Cretaceous Freshwater Research Articles

The first record of gars (Ginglymodi: Lepisosteidae) from the Upper Cretaceous of Bulgaria reveals a wider paleogeographic distribution of lepisosteids within the European Archipelago

The bony-fishes of the clade Lepisosteidae, commonly called ‘gars’ or ‘gar-fish’, are a lineage of proficient piscivores with evolutionary history spanning about 150 million years, which are today represented by two genera inhabiting the freshwater and brackish areas of southeastern North America, Central America, and Cuba. This was not the case during the Late Cretaceous when lepisosteids were more diverse and had much wider geographical distribution. Gar fossils, albeit very fragmentary, are a common component of the Upper Cretaceous freshwater and brackish vertebrate assemblages in Europe, yet all of the pre-upper Campanian records come from the western and central parts of the Late Cretaceous European Archipelago. Here we describe new lepisosteid material from Bulgaria, comprising nine teeth and three scales found at the uppermost Santonian–lowermost Campanian Vrabchov Dol vertebrate locality. These fossils represent the easternmost record of gars within the European Archipelago to date. Despite being found in a lagoonal to foreshore deposits, the paleontological content of the locality, the incompleteness and preservational state of the material, as well as the predominantly non-marine ecology of modern and fossils gars suggest that these fossils belong to fishes which inhabited more inland, freshwater environments. The Vrabchov Dol lepisosteids remains are the first record of gars in Bulgaria and one of the rare documented occurrences of Mesozoic osteichthyans in the country. This material expands the paleobiogeographic distribution of the Lepisosteidae within the European Archipelago.

A rich coprolite assemblage from Angeac-Charente (France): A glimpse into trophic interactions within an Early Cretaceous freshwater swamp

Coprolites (fossil droppings) are common in the Mesozoic fossil record. However, coprolite assemblages from continental settings have more rarely been quantitatively analysed than the marine ones. The excavation of the Berriasian continental Lagerstätte of Angeac-Charente (France) during the last decade has resulted in unearthing a vast number of fossils, including ca. 6000 coprolite specimens. This large collection, accompanied by spatially landmarked depositional data, offers a unique opportunity to assess the organism interactions in an Early Cretaceous freshwater swamp ecosystem. We assign the coprolites to nine morphotypes based on their morphology and contents (using tomography and thin-sections), and to four taphonomic categories. We compare the specimens to other fossil and recent droppings, including newly imaged crocodilian faeces. While the Angeac-Charente biota comprised animals of a range of different ecologies, including waters primary inhabitants (chelonians, crocodylomorphs, actinopterygians) and a variety of dinosaurs, the coprolites, together with other ichnological evidence, such as bite marks, seem to largely illustrate the activity of the crocodylomorphs in this ancient swamp. The assemblage is compared to previously analysed continental Cretaceous copro-assemblages from Europe and North Africa.

A new condensed freshwater-brackish water bivalve-dominated assemblage in the Aptian Crato Formation, Araripe Basin, NE Brazil and its paleoenvironmental significance

The known occurrences of Early Cretaceous freshwater bivalves in the Araripe Basin are confined to the 0.85- to 2-m-thick Caldas Bed of the Crato Formation. Herein, we record a new bivalve-dominated assemblage in a siltstone bed nearly 30 m below the upper boundary with the overlying Romualdo Formation. The assemblage is composed of ecologically incompatible bivalve mollusk species, being dominated by tiny individuals of Modiolus? sp., a brackish water form. These are directly associated with the freshwater bivalves Araripenaia elliptica and Monginellopsis bellaradiata, the latter two species commonly found in the Caldas Bed. Those bivalves are mixed with carbonized, partially comminuted plant remains. Specimens of Modiolus? sp. are usually articulated and are dispersed to loosely packed or forming cm-long clusters. A few shells of the freshwater forms are also articulated. The bivalves lived in a semi-confined embayment setting with fluctuating salinity. Background brackish water conditions must have existed long enough for the Modiolus? sp. larvae to settle and develop as juvenile individuals. Rapid salinity fluctuations associated with sea level and climate variations allowed A. elliptica and M. bellaradiata to thrive in the same area. Hence, the numerically dominant shells of the brackish water forms and the freshwater bivalves have been telescoped into the same bedding plane, generating an environmentally condensed, time-averaged benthic assemblage. Finally, the presence of these taxa considerably expands the vertical/temporal distribution of bivalve mollusks within the Crato Formation.

