dominated during the Albian, Cenomanian, Campanian and at the Maastrichtian/Danian boundary, and that local influences of sea-floor spreading are identified from the Albian/Cenomanian boundary onwards. To this we must add the local differential crustal movements modulating the global and regional ocean level changes. Geoidal-eustasy is mainly expressed as a latitudinal differentiation of the sea-level with out-of-phase changes between the hemispheres or the both high latitude regions. Furthermore, sedimentological records seem to record short-period geoidal-eustatic cycles.     

Calcareous nannofossil assemblage changes and stable isotope data from Maastrichtian to Selandian in the Akveren Formation,Western Black Sea,Turkey

This study is based on calcerous nannofossil assemblages changes and fluctutions of stable carbon and oxygen isotopes was collected clayey limestones, limestones, and marls in the Maastrichtian to Selandian from Akveren Formation (Western Black Sea). As the relative abundances of species of Micula spp, Watznauera barnesiae, and Arkhangelskiella cymbiformis, which tolerated changes of temperature and nutrition, carbon and oxygen isotopes compositions, and low species richness imply strong diagenesis effect at the Maastrichtian, there is no important diagenesis effect at Paleocene. Just after the Cretaceous/Tertiary (K-Pg) boundary, Thoracosphaera spp. and Braarudosphaera bigelowi were dominant species; Danian is characterized by Thoracosphaera, Ericsonia ovalis, Cruciplacolithus spp., Coccolithus pelagicus, and Ericsonia subpertusa. Generally, the nutrition productivity is good–moderate in Lower Maastrichtian, and decreasing carbon isotope values during the Uppermost Maastrichtian shows the presence of oligotrophic environmental conditions suitable with global nutrition crises before the K-Pg boundary and diagenesis in study area. Throughout the Danian, mesotrophic–oligotrophic environmental conditions dominate; however, the decrease in nutrition before Selandian represents oligotrophic environmental conditions. The increasing nutrition at Selandian is related to the change in the environmental conditions.     

Microfacies and depositional environment of Late Cretaceous to Early Paleocene oil shales from Jordan

The purpose of this study is to better understand the depositional setting of Late Cretaceous to Early Paleocene oil shales from southeast and central-east Jordan. One core from both the Jafr and the Azraq-Hamza Basins was logged, and their lithology, texture, and ichnofabrics were recorded. A total of 79 thin sections were analyzed petrographically, and eight microfacies types recognized. Both cores show lithologic and petrographic similarities. The oil shales are Maastrichtian to Danian in age and can be described as organic matter and calcite-rich mudrocks. The most abundant granular components are foraminifera and various types of phosphatic bio- and lithoclasts. Macrofossils (bivalves, ostracods, echinoderms) were recorded in some intervals. The current results were compared with data from a previous publication on Maastrichtian oil shales from the Jafr Basin. A new model explaining the deposition of the oil shales of the Jafr and Azraq-Hamza Basins is proposed. The onset of the Maastrichtian oil shale deposition in both basins coincides with the early Maastrichtian transgression in this region. The organic matter-rich sediments were deposited in a mid to outer ramp setting below the storm wave base. Younger oil shales of Late Maastrichtian to Danian age were deposited in a shallower environment, below the fair weather wave base. The Cretaceous/Paleogene boundary is marked by a hiatus in both cores. The Danian oil shales show relatively lower “total organic carbon” content than the Maastrichtian ones. The former are believed to have been deposited in more oxygenated bottom waters of a mid-ramp zone.     

Evolution of Dentary Diastema in Iguanodontian Dinosaurs

The dentary diastema of iguanodontians has been considered to be related to its unique jaw mechanism for herbivorous adaptation. The dentary and diastema lengths of iguanodontians were measured and compared to elucidate the evolution of iguanodontian diastema. A gap in ratios between most non-hadrosaurid iguanodontians and hadrosaurids was observed, suggesting that all non-hadrosaurid iguanodontians, expect for Ouranosaurus nigeriensis and Protohadros byrdi, lack a diastema or have a short diastema, although some other taxa have been considered to have a long diastema in previous studies. In non-hadrosaurid iguanodontians, some large-sized forms, such as Iguanodon bernissartensis, Shuangmiaosaurus gilmorei, and possibly Eolambia caroljonesa, had a short diastema through ontogeny, whereas Ouranosaurus nigeriensis and Protohadros byrdi developed a long diastema convergently. The development of a long diastema of hadrosaurine hadrosaurids may be different from that of lambeosaurine hadrosaurids. Some hadrosaurines (Edmontosaurus annectens, Edmontosaurus regalis, and Saurolophus angustirostris) may have developed a long diastema in the subadult stage and showed little elongation of dentary diastema through ontogeny. Lambeosaurines (Corythosaurus casuarius and Lambeosaurus lambei) tend to have a short diastema in the embryonic and subadult stages, and an elongated diastema from the subadult to adult stages.