Historical perspective on the origin of Dinosauria

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

In the nineteen 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). This characteristics were more mammalian. He even speculated that dinosaur had four-chambered