Case study of the convergent evolution in the color patterns in the freshwater bivalves

The class Bivalvia (phylum Mollusca) is one of the most successful at survival groups of animals with diverse color patterns on their shells, and they are occasionally preserved in the fossil record as residual color patterns. However, the fossil record of the residual color patterns in freshwater bivalves could be traced only to the Miocene, greatly limiting color pattern evolution knowledge. We present the color patterns of the Cretaceous freshwater bivalves belonging to three extinct families of the order Trigoniida (hereinafter the Kitadani Freshwater Bivalves) from Japan, which is the oldest and the second fossil record of freshwater molluscan color patterns. The Kitadani Freshwater Bivalves consists of two types of color patterns: stripes along the growth lines and radial rays tapered toward the umbo, which resemble that of the colored bands of extant freshwater bivalves. This resemblance of the color patterns between the Kitadani Freshwater Bivalves and the extant species indicates that the color patterns of the freshwater bivalves represent the convergent evolution between Trigoniida and Unionida. To explain this convergent evolution, we advocate three conceivable factors: the phylogenetic constraints, monotonous habitats typical of freshwater ecosystems, and the predation pressure by visual predators in freshwater sediments.

Palynoflora from an Upper Cretaceous freshwater paleolake in central India: paleoecological implications

An Upper Cretaceous paleolake near Jamsavli in the Mandla Lobe of central India has provided a unique opportunity to understand paleoecological conditions during Deccan volcanism. The sedimentary deposits of this paleolake are dominated by freshwater aquatic and semiaquatic flora, such as algae (Pediastrum, Lecaniella), dinocysts (Pierceites deccanensis), diatoms (Aulacoseira), aquatic ferns of Salviniaceae (Azolla), Marsileaceae (Crybelosporites), and pollen grains of Sparganiaceae/Typhaceae (Sparganiaceaepollenites). Such paleolakes, representing a few hundred years of lake history with only aquatic and semiaquatic biota, have rarely been documented from the Deccan volcanic province.An overview of aquatic and semiaquatic flora from the infratrappean and intertrappean beds associated with the Deccan volcanic province across the Cretaceous-Paleogene shows that the floral communities include algal remains of Botryococcaceae, Chlorophyceae, Chlorellaceae, Cyanophyceae, Hydrodictyaceae, Oedogoniaceae, Ulotrichaceae, Zygnemataceae, dinocysts, and diatoms. Bryophytes are scarce, whereas aquatic ferns of Salviniaceae (Azolla) and Marsileaceae (Marsilea and Regnellidium) are the dominating groups in the intertrappean deposits. Aquatic and semiaquatic angiosperms show less taxonomic diversity and are represented by only five families, namely Nymphaeaceae, Typhaceae, Liliaceae, Acanthaceae, and Pontederiaceae. The ubiquitous presence of macrophytes and microfloral fossil remains of Zygnemataceae (Ovoidites), Salviniaceae (Azolla), Marsileaceae (Crybelosporites and Gabonisporis), and palms, along with paleosol, indicate frost-free, warm, humid tropical to subtropical climates with intermittent dry and wet semi-arid conditions across the Late Cretaceous–early Paleocene.

