Introducing Caihong juji

Caihong juji holotype specimen (Hu, et al., 2018)

Over the last 10 years, theropod dinosaurs from the Middle-Late Jurassic Yanliao Biota have offered rare glimpses of the early paravian evolution and particularly the origin of birds. The first discovered Yanliao non-scansoriopterygid theropod was Anchiornis huxleyi, and since then several other extremely similar species have also been reported. Caihong juji, a newly discovered Yanliao specimen, exhibits an array of osteological features, plumage characteristics, and putative melanosome morphologies not previously seen in other Paraves. The name Caihong is from the Mandarin ‘Caihong’ (rainbow). The specific name, juji is from the Mandarin ‘ju’ (big) and ‘ji’ (crest), referring to the animal’s prominent lacrimal crests.

The holotype (PMoL-B00175) is a small, articulated skeleton with fossilized soft tissues, preserved in slab and counter slab, collected by a local farmer from Qinglong County, Hebei Province, China, and acquired by the Paleontological Museum of Liaoning in February, 2014. The specimen (estimated to be ~400 mm in total skeletal body length with a body mass of ~475 g) exhibits the following autapomorphies within Paraves: accessory fenestra posteroventral to promaxillary fenestra, lacrimal with prominent dorsolaterally oriented crests, robust dentary with anterior tip dorsoventrally deeper than its midsection and short ilium.

Caihong juji differs from Anchiornis huxleyi in having a shallow skull with a long snout, forelimb proportionally short, and forearm proportionally long. Caihong also resembles basal troodontids and to a lesser degree basal dromaeosaurids in dental features (anterior teeth are slender and closely packed, but middle and posterior teeth are more stout and sparsely spaced; and serrations are absent in the premaxilla and anterior maxilla).

Platelet-like nanostructures in Caihong juji and melanosomes in iridescent extant feathers (Hu, et al., 2018)

Feathers are well preserved over the body, but in some cases, they are too densely preserved to display both gross and fine morphological features. The contour feathers are proportionally longer than those of other known non-avialan theropods. The tail feathers resemble those of Archaeopteryx, and the troodontid Jinfengopteryx in having large rectrices attaching to either side of the caudal series forming a frond-shaped tail, a feature that has been suggested to represent a synapomorphy for the Avialae.

But, the most remarkable feature observed in Caihong, is the presence of some nanostructures preserved in the head, chest, and parts of its tail, that have been identified as melanosomes. They are long, flat, and organized into sheets, with a pattern similar of those of the iridescent throat feathers of hummingbirds.

Recovered as a basal deinonychosaur, Caihong shows the earliest asymmetrical feathers and proportionally long forearms in the theropod fossil record wich indicates locomotor differences among closely related Jurassic paravians and has implications for understanding the evolution of flight-related features.

References:

Hu, et al. A bony-crested Jurassic dinosaur with evidence of iridescent plumage highlights complexity in early paravian evolution. Nature (2018) doi:10.1038/s41467-017-02515-y

Godefroit, P. et al. A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds. Nature 498, 359–362 (2013).

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The last terror birds

Skeleton of the terror bird Titanis walleri at the Florida Museum of Natural History.

In 1887, Florentino Ameghino, the “father of Argentinian Palaeontology”, described a large, toothless jaw from the Miocene of the Province of Santa Cruz, naming it Phorusrhacos longissimus and assigning it to a new family of edentulous mammal. He used this finding as a critical evidence for his contention that modern mammalian lineages originated in Argentina and later spread across the globe. Four years later, Moreno and Mercerat recognized for the first time that the mandible described by Ameghino was really that of a bird.

The Phorusrhacidae, the so-called “terror birds”, were a group of medium-to large sized extinct ground-dwelling birds, which lived during the Cenozoic, and became the dominant carnivores of South America while it was an isolated continent. They are characterized by their elongated hindlimbs, narrow pelvis, reduced forelimbs, and their huge skull with their tall, long, narrow, and hollow beaks ended in a hook. Kelenken guillermoi, is the largest known phorusrhacid and lived in the Miocene of Argentina. The skull reaches a length of 71.6 cm and the whole animal would reach 3 m high. Kelenken is also represented by a tarsometatarsus and a broken phalanx and proceeds from the locality of Comallo (Río Negro Province, Argentina). Titanis walleri, lived during the Pliocene and Pleistocene of North America. It was 2.5 metres tall and weighed approximately 150 kilograms. This giant bird is interpreted as an early immigrant during the Great American Interchange.

Proximal portion of the left humerus of Psilopterus sp. Caudal, b ventral, c cranial and d dorsal views (From Jones et. al., 2017)

At the end of the Pliocene, Phorusrhacids decline in diversity. Two new specimens support the hypothesis that the latest geologic occurrence of the Phorusrhacidae comes from late Pleistocene sediments of Uruguay. The remains comprise the distal portion of right tarsometatarsus and a left humerus; the latter is assigned to the genus Psilopterus. The first material (MPAB-520) comes from Soriano, Uruguay, and the sediments belong to the Dolores Formation (Lujanian Stage-Age, late Pleistocene/early Holocene). The following features identify the specimen as a phorusrhacid bird. (1) a large and distally expanded trochlea metatarsi III; (2) a very narrow trochlea metatarsi II with the articular surface transversally convex and without any longitudinal sulcus (in dorsal and distal views); (3) in dorsal view the trochlea metatarsi II is almost parallel and much shorter than the middle trochlea, and forming a narrow notch between trochleae II and III. The second material consists of a left humerus without distal epiphysis belonged to Museo Paleontológico Alejandro Berro (MPAB-2024).

There are two explanatory hypotheses proposed for the decline of the terror birds: environmental reasons or direct competition (at least for the larger specimens) with placental carnivore’s immigrants to South America after the setting of the Panamanian bridge. 

 

References:

Jones, W., Rinderknecht, A., Alvarenga, H. et al. PalZ (2017), The last terror birds (Aves, Phorusrhacidae): new evidence from the late Pleistocene of Uruguay, https://doi.org/10.1007/s12542-017-0388-y

ALVARENGA, Herculano M.F.  and  HOFLING, Elizabeth. Systematic revision of the Phorusrhacidae (Aves: Ralliformes). Pap. Avulsos Zool. (São Paulo) [online]. 2003, vol.43, n.4 [cited  2015-03-24], pp. 55-91 .

Top fossils discoveries of 2017

 

Skeletal anatomy of Shringasaurus indicus (From Sengupta et al., 2017)

On April 22, 2017, the March for Science reunited more than a million persons worldwide to protest against the attack on science, budget cuts and censorship. A fight that doesn’t over yet. But despite all that, 2017 was also an extraordinary year for palaeontological discoveries. My top list includes:

  • Borealopelta

Holotype of Borealopelta markmitchelli

Borealopelta markmitchelli, from the Early Cretaceous of Alberta, is a three-dimensionally preserved ankylosaurian,  discovered in the Suncor Millennium Mine in Canada. The generic name Borealopelta is derived from “borealis” (Latin, “northern”) and “pelta” (Greek, “shield”). The holotype (TMP 2011.033.0001), with an estimated living mass of 1,300 kg, is an articulated specimen preserving the head, neck, most of the trunk and sacrum, a complete right and a partial left forelimb and manus, and partial pes. The skull is covered in dermal plates, which are overlain by their associated epidermal scales. Cervical and thoracic osteoderms form continuous transverse rows completely separated by transverse rows of polygonal basement scale. Osteoderms are covered by a thick, dark gray to black organic layer, representing the original, diagenetically altered, keratinous epidermal scales. The distribution of the film correlates well to the expected distribution of melanin, a pigment present in some vertebrate integumentary structures. The keratinized tissues in this nodosaur are heavily pigmented. The possible presence of eumelanin and pheomelanin, suggested it had reddish-brown camouflage. The evidence of countershading in a large, heavily armored herbivorous dinosaur also provides a unique insight into the predator-prey dynamic of the Cretaceous Period.

  • Isaberrysaura

 

Isaberrysaura skull in lateral view and maxillary teeth (Adapted from Salgado et al., 2017)

 

Isaberrysaura mollensis gen. et sp. nov. is the first dinosaur recovered in the marine-deltaic deposits of the Los Molles Formation (Neuquén Province, Argentina), and the first neornithischian dinosaur known from the Jurassic of South America. The holotype of Isaberrysaura is an incomplete articulated skeleton with an almost complete skull, and a partial postcranium consisting of 6 cervical vertebrae, 15 dorsal vertebrae, a sacrum with a partial ilium and an apparently complete pubis, 9 caudal vertebrae, part of a scapula, ribs, and unidentifiable fragments. One of the most notable features of the discovery is the presence of permineralized seeds in the middle-posterior part of the thoracic cavity. The seeds were assigned to the Cycadales (Zamiineae) on the basis of a well-defined coronula in the micropylar region. The findings suggest the hypothesis of interactions (endozoochory) between cycads and dinosaurs, especially in the dispersion of seeds.

  •  Junornis 

 

Holotype of Junornis houi. From Chiappe et. al; 2017)

Junornis houi, from the Yixian Formation of eastern Inner Mongolia, represents a new addition to the enantiornithine diversity of the Jehol BiotaThe holotype (BMNHC-PH 919; Beijing Museum of Natural History), from the Early Cretaceous (~ 126±4 mya) of Yixian Formation,  is a nearly complete and articulated skeleton contained in two slabs, and surrounded by feather impressions defining the surface of its wings and body outline. Based on the well-preserved skeleton and exquisite plumage of Junornis, it was possible  make some estimation of its flight capacity. The body and wings of this bird were similar to those of modern passeriforms such as Alauda arvensis. 

  • Shringasaurus

Cranial anatomy of Shringasaurus indicus

In the aftermath of the Permo-Triassic mass extinction (~252 Ma), well diversified archosauromorph groups appear for the first time in the fossil record, including aquatic or semi aquatic forms, highly specialized herbivores, and massive predators. Allokotosaurians, meaning “strange reptiles” in Greek, comprise a bizarre suite of herbivorous archosauromorphs with a high disparity of craniodental features. Shringasaurus indicus, from the early Middle Triassic of India, is a new representative of the Allokotosauria. The generic name is derived from ‘Śṛṅga’ (Shringa), horn (ancient Sanskrit), and ‘sauros’ (σαῦρος), lizard (ancient Greek), referring to the horned skull.  The species name ‘indicus’, refers to the country where it was discovered. The holotype ISIR (Indian Statistical Institute, Reptile, India) 780, consist of a partial skull roof (prefrontal, frontal, postfrontal, and parietal) with a pair of large supraorbital horns. The fossil bones have been collected from the Denwa Formation of the Satpura Gondwana Basin. At least seven individuals of different ontogenetic stages were excavated in the same area. Most of them were disarticulated, with exception of a partially articulated skeleton.

  • Patagotitan 

Patagotitan reconstruction (Image: Diego Pol)

Patagotitan mayorum, originally discovered in 2010 by the rural farmer Aurelio Hernandez  is the largest and the most complete titanosaur taxa recovered to date. The generic name Patagotitan is derived from Patago (in reference to the geographic origin of the fossils, Patagonia), and titan (symbolic of its large size). The species name honours the Mayo family (owner of La Flecha Farm, the place where the fossils were found). The holotype (MPEF-PV 3400), includes an anterior and two middle cervical vertebrae, three anterior, two middle and two posterior dorsal vertebrae, six anterior caudal vertebrae, three chevrons, dorsal ribs, both sternal plates, right scapulocoracoid, both pubes and both femora. Six individuals were found in the same quarry, distributed in three distinct but closely spaced horizons, corresponding to  three different burial events. The first estimations of Patagotitan body mass suggest that it would weigh around 70 tons. The dorsal vertebrae preserved in Patagotitan, Argentinosaurus and Puertasaurus allows distinguishing the new taxon from previously known giant titanosaurs from the ‘mid-Cretaceous’ of Patagonia.