The influence of Early Cretaceous paleoclimate warming event on sedimentary environment evolution and organic matter sources in Yin'e Basin: Evidence from petrology and molecular geochemistry

The lacustrine organic-rich fine-grained sedimentary rocks of the Early Cretaceous Bayingebi Formation are widely distributed in the Yin'e Basin and they have become important source rocks for oil and gas exploration. To reveal the influence of paleoclimate warming event on sedimentary environment evolution and organic matter sources, the petrological and molecular geochemical analyses of the upper member of the Bayingebi Formation (K1b2) were carried out using optical microscopy, X-ray diffraction, scanning electron microscopy, Rock-Eval pyrolysis and gas chromatography–mass spectrometry, in order to establish the paleoecological models of the Early Cretaceous freshwater and saline lakes. The oil shale and dark mudstone of K1b2 are both in the immature-early mature stage and have good potential for hydrocarbon generation, with a dominance of type I and type II1 kerogen. Two types of primary dolomite were found in this formation: microcrystalline dolomite with uniform structure, and the microbially originated spherical dolomite aggregates. The K1b2 can be divided into four units based on its lithology and logging curve characteristics. Unit II was deposited in the peak of lake development, under the warm-humid paleoclimate with abundant annual precipitation. There was greater influx of freshwater than other units, which resulted in the unstable stratification of the water column and less reducing condition of the lake bottom water body. Lower aquatic organisms and vascular plants were unprecedentedly prosperous, which were indicated by short-term thriving phenomena of telalginite, and the large input of terrestrial OM and highest abundance of the OM compared to other units. A lake water salinization event occurred during the sedimentation period of Unit III under the cooling climate, causing a transition from a freshwater lake to a high-salinity lake with reducing condition and stable stratification of the water column. Salinity of lake water increased the competitiveness of salt-tolerant algae and bacteria. The salty environment is beneficial for the thriving of single to halophilic genus of microorganism and lead to significantly lower abundance of telalginite. The biological sources of OM were mainly bacteria, algae and emergent plants, with limited contribution of terrestrial higher plants. The anoxic alkaline environment and high salinity condition at the lake bottom were beneficial for the formation of carbonate minerals and the preservation of OM. This were characterised by medium abundance of OM and high hydrocarbon generation potential. The study on the petrological and molecular geochemical characteristics of these organic-rich fine-grained sedimentary rocks is of great significance for investigating the genetic mechanisms and resource potential of oil and gas.

Early Cretaceous freshwater fishes from the Tetori Group, central Japan

Early Cretaceous freshwater fishes from the Tetori Group, central Japan

Late Jurassic-Early Cretaceous freshwater bivalves from Turiasaurus riodevensis bearing strata of Teruel (Spain)

Three freshwater bivalve taxa are described from the Villar del Arzobispo Formation (Late Jurassic-Early Cretaceous) of Riodeva, Teruel, Spain. The stratigraphic range of the large and geographically widely distributed bivalve Margaritifera, previously recorded from the Early Cretaceous of Spain and southern England, is recognised to extend back probably into the Late Jurassic. This new record extends the palaeogeographic and temporal distribution of the family Margaritiferidae in Europe

Characterization of coelacanth scales from the Early Cretaceous freshwater locality of Las Hoyas, upper Barremian (Cuenca, Spain)

Coelacanths are rare, mostly marine fishes, but the species from the Lower Cretaceous Spanish locality of Las Hoyas (Barremian) is a freshwater form and we know almost nothing about it. The Las Hoyas specimens are very rare and relatively incomplete, but there are still many things we can learn from the isolated skeletons and scales. First, the coelacanth scales were distinguished from other superficially similar scales (i.e., other “amioid” scales). Coelacanth scales are distinguished by the presence of a smooth central surface, a particular pattern of arrangement of concentric growth cessation marks, and mainly a relatively short posterior field with thick elongated ridges. Only a few articulated coelacanth specimens have been recovered from Las Hoyas to date, and only 7.3% (n = 11) of the total isolated scales are coelacanth. The Las Hoyas coelacanth scales represent relatively large individuals. This suggests that a natural population of the coelacanth may have not inhabited permanently the freshwater pool represented by the excavated area of Las Hoyas because small juveniles should be the most common sizes.