 

References:

Brown, C.M.; Henderson, D.M.; Vinther, J.; Fletcher, I.; Sistiaga, A.; Herrera, J.; Summons, R.E. “An Exceptionally Preserved Three-Dimensional Armored Dinosaur Reveals Insights into Coloration and Cretaceous Predator-Prey Dynamics”. Current Biology. doi:10.1016/j.cub.2017.06.071

Salgado, L. et al. A new primitive Neornithischian dinosaur from the Jurassic of Patagonia with gut contents. Sci. Rep. 7, 42778; doi: 10.1038/srep42778 (2017)

Liu D, Chiappe LM, Serrano F, Habib M, Zhang Y, Meng Q (2017) Flight aerodynamics in enantiornithines: Information from a new Chinese Early Cretaceous bird. PLoS ONE12(10): e0184637. https://doi.org/10.1371/journal.pone.0184637

Saradee Sengupta, Martín D. Ezcurra and Saswati Bandyopadhyay. 2017. A New Horned and Long-necked Herbivorous Stem-Archosaur from the Middle Triassic of India. Scientific Reports. 7, Article number: 8366. DOI: s41598-017-08658-8

Carballido JL, Pol D, Otero A, Cerda IA, Salgado L, Garrido AC, Ramezani J, Cúneo NR, Krause JM. 2017 A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs. Proc. R. Soc. B 284: 20171219.
DOI: 10.1098/rspb.2017.1219

Pisanosaurus and the Triassic ornithischian crisis

Pisanosaurus mertii holotype. Dorsal vertebrae in left lateral (A) and right lateral (B) views. Scale bar: 5 cm. From Agnolín and Rozadilla, 2017.

In 1887, Harry Govier Seeley was the first to subdivide dinosaurs into Saurischians and the Ornithischians based on the nature of their pelvic bones and joints. While the clade Saurischia is well represented in the Late Triassic, the record of the Ornithischia is certainly more problematic. Only a single Triassic ornithischian taxon was generally considered to still be valid: Pisanosaurus mertii, originally described by Argentinian paleontologist Rodolfo Casamiquela in 1967, based on a poorly preserved but articulated skeleton from the upper levels of the Ischigualasto Formation (Late Triassic).

The holotype and only known specimen (PVL 2577) is a fragmentary skeleton including partial upper and lower jaws, seven articulated dorsal vertebrae, four fragmentary vertebrae of uncertain position in the column; the impression of the central portion of the pelvis and sacrum; an articulated partial hind limb including the right tibia; fibula; proximal tarsals and pedal digits III and IV; the distal ends of the right and left femora; a left scapular blade (currently lost); a probable metacarpal III;  and the impressions of some metacarpals (currently lost).

Reconstructed skeleton reflecting the traditional interpretation of Pisanosaurus (Royal Ontario Museum)

But Pisanosaurus shows some derived traits that resulted as unambiguous synapomorphies of the Silesauridae clade, and include: reduced to absent denticles on maxillary and dentary teeth; sacral ribs shared between two sacral vertebrae; lateral side of proximal tibia with a fibular flange; dorsoventrally flattened ungual phalanges; and ankylothecodonty, teeth partially fused to maxilla and dentary bone. The inclusion of Pisanosaurus within Silesauridae implies that this taxon does not constitute the oldest ornithischian. This is consistent with previous interpretations proposing that ornithischian radiation occurred after the Triassic–Jurassic boundary.

To explain the relatively low diversity exhibited by Ornithischia in the Late Triassic-Early Jurassic, several hypotheses have been proposed. One, suggests that Ornithischia is the sister-taxon of Neotheropoda (the least inclusive clade that includes Coelophysis and modern birds) within the clade of ‘traditional theropod taxa’. In this model, a ‘transitional’ ornithischian may possess some anatomical features of theropods that appear to be more like those in more derived than Eodromaeus murphi and Tawa hallae.

Hypothesis 4, in which Ornithischia forms the sister-taxon of Averostra (From Baron 2017)

In a second hypothesis, Ornithischia is positioned as the sister-taxon to the coelophysids. In this model, Neotheropoda and Ornithoscelida would encompass the same set of taxa, but Ornithoscelida would, theoretically, take priority of Neotheropoda as it is the older name. In a third hypothesis, Ornithischia is positioned as the sister-taxon to the ‘other neotheropods’ not contained in the coelophysid clade.

Another hypothesis proposes that Ornithischia forms the sister-taxon of Averostra. Like Ornithischia, Averostra is only known from the Jurassic and Cretaceous Periods, and both share a number of anatomical features, such as fusion of the sacral neural. Another anatomical traits that could unite such a group include the possession of six or more sacral vertebrae; and the fusion of the sacral neural spines into a broad and continuous sheet, as in early ornithischians like Lesothosaurus diagnosticus and tetanuran theropods like Megalosaurus bucklandii. It’s worth mentioning the fact the earliest known unambiguous members of both Ornithischia and Averostra, are found in the same formation in South America: Laquintasaura venezuelae and Tachiraptor admirabilis.

Laquintasaura venezuelae gen. et sp. nov (From Barret et al., 2014)

 

It was suggested (Baron and Barrett 2017) that Chilesaurus diegosaurezi from the Late Jurassic, might represent the earliest diverging member of Ornithischia. Chilesaurus shows several characters typical of ornithischians. The features include a premaxilla with an edentulous anterior region;  loss of recurvature in maxillary and dentary teeth; a postacetabular process that is 25–35% of the total anteroposterior length of the ilium; possession of a retroverted pubis; a pubis with a rod-like pubic shaft; a pubic symphysis that is restricted to the distal end of the pubis; and a femur that is straightened in anterior view. The unique combination of ‘primitive’ and ‘derived’ characters for Chilesaurus has the potential to illuminate the order in which traditional ornithischian synapomorphies were acquired.

The Phytodinosauria hypothesis suggest that Ornithischia is nested among the taxa traditionally termed as sauropodomorphs could also offer a solution to the problem of the lack of unambiguous ornithischians in the Carnian and Late Triassic in general.

 

References:

Baron, M. G. (2017): Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?, Historical Biology, DOI: 10.1080/08912963.2017.1410705

Agnolín FL, Rozadilla S. (2017): Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina, Journal of Systematic Palaeontology DOI: 10.1080/14772019.2017.1352623

Baron M. G, Barrett P. M. 2017, A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution.  Nature 543, 501–506  (2017).  doi:10.1038/nature21700

Barrett, Paul M.; Butler, Richard J.; Mundil, Roland; Scheyer, Torsten M.; Irmis, Randall B.; Sánchez-Villagra, Marcelo R. (2014). A palaeoequatorial ornithischian and new constraints on early dinosaur diversification, Proceedings of the Royal Society B, DOI: 10.1098/rspb.2014.1147

Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol & Stephen L. Brusatte, Untangling the dinosaur family tree, Nature 551 (2017) doi; oi:10.1038/nature24012

Padian K. Dividing the dinosaurs. Nature 543, 494–495 (2017) doi:10.1038/543494a

 

The bizarre Halszkaraptor escuilliei

H. escuilliei MPC D-102/109. From Cau et al., 2017.

Maniraptoran lineages evolved novel ecomorphologies during the Cretaceous period, including active flight, gigantism, cursoriality and herbivory. This group share the following characteristics: large brain but a reduced skull in comparison to their body size, beaks, and smaller teeth. Now, a well-preserved maniraptoran from Mongolia, revealed a mosaic of features, most of them absent among non-avian maniraptorans but shared by reptilian and avian groups with aquatic or semiaquatic ecologies. This new theropod, Halszkaraptor escuilliei gen. et sp. nov., adds an amphibious ecomorphology to those evolved by maniraptorans.

The holotype, MPC (Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia) D-102/109, is an articulated and almost complete skeleton preserved three-dimensionally. The generic name, honours Halszka Osmólska (1930–2008) for her contributions to theropod palaeontology. The species name, ‘escuilliei’ refers to François Escuillié, who returned the holotype to Mongolia.

Reconstruction of Halszkaraptor escuilliei. Photograph: Lukas Panzarin/Andrea Cau

Halszkaraptor is related to other enigmatic Late Cretaceous maniraptorans from Mongolia in a novel clade at the root of Dromaeosauridae. It was the size of a mallard. Originally poached from Ukhaa Tolgod, Mongolia, the fossil was in private collections in Japan and England for an unknown amount of time, and later it  was transferred to the Royal Belgian Institute of Natural Sciences (RBINS). Thanks to a cooperation agreement between the Ministry of Education, Culture and Science of Mongolia, the Belgian Science Policy Office and the RBINS, the specimen returned to the Institute of Paleontology and Geology, Mongolian Academy of Science.

The skeleton is almost complete. The skull is lightly built, and is still articulated with the first cervical vertebra. The preorbital region forms 60% of basicranial length, and each premaxilla is elongate, bearing eleven teeth, the highest number among dinosaurs. The presacral vertebrae include 10 cervicals and 12 dorsals. The neck forms 50% of snout–sacrum length.

Skull of H. escuilliei. From Cau et al., 2017

The forelimb is relatively shorter than in most dromaeosaurids. The ulna is flattened and possesses an acute posterior margin. The hand has a morphology that is unique among theropods, with a progressive elongation of the lateral fingers, with the third being the longest and most robust. The 76 mm long femur has a robust greater trochanter. The metatarsus lacks cursorial adaptations and measures 80% of femoral length. The feet are complete and articulated, although some elements are poorly visible.

Based on the neck hyperelongation for food procurement, the forelimb proportions that may support a swimming function, and postural adaptations convergent with short-tailed birds, Halszkaraptor may represent the first case among non-avian dinosaurs of a double locomotory module.

References:

Cau, A.; Beyrand, V.; Voeten, D.; Fernandez, V.; Tafforeau, P.; Stein, K.; Barsbold, R.; Tsogtbaatar, K.; Currie, P.; Godrfroit, P.; “Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs”. Nature. doi:10.1038/nature2467

A brief history of Mesozoic theropods research in Gondwana

Snout of the ceratosaurian Genyodectes serus

In the last decades, the study of Gondwanan non-avian theropods has been highly prolific, showing that the group reached a great taxonomic and morphological diversity comparable to that of Laurasia. The Mesozoic Gondwanan neotheropod record includes: coelophysoids, basal averostrans, ceratosaurids, abelisauroids, megalosauroids, carcharodontosaurids, megaraptorans, basal coelurosaurs, compsognathids, alvarezsauroids, unenlagiids, and basal avialans, as well as putative tyrannosauroids, ornithomimosaur-like forms, and troodontid. Therefore, the Gondwanan fossil record has been crucial to understand the evolution and global biogeography of dinosaurs during the Mesozoic.

The first probable theropod remains from Gondwana were discovered in Colombia by Carl Degenhard, a German engineer, in 1839. At that time the word “dinosaur” did not even exist yet. Although Degenhard identified them as bird footprints, his brief description suggests that they were tracks of bipedal dinosaurs. But it was not until 1896 that the first Gondwanan theropod was named by the French palaeontologist Charles Depéret as “Megalosaurus” crenatissimus from the Upper Cretaceous of Madagascar. Several theropod remains were described from India, Africa, and South America during the 19th century. These early fragmentary discoveries lead the authors of the late XIX and early XX centuries to interpret them as belonging the same lineages present in Europe and North America.

Elaphrosaurus bambergi (Museum für Naturkunde 4960, holotype) from the Upper Jurassic of Tanzania (Janensch, 1920)

In 1901, A. Smith Woodward described Genyodectes, based on fragmentary skull bones, including portions of both maxillas, premaxillae,  parts of the supradentaries, and some teeth, discovered by Santiago Roth in Chubut, at the end of the 1880s. Genyodectes remained as the most completely known  theropod from South American until the 1970s. In 2004, O. Rauhut concluded that Genyodectes is more closely related to Ceratosaurus than the more derived abelisaurs.

Between 1915 and 1933, the most relevant Gondwanan theropod discoveries were produced by the work of the German palaeontologists Frederich von Huene, Ernst Stromer, and Werner Janensch, including for the first time the publication of very informative partial skeletons, such as those of Spinosaurus aegyptiacus and Elaphrosaurus bambergi (Stromer, 1915; Janensch, 1920). Despite its low fossil record, Spinosaurus is one of the most famous dinosaur of all time. This gigantic theropod possessed highly derived cranial and vertebral features sufficiently distinct for it to be designated as the nominal genus of the clade Spinosauridae. But during and after the Second World War the influence of the German palaeontology in the research of Gondwanan theropods abruptly declined.