A reappraisal of the type series of the early Cretaceous margaritiferid ‘Unio’ valdensis Mantell, 1844

This study re-examines the type series of the Early Cretaceous freshwater bivalve Unio valdensisMantell, 1844, in relation to the assignment of the species to the genus ParaheudeanaStarobogatov (1970), and the subgenus Pseudunio by Mongin (1961). For the first time, we recognise the type specimens as incomplete lacking critical posterior portions of the shell. We observe that the lectotype, superficially oval in outline, has a shallow, but distinct postero-ventral sinus, seemingly out-grown, the broken ventral margin not preserving actual shell shape or the commissure. Significantly, many other specimens confirm the presence of the postero-ventral sinus and elongate-oval outline. It is unclear why Starobogatov removed Margaritifera (Pseudunio) valdensis but maybe the perceived oval outline was sufficient for separation. Combined with diagnostic characters such as mantle pits, we retain the freshwater mussel Unio valdensis in the genus Margaritifera and refute the validity of the genus ParaheudeanaStarobogatov (1970). Araujo et al. (2016) reviewing the phylogeny of the Margaritiferidae, concluded only the genus Margaritifera was valid, discounting Pseudunio as a valid genus or subgenus, placing further doubt upon the use of the then subgenus, conversely more recent studies have supported the genus Pseudunio promoting it to a genus.

First occurrence of a mawsoniid (Sarcopterygii: Actinistia), Mawsonia soba sp. nov., in pre-Aptian Cretaceous deposits from Cameroon

Coelacanth remains from the Lower Cretaceous (pre-Aptian) freshwater deposits of the Babouri Figuil Basin, Northern Cameroon are described as a new taxon Mawsonia soba sp. nov. based on two characters: four openings in the dentary for the branch of the trigeminal nerves and an extremely rugose and disorganized ornamentation of the bones. M. soba sp. nov. known by some three-dimensionally partially preserved specimens represents the first actinistian described from Cameroon. This discovery attests the presence of Mawsonia in the African counterpart of the continental pre-rift and rift environments where only Brazilian members were known so far. The fact that the Cameroonian coelacanth belongs to a new species suggest a certain endemism at least in this African basin.

New articulated osteoglossomorph from Late Cretaceous freshwater deposits (Maastrichtian, Scollard Formation) of Alberta, Canada

ABSTRACTArticulated fishes are rare in Late Cretaceous non-marine deposits of the Western Interior; fishes are more often represented by disarticulated elements in vertebrate microfossil localities. A new Maastrichtian locality of the Scollard Formation in central Alberta, Canada, has been named Pisces Point to reflect the diversity of articulated fishes that are now being recovered from this site. Material collected from the Pisces Point locality includes a percopsiform, a semionotiform, at least one esociform, and at least two osteoglossomorphs. One of these is described here as a new genus and species, †Wilsonichthys aridinsulensis. A phylogenetic analysis shows this new taxon to be the sister group of most other Osteoglossiformes. The new articulated material allows us to identify some of the microfossil remains previously collected from other Late Cretaceous sites as belonging to the same or a similar taxon.http://zoobank.org/urn:lsid:zoobank.org:pub:C35449C0-5711-4618-9084-177ADA83FEE7Citation for this article: Murray, A. M., M. G. Newbrey, A. G. Neuman, and D. B. Brinkman. 2016. New articulated osteoglossomorph from Late Cretaceous freshwater deposits (Maastrichtian, Scollard Formation) of Alberta, Canada. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1120737.