Skull and neck of Carnotaurus sastrei

By the 1960s, the Argentine biologist Osvaldo Reig, together with Rodolfo Casamiquela and José Bonaparte, began to explore the Mesozoic rocks of Argentina looking for fossil tetrapods. In 1985, Bonaparte published a note presenting Carnotaurus sastrei as a new genus and species and briefly describing the skull and lower jaw. It was collected in the lower section of La Colonia Formation, Chubut Province. The discoveries of Bonaparte and his collaborators resulted in the recognition of the Patagonian theropod record as the most relevant and informative among Gondwanan continents. Some of the theropod species discovered in Patagonia are known on the basis of skulls and fairly complete skeletons offering insights into the anatomy and phylogeny of abelisaurids, carcharodontosaurids, and maniraptorans.

References:

Martín D. Ezcurra, and Federico L. Agnolín (2017). Gondwanan perspectives: Theropod dinosaurs from western Gondwana. A brief historical overview on the research of Mesozoic theropods in Gondwana. Ameghiniana 54: 483–487. Published By: Asociación Paleontológica Argentina https://doi.org/10.5710/102.054.0501

Novas, F.E., et al., Evolution of the carnivorous dinosaurs during the Cretaceous: The evidence from Patagonia, Cretaceous Research (2013), http://dx.doi.org/10.1016/j.cretres.2013.04.001 

Buffetaut, E. 2000A forgotten episode in the history of dinosaur ichnology; Carl Degenhardt’s report on the first discovery of fossil footprints in South America (Colombia, 1839). Bulletin de la Société Géologique de France 171: 137140Google Scholar

 

Historical perspective on the dinosaur family tree

Megalosaurus at Crystal Palace Park, London. From Wikimedia Commons.

In the 19th century, the famous Victorian anatomist Richard Owen diagnosed Dinosauria using three taxa: Megalosaurus, Iguanodon and Hylaeosaurus, on the basis of three main features: large size and terrestrial habits, upright posture and sacrum with five vertebrae (because the specimens were from all Late Jurassic and Cretaceous, he didn’t know that the first dinosaurs had three or fewer sacrals). These characteristics were more mammalian than reptilian. But new fossil findings from Europe and particularly North America forced to a new interpretation about those gigantic animals.

In 1887, Harry Govier Seeley summarised the works of Cope, Huxley and Marsh who already subdivided the group Dinosauria into various orders and suborders. However, he was the first to subdivide dinosaurs into Saurischians and the Ornithischians, based on the nature of their pelvic bones and joints. He wrote: The characters on which these animals should be classified are, I submit, those which pervade the several parts of the skeleton, and exhibit some diversity among the associated animal types. The pelvis is perhaps more typical of these animals than any other part of the skeleton and should be a prime element in classification. The presence or absence of the pneumatic condition of the vertebrae is an important structural difference…” Based on these features, Seeley denied the monophyly of dinosaurs.

Seeley’s (1901) diagram of the relationships of Archosauria. From Padian 2013

At the mid 20th century, the consensual views about Dinosauria were: first, the group was not monophyletic; second almost no Triassic ornithischians were recognised, so they were considered derived morphologically, which leads to the third point, the problem of the ‘‘origin of dinosaurs’’ usually was reduced to the problem of the ‘‘origin of Saurischia,’’ because theropods were regarded as the most primitive saurischians. A great influence on the views about the dinosaur origins was Alan Charig. He was Curator of Amphibians, Reptiles and Birds at the British Museum (Natural History), now the Natural History Museum, in London for almost thirty years. Charig thought that the first dinosaurs were quadrupedal, not bipedal. He based this on the kinds of animals that he and his colleagues found in the early Triassic localities of eastern and South Africa. He thought that forms such as ‘‘Mandasuchus’’ were related to dinosaurs, but that they had a posture intermediate between a sprawling and upright gait that he called ‘‘semi-improved” or ‘‘semi-erect’’.

Herrerasaurus skull. From Wikimedia Commons.

The discovery of Lagosuchus and Lagerpeton from the Middle Triassic of Argentina induced a change in the views of dinosaurs origins. Also from South America came Herrerasaurus from the Ischigualasto Formation, the basal sauropodomorphs Saturnalia, Panphagia, Chromogisaurus, and the theropods Guibasaurus and Zupaysaurus, but no ornithischians except a possible heterodontosaurid jaw fragment from Patagonia. The 70s marked the beginning of a profound shift in thinking on nearly all aspects of dinosaur evolution, biology and ecology. Robert Bakker and Peter Galton, based on John Ostrom’s vision about Dinosauria, proposed, for perhaps the first time since 1842, that Dinosauria was indeed a monophyletic group and that it should be separated (along with birds) from other reptiles as a distinct ‘‘Class”. In 1986, the palaeontologist Jacques Gauthier showed that dinosaurs form a single group, which collectively has specific diagnostic traits that set them apart from all other animals.

From Baron et al., 2017.

Phylogenetic analyses of early dinosaurs have  supported the traditional scheme. But back in March of this year, a paper, authored by Matthew Baron, David Norman and Paul Barrett, challenged this paradigm with a new phylogenetic analysis that places theropods and ornithischians together in a group called Ornithoscelida. The team analysed a wide range of dinosaurs and dinosauromorphs (74 taxa were scored for 457 characters), and they arrived at a dinosaur evolutionary tree containing one main branch that subdivides into the groupings of Ornithischia and Theropoda, and a second main branch that contains the Sauropoda and Herrerasauridae (usually positioned as either basal theropods or basal Saurischia, or outside Dinosauria but close to it). The term Ornithoscelida was coined in 1870 by Thomas Huxley for a group containing the historically recognized groupings of Compsognatha, Iguanodontidae, Megalosauridae and Scelidosauridae. The synapomorphies that support the formation of the clade Ornithoscelida includes: an anterior premaxillary foramen located on the inside of the narial fossa; a sharp longitudinal ridge on the lateral surface of the maxilla; short and deep paroccipital processes; a post-temporal foramen enclosed within the paroccipital process; a straight femur, without a sigmoidal profile; absence of a medioventral acetabular flange; a straight femur, without a sigmoidal profile; and fusion of the distal tarsals to the proximal ends of the metatarsals.

Of course, those results have great implications for the very origin of dinosaurs. Ornithischia don’t begin to diversify substantially until the Early Jurassic. By contrast, the other dinosaurian groups already existed by at least the early Late Triassic. If the impoverished Triassic record of ornithischians reflects a true absence, ornithischians might have evolved from theropods in the Late Triassic (Padian, 2017). The study also suggest that dinosaurs might have originated in the Northern Hemisphere, because most of their basal members, as well as their close relatives, are found there. Furthermore, their analyses places the origin of dinosaurs at the boundary of the Olenekian and Anisian stages (around 247 Ma), slightly earlier than has been suggested previously.

 

The dinosaur family tree Credit: Max Langer

More recently, an international team of early dinosaur evolution specialists, led by Max Langer, highlighted that the lack of some important taxa (for example, the early thyreophoran Scutellosaurus, the possible theropod Daemonosaurus, and the newly described Ixalerpeton and Buriolestes) may have a substantial effect on character optimizations near the base of the dinosaur tree, and thus on the interrelationships of early dinosaurs. The study did not find strong evidence to discard the traditional Ornithischia–Saurischia division. But they reintroduced a third possibility that was articulated in the 1980s but rarely discussed since: that sauropodomorphs and ornithischians may form their own herbivorous group, separate from the ancestrally meat-eating theropods. The Phytodinosauria hypothesis was coined by Robert T. Bakker in his book The Dinosaur Heresies: “Therefore all the plant-eating dinosaurs of every sort really constitute one, single natural group branching out from one ancestor, a primitive anchisaurlike dinosaur. And a new name is required for this grand family of vegetarians. So I hereby christen them the Phytodinosauria, the “plant dinosaurs”‘

References:

Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol & Stephen L. Brusatte, Untangling the dinosaur family tree, Nature 551 (2017) doi; oi:10.1038/nature24012

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution.  Nature 543, 501–506  (2017).  doi:10.1038/nature21700

Padian K. Dividing the dinosaurs. Nature 543, 494–495 (2017) doi:10.1038/543494a

Padian K. The problem of dinosaur origins: integrating three approaches to the rise of Dinosauria. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, Available on CJO 2013 doi:10.1017/S1755691013000431 (2013).

Seeley, H. G. On the classification of the fossil animals commonly named DinosauriaProc. R. Soc. Lond. 43165171 (1887).

Huxley, T. H. On the classification of the Dinosauria, with observations on the Dinosauria of the Trias. Quarterly Journal of the Geological Society, London 26, 32-51. (1870).

The Winds of Winter

Gravity anomaly map of the Chicxulub impact structure (From Wikimedia Commons)

Almost thirty years ago, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that an asteroid collided with the Earth and caused one of the most devastating events in the history of life. The impact created the 180-kilometre wide Chicxulub crater causing widespread tsunamis along the coastal zones of the surrounding oceans and released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere.

Three-quarters of the plant and animal species on Earth disappeared, including non-avian dinosaurs, other vertebrates, marine reptiles and invertebrates, planktonic foraminifera and ammonites. Marine ecosystems lost about half of their species while freshwater environments shows low extinction rates, about 10% to 22% of genera.

A time-lapse animation showing severe cooling due to sulfate aerosols from the Chicxulub asteroid impact 66 million years ago (Credit: PKI)

The decrease of sunlight caused a drastic short-term global reduction in temperature (15 °C on a global average, 11 °C over the ocean, and 28 °C over land). While the surface and lower atmosphere cooled, the tropopause became much warmer, eliminate the tropical cold trap and allow water vapor mixing ratios to increase to well over 1,000 ppmv in the stratosphere. Those events accelerated the destruction of the ozone layer. During this period, UV light was able to reach the surface at highly elevated and harmful levels. This phenomenon is called “impact winter”.

Recent drilling of the peak ring of the Chicxulub impact crater has been used to create 3-D numerical simulations of the crater formation. It was estimate that the angle of impact at Chicxulub was ~60° with a downrange direction to the southwest. The new study indicates that the impact may have released around three times as much sulfur and much less carbon dioxide compared with previous calculations, suggesting that surface temperatures were likely to have been significantly reduced for several years and ocean temperatures affected for hundreds of years after the Chicxulub impact.

 

References:

Artemieva, N., Morgan, J., & Expedition 364 Science Party (2017). Quantifying the release of climate-active gases by large meteorite impacts with a case study of Chicxulub. Geophysical Research DOI: 10.1002/2017GL074879

 

Charles G. Bardeen, Rolando R. Garcia, Owen B. Toon, and Andrew J. Conley, On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections, PNAS 2017 ; published ahead of print August 21, 2017 DOI: 10.1073/pnas.1708980114

Brugger J.G. Feulner, and S. Petri (2016), Baby, it’s cold outside: Climate model simulations of the effects of the asteroid impact at the end of the CretaceousGeophys. Res. Lett.43,  doi:10.1002/2016GL072241.

 

Halloween special V: Lovecraft’s paleontological Journey

H.P. Lovecraft’s love for astronomy is well known. As an amateur astronomer, Lovecraft attended several lectures from leading astronomers and physicists of his time. In 1906 he wrote a letter to the Scientific American on the subject of  finding planets in the solar system beyond Neptune. Around this time he began to write two astronomy columns for the Pawtuket Valley Gleaner and the Providence Tribune. He also wrote a treatise, A Brief Course in Astronomy – Descriptive, Practical, and Observational; for Beginners and General Readers. In several of his astronomical articles he describes meteors as  “the only celestial bodies which may be actually touched by human hands”. But Lovecraft was also obsessed with the concept of deep time, so geology and paleontology were also present in his writings.

Lovecraft’s monsters are certainly titanic, biologically impossible beings, from dimensions outside our own. He began conjuring monsters almost from the start of his career. In “The Nameless City”, published in the November 1921 issue of the amateur press journal The Wolverine, and often considered the first Cthulhu Mythos story, he describes an ancient race of reptiles that built the city: “They were of the reptile kind, with body lines suggesting sometimes the crocodile, sometimes the seal, but more often nothing of which either the naturalist or the palaeontologist ever heard.”  