Why so many dipnoans? A multidisciplinary approach on the Lower Cretaceous lungfish record from Tunisia

The Lower Cretaceous record of vertebrates from Africa is problematic as the majority of fossil localities lack adequate stratigraphic and paleoecological data when compared with coeval Laurasian deposits. Thereby, our comprehension of paleocommunities and paleobiogeographic patterns may be affected by the lack of multidisciplinary approach. Among taxonomically and paleoecological significant clades, lungfishes (Sarcopterygii, Dipnoi) are commonly found in the Cretaceous fresh water, brackish and marginal-marine deposits of Gondwana, although identifiable elements are limited to isolated tooth plates. We provide the first taxonomic identification of dipnoans from the Ain el Guettar Formation of southern Tunisia (Oum ed Diab member, Albian). Identification of tooth plates based on morphological parameters and phylogenetic analyses indicates the co-occurrence in a discrete stratigraphic unit of at least five lineages referable to Equinoxiodus, Neoceratodus, Asiatoceratodus and/or Ferganoceratodus, Ceratodus, and Lavocatodus. This unusually high diversity is unparalleled in the fossil record and is also challenged by an actualistic comparison with extant taxa. We suggest that a series of taphonomic factors significantly inflated observed lungfish diversity in the estuarine and marginal-marine deposits of the Oum ed Diab member. Therefore, we recognize the fossil fauna as representative of a larger, inland paleo-hydrographic system. This study confirms the paleoecological scenario resulted from the analyses on terrestrial reptiles from the Oum ed Diab member.

Sinamia kukurihime, a New Early Cretaceous Amiiform Fish from Ishikawa, Japan

Abstract. A new species of the genus Sinamia of the family Sinamiidae of the order Amiiformes, S. kukurihime is described on the basis of a hyomandibula and many isolated bones and scales found from the Lower Cretaceous freshwater deposits of the Kuwajima Formation, Itoshiro Subgroup, Tetori Group in Kuwajima, Hakusan City, Ishikawa Prefecture, Japan. The new species is distinguished from other congeners by the forms of the hyomandibular, frontal, dermopterotic, maxilla, gular plate, preopercle, supracleithrum and scales. Among the genus Sinamia, S. kukurihime sp. nov. is considered to be closest to S. liaoningensis. This is the first record of the genus Sinamia from Japan. The presence of a new species of the genus from Japan demonstrates the wide distribution and diversity of the genus during the Cretaceous in East Asia.

Mid-Cretaceous (Albian–Cenomanian) freshwater Mollusca from the Clarence Valley, Marlborough, New Zealand, and their biogeographical significance

Mollusca from the Warder Formation in the middle Clarence Valley, Marlborough, are the first well-preserved Cretaceous freshwater Mollusca recorded from New Zealand. They include a common mytilid (?Mytilus), a rare second mytilid (?Xenostrobus), a rare hyriid (?Echyridella), a common smooth ?Corbicula species and a rare commarginally sculptured one, an uncommon thiarid (?Melanoides), a rare lymnaeid (?Austropeplea), and abundant Clarenciella hippopedion n. gen., n. sp., a rissooidean gastropod referred to Cochliopidae. The Warder Formation underlies Gridiron Volcanics (97–95 Ma) and is laterally equivalent to the Bluff Sandstone, which contains a marine Ngaterian fauna (99.5–95.2 Ma, late Albian–Cenomanian). The freshwater fauna lived before New Zealand began to separate from the rest of Gondwana (85 Ma). Faunal elements lost from New Zealand since the Cretaceous include Cyrenidae and Cochliopidae, which are diverse in the South American Cenozoic and living freshwater faunas. In contrast, Hyriidae have a continuous Jurassic–Recent record in New Zealand and are among freshwater taxa that survived both the fragmentation of Gondwana and the putative Oligocene submergence. These and other archaic endemic freshwater taxa such as Latia (Gastropoda, Chilinoidea) remain a vestige of New Zealand’s Gondwanan heritage, modified by extinction, evolution and dispersal.

Age and growth in Myledaphus bipartitus, a Late Cretaceous freshwater guitarfish from Alberta, Canada

Myledaphus bipartitus is a Late Cretaceous (75 Ma) freshwater guitarfish, commonly represented by teeth and vertebral centra. The species was first described in 1876, but until now, very little has been known about its ecology and life history. We analyzed the age and growth using 117 fossilized centra (74 hemisectioned) from the Oldman and Dinosaur Park formations of southern Alberta. The resulting estimated maximum age was 16 years. Radial distance (RD) at the estimated asymptotic growth was calculated, and average RD at birth and 3 years were compared with two extant marine species of Rhinobatos (R. productus and R. rhinobatos). Birth ring size ranged from 0.9 to 1.8 mm RD. At age three, centrum RD was between 2.8 and 4.6 mm RD for M. bipartitus and significantly less than R. productus and R. rhinobatos (5.0–6.6 mm RD). Myledaphus approached “asymptotic” growth at 7.8 mm RD in the growth equation. The growth trajectory of Myledaphus had a similar slope to the Rhinobatos species, but with significantly smaller centrum RD. Rhinobatos rhinobatos also outlived Myledaphus by 8 years.

The maxillary depression of Pholidosauridae: an anatomical study

Goniopholidids and pholidosaurids are common crocodilians in Late Jurassic and Early Cretaceous freshwater environments in many parts of the world, although evidence that goniopholidids ever reached Gondwanan regions needs to be substantiated. In the ‘mid’-Cretaceous, some pholidosaurids (Terminonaris, Oceanosuchus) became adapted to life in the sea, a domain that goniopholidids apparently never occupied. Whereas goniopholidids were mainly mesorostral forms, pholidosaurids were longirostral. There is currently no consensus about the phylogenetic relationships between goniopholidids and pholidosaurids. They have been considered as closely related on the basis of various shared characters (Buffetaut, 1982), but in some recent phylogenetic analyses (e.g., Jouve et al., 2006; Jouve, 2009; Pol et al., 2009) at least some pholidosaurids cluster with other longirostral forms such as dyrosaurids and thalattosuchians, which may suggest that the family Pholidosauridae is not monophyletic. Nevertheless, a number of characters uniting Pholidosauridae have recently been proposed (Hua et al., 2007), but were not tested in a phylogenetic framework. On the other hand, only one unambiguous character diagnoses the clade Goniopholididae in phylogenetic studies: the presence of a depression on the posterior end of the maxilla, described as the ‘depression maxillaire’ (maxillary depression) by Buffetaut (1982; see also Lauprasert et al., 2007). On this basis, Goniopholididae is accepted as a monophyletic entity comprising the generaCalsoyasuchus,Goniopholis,Eutretauranosuchus, Siamosuchus, and Sunosuchus (Tykoski et al., 2002; Lauprasert et al., 2007; Smith et al., 2010), thus agreeing with pre-cladistic views (Buffetaut, 1982). Functionally, the maxillary depression may have housed a sensory organ helping to localize and capture prey in the aquatic environment (Brandalise de Andrade, 2009). Here, we report for the first time the presence of a maxillary depression in Pholidosauridae. Its morphology and position is for the first time described in taxa where the maxillojugal area is preserved. Implications for a close relationship between Goniopholididae and Pholidosauridae are discussed on the basis of this previously overlooked character. Institutional Abbreviations—AMNH, American Museum of Natural History, New York, U.S.A.; BYU, Brigham Young University, Provo, Utah, U.S.A.; CHE, Universite Lyon 1, Lyon, France, specimen from Cherves de Cognac; CMNH, Cleveland Museum of Natural History, Cleveland, Ohio, U.S.A.; DORCM, Dorset County Museum, Dorchester, United Kingdom; MNHN, Museum National d’Histoire Naturelle, Paris, France;MNN, Musee National du Niger, Niamey, Republique du Niger; NHMUK, The Natural History Museum, London, United Kingdom.

Late Cretaceous (Campanian/Maastrichtian) freshwater to restricted marine mollusc fauna from the Loncoche Formation, Neuquén Basin, west-central Argentina

This study is the first detailed account of freshwater to restricted marine molluscs from the Upper Cretaceous sedimentary rocks in the northern sector of the Neuquén Basin. The fossils are from the Campanian–Maastrichtian Loncoche Formation in southern Mendoza, west-central Argentina, which records the initial connection of the Neuquén Basin to the Atlantic Ocean. Six species of bivalves (Diplodon bodenbenderi, Pleiodon? sp., Isognomon? mechanquilensis, Mactridae? indet., Panopea? sp., and Laternula sp.) and three of gastropods (Paleoanculosa macrochilinoides, Paleoanculosa ameghiniana, and a possible cerithioidean) are described. Specimens were collected from fine to coarse sandstones, which may be massive or with planar stratification, planar-cross stratification or trough-cross stratification, and a few from bioclastic limestones and mudstones. Although the sections are from 50 to 300 m thick, the specimens are found only in the lower 120 m. Molluscs represent autochthonous/parautochthonous assemblages composed of mostly non-broken gastropods and articulated bivalve specimens, some of which show signs of postmortem transport; however, they were not removed far from their original habitat. Review of the habitats of living genera supports the inference of dominantly freshwater palaeoenvironments in the lower and middle part of the Loncoche Formation, with restricted marine influence in the southernmost localities studied where there are a few samples that contain specimens belonging to predominantly marine groups (e.g., Laternula, Panopea).

Taphonomy of Early Cretaceous freshwater bivalve concentrations from the Sihetun area, western Liaoning, NE China

The Early Cretaceous Yixian Formation at Sihetun, Beipiao, western Liaoning, northeastern China, is well-known for yielding diverse and excellently preserved fossils of the Jehol Biota. The lower unit of Yixian Formation, dominated by lacustrine deposits, is rich in concentrations of two freshwater bivalves: Sphaerium anderssoni and Arguniella ventricosa. These bivalve concentrations can be divided into three types that comprise either paucispecific A. ventricosa or S. anderssoni, or both bivalves in similar amounts. The lithological, biotic, and taphonomic features of 12 bivalve concentrations are recorded, and the taphonomic signatures (such as shell articulation, size-frequency distribution, and orientation) are analyzed. Autochthonous as opposed to allochthonous bivalve concentrations are discriminated. A very short time-averaging effect is recognized in some concentrations, which was probably caused by seasonal or episodic water-level fluctuations and hypoxia. Three factors operated on the bivalve concentrations before final burial: in-situ reworking, transport, and time-averaging. Although time-averaging of the death assemblages was limited to several years, it is in this way that several generations of the two bivalve species could become preserved together. Reworking of the bivalve concentrations was most likely caused by storm action.

Early Aptian (Early Cretaceous) freshwater bivalves from the Australian–Antarctic rift, southeast Victoria

Australian Mesozoic freshwater and estuarine bivalves belonging to the Hyriidae, Glauconomidae and Sphaeriidae have been described from only a few localities in Queensland, New South Wales and South Australia, revealing a low-diversity fauna. The hyriids Megalovirgus flemingi McMichael, 1956, and Megalovirgus stirlingi sp. nov. are among the first Cretaceous freshwater bivalves to be described from Victoria and were recovered from upper Strezlecki Group (lower Aptian) strata, ca 4 km southwest of Inverloch, Gippsland Basin. These strata represent major fluvial flood deposits that accumulated in the Australia–Antarctica rift during the Early Cretaceous breakup of eastern Gondwana. The distribution of Megalovirgus flemingi in the Cretaceous of both New Zealand and Victoria indicates close links in the freshwater faunas between Zealandia and southeastern Australia during the Early Cretaceous.