Panorama of Ross Island showing Hut Point Peninsula (foreground), Mount Erebus (left) and Mount Terror (right), Antarctica. Photo: John Bortniak, NOAA

According to his biographer S. T. Joshi, Lovecraft was fascinated by Antarctica since an early age. Much of this fascination is recognizable in his famous novel “At the Mountains of Madness”, written in 1931. The novel was rejected by Weird Tales and finally was published by Astounding Stories in a serial form in 1936. “At the Mountains of Madness” is told from the perspective of William Dyer, a geologist from Miskatonic University who flies into an unexplored region of Antarctica. He’s accompanied by Professor Lake, a biologist, Professor Pabodie, an engineer, and some graduate students. The basic plot of the novel is the discovery of the frozen remains of bizarre entities from the deep space and their even more terrifying “slaves”:  the  shoggoths. The story could be divided in two parts. The first one is particularly rich, detailed and shows an impressive scientific erudition. This is clear in the following paragraph when he describes something that Professor Lake found: “He  was strangely convinced that the marking was the print of some bulky, unknown, and radically unclassifiable organism of considerably advanced evolution, notwithstanding that the rock which bore it was of so vastly ancient a date—Cambrian if not actually pre-Cambrian— as to preclude the probable existence not only of all highly evolved life, but of any life at all above the unicellular or at most the trilobite stage. These fragments, with their odd marking, must have been 500 million to a thousand million years old”. 

Of course, one of the most fascinating parts of the novel is the description of the Elder Things: “Cannot yet assign positively to animal or vegetable kingdom, but odds now favour animal. Probably represents incredibly advanced evolution of radiata without loss of certain primitive features. Echinoderm resemblances unmistakable despite local contradictory evidences. Wing structure puzzles in view of probable marine habitat, but may have use in water navigation. Symmetry is curiously vegetable-like, suggesting vegetable’s essentially up-and-down structure rather than animal’s fore-and-aft structure. Fabulously early date of evolution, preceding even simplest Archaean protozoa hitherto known, baffles all conjecture as to origin.” According with  S.T. Joshi, Lovecraft based his description of the Elder Thing in the fossil crinoids drawn by E. Haeckel in  Kunstformen der Natur.

E. Haeckel’s Kunstformen der Natur (1904), plate 90: Cystoidea. From Wikimedia Commons

“The Shadow Out of Time” (1935) was H. P. Lovecraft’s last major story. It’s told from the perspective of Nathaniel Wingate Peaslee, a professor of political economy at Miskatonic University. During five years, this man suffers a bizarre form of amnesia  followed by vivid dreams of aliens cities in ancient landscapes.  Later, Peaslee discovered that a small number of people throughout history suffered the same type of amnesia. They were possessed by the Great Race, a group of cone shaped creatures who developed the technique of swapping minds with creatures of another era with the purpose of learn the secrets of the Universe. Peaslee describes the gardens that surround the cities of his visions with detail. There was calamites, cycads, trees of coniferous aspect, and small, colourless flowers: “The far horizon was always steamy and indistinct, but I could see that great jungles of unknown tree-ferns, calamites, lepidodendra, and sigillaria lay outside the city, their fantastic frondage waving mockingly in the shifting vapours.”

Calamites was a genus of tree-sized, spore-bearing plants that lived during the Carboniferous and Permian periods (about 360 to 250 million years ago), closely related to modern horsetails. They reached their peak diversity in the Pennsylvanian and were major constituents of the lowland equatorial swamp forest ecosystems. The Cycadales are an ancient group of seed plants that can be traced back to the Pennsylvanian. Cycads have a stem or trunk that commonly looks like a large pineapple and composed of the coalesced bases of large leaves.  Today’s cycads are found in the tropical, subtropical and warm temperate regions of both the north and south hemispheres.

Lepidodendron (fossil tree) on display at the State Museum of Pennsylvania, From Wikimedia Commons

Lepidodendron was a tree-like (‘arborescent’) tropical plant, related to the lycopsids. The name of the genus comes from the Greek lepido, scale, and dendron, tree, because of the distinctive diamond shaped pattern of the bark. The name Lepidodendron is a generic name given to several fossil that clearly come from arborescent lycophytes but are difficult to assign to one species. Fossil remains indicate that some trees attained heights in excess of 40 m and were at least 2m in diameter at the base. They were dominant components of swamp ecosystems in the Carboniferous and frequently associated with Sigillaria, another extinct genus of tree-sized lycopsids from the Carboniferous Period. The absence of extensive branching and the structure of the leaf bases are the principal feature that distinguish Sigillaria from other lycopsids (Taylor et al, 2009). Sigillariostrobus is the name assigned to the reproductive organs or cones of Sigillaria. Unlike Lepidodendron cones, which were attached attached individually near the tip of the branches, Sigillaria cones occurred in clusters attached in certain places along the upper stem.

Tunguska forest (Photograph taken by Evgeny Krinov near the Hushmo river, 1929).

“The Colour Out of Space” is a short story written by  H. P. Lovecraft in 1927.  The story is set in the fictional town of Arkham, Massachusetts, where an unnamed narrator investigates a local area known as the “blasted heath”. Ammi Pierce, a local man, relates him the tragic story of a man named Nahum Gardner and how his life crumbled when a great rock fell out of the sky onto his farm. Within the meteorite there was a coloured globule impossible to describe that infected Gardner’s family, and spread across the property, killing all living things. It’s the first of Lovecraft’s major tales that combines horror and science fiction. The key question of the story of course is the meteorite. Although “the coloured globule” inside the meteorite has mutagenic properties we cannot define their nature. But as Lovecraft stated once, the things we fear most are those that we are unable to picture.

“The Colour Out of Space” was published nineteen year after the Tunguska Event. On the morning of June 30, 1908, eyewitnesses reported a large fireball crossing the sky above Tunguska in Siberia. The object entered Earth’s atmosphere traveling at a speed of about 33,500 miles per hour and released the energy equal to 185 Hiroshima bombs. The night skies glowed and the resulting seismic shockwave was registered with sensitive barometers as far away as England. In 1921, Leonid Kulik, the chief curator for the meteorite collection of the St. Petersburg museum led an expedition to Tunguska, but failed in the attempt to reach the area of the blast. Later, in 1927, a new expedition, again led by Kulik, discovered the huge area of leveled forest that marked the place of the Tunguska “meteorite” fall. At the time, Kulik mistook shallow depressions called thermokarst holes for many meteorites craters. However, he didn’t find remnants of the meteorite, and continued to explore the area until World War II. In the early 1930s, British astronomer Francis Whipple suggested that the Tunguska Event was caused by the core of a small comet, while Vladimir Vernadsky, suggested the cause was a lump of cosmic matter. (Rubtsov, 2009). More than a century later the cause of the Tunguska Event remains a mystery.

 

References:

Lovecraft, H. P, “At the Mountains of Madness”, Random House, 2005.

Lovecraft, H. P, “The Dreams in the Witch House and Other Weird Stories”, Penguin Books, 2004.

Joshi, S. T. (2001). A dreamer and a visionary: H.P. Lovecraft in his time. Liverpool University Press, 302.

LONG, J. (2003): Mountains of Madness – A Scientist’s Odyssey in Antarctica. Jospeh Henry Press, Washington: 252

N. Taylor, Edith L. Taylor, Michael Krings: “Paleobotany: The Biology and Evolution of Fossil Plants”. 2nd ed., Academic Press 2009.

Kathy Willis, Jennifer McElwain, The Evolution of Plants, Oxford University Press, 2013

 

 

Brief history of the Ocean Acidification through time: an update

Corals one of the most vulnerable creatures in the ocean. Photo Credit: Katharina Fabricius/Australian Institute of Marine Science

About one third of the carbon dioxide released by anthropogenic activity is absorbed by the oceans. Once dissolved in seawater, most of the CO2 is transported into deep waters via thermohaline circulation and the biological pump. But a smaller fraction of the CO2, forms carbonic acid and causes a decline in pH in the surface ocean. This phenomenon is called ocean acidification, and is occurring at a rate faster than at any time in the last 300 million years.

Acidification affects the biogeochemical dynamics of calcium carbonate, organic carbon, nitrogen, and phosphorus in the ocean and interferes with a range of processes, including growth, calcification, development, reproduction and behaviour in a wide range of marine organisms like planktonic coccolithophores, foraminifera, pteropods and other molluscs,  echinoderms, corals, and coralline algae.

The pH within the ocean surface has decreased ~0.1 pH units since the industrial revolution and is predicted to decrease an additional 0.2 – 0.3 units by the end of the century. An eight-year study carried out by the Biological Impacts of Ocean Acidification group (Bioacid), with the support of the German government, has contributed to quantifying the effects of ocean acidification on marine organisms and their habitats. Among the many effects of ocean acidification on marine organisms are including: decreased rate of skeletal growth in reef-building corals, reduced ability to maintain a protective shell among free-swimming zooplankton, and reduced survival of larval marine species, including commercial fish and shellfish. Even worse, the effects of acidification can intensify the effects of global warming, in a dangerous feedback loop.

Coccolithophores exposed to differing levels of acidity. Adapted by Macmillan Publishers Ltd: Nature Publishing Group, Riebesell, U., et al., Nature 407, 2000.

The geologic record of ocean acidification provide valuable insights into potential biotic impacts and time scales of recovery.  Rapid additions of carbon dioxide during extreme events in Earth history, including the end-Permian mass extinction (252 million years ago) and the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago) may have driven surface waters to undersaturation. But, there’s  no perfect analog for our present crisis, because we are living in an “ice house” that started 34 million years ago  with the growth of ice sheets on Antarctica, and this cases corresponded to events initiated during “hot house” (greenhouse) intervals of Earth history.

The end-Permian extinction is the most severe biotic crisis in the fossil record, with as much as 95% of the marine animal species and a similarly high proportion of terrestrial plants and animals going extinct . This great crisis ocurred about 252 million years ago (Ma) during an episode of global warming. The cause or causes of the Permian extinction remain a mystery but new data indicates that the extinction had a duration of 60,000 years and may be linked to massive volcanic eruptions from the Siberian Traps. The same study found evidence that 10,000 years before the die-off, the ocean experienced a pulse of light carbon that most likely led to a spike of carbon dioxide in the atmosphere. This could have led to ocean acidification, warmer water temperatures that effectively killed marine life.

Taxonomic variation in effects of ocean acidification (From Kroeker et al. 2010)

The early Aptian Oceanic Anoxic Event (120 million years ago) was an interval of dramatic change in climate and ocean circulation. The cause of this event was the eruption of the Ontong Java Plateau in the western Pacific, wich led to a major increase in atmospheric pCO2 and ocean acidification. This event was characterized by the occurrence of organic-carbon-rich sediments on a global basis along with evidence for warming and dramatic change in nanoplankton assemblages. Several oceanic anoxic events (OAEs) are documented in Cretaceous strata in the Canadian Western Interior Sea.

The Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago) was a short-lived (~ 200,000 years) global warming event. Temperatures increased by 5-9°C. It was marked by the largest deep-sea mass extinction among calcareous benthic foraminifera in the last 93 million years. Similarly, planktonic foraminifer communities at low and high latitudes show reductions in diversity. The PETM is also associated with dramatic changes among the calcareous plankton,characterized by the appearance of transient nanoplankton taxa of heavily calcified forms of Rhomboaster spp., Discoaster araneus, and D. anartios as well as Coccolithus bownii, a more delicate form.

The current rate of the anthropogenic carbon input  is probably greater than during the PETM, causing a more severe decline in ocean pH and saturation state. Also the biotic consequences of the PETM were fairly minor, while the current rate of species extinction is already 100–1000 times higher than would be considered natural. This underlines the urgency for immediate action on global carbon emission reductions.

References:

David A. Hutchins & Feixue Fu, Microorganisms and ocean global change, Nature Microbiology 2, Article number: 17058 (2017) doi:10.1038/nmicrobiol.2017.58 

Kump, L.R., T.J. Bralower, and A. Ridgwell. 2009. Ocean acidification in deep time. Oceanography 22(4):94–107, http://dx.doi.org/10.5670/oceanog.2009.100

Parker, L. M. et al. Adult exposure to ocean acidification is maladaptive for larvae of the Sydney rock oyster Saccostrea glomerata in the presence of multiple stressors. Biology Letters 13 (2017). DOI: 10.1098/rsbl.2016.0798

Kristy J. Kroeker, Rebecca L. Kordas, Ryan N. Crim, Gerald G. Singh, Meta-analysis reveals negative yet variable effects of ocean acidification on marine organisms, Ecology Letters (2010) 13: 1419–1434
DOI: 10.1111/j.1461-0248.2010.01518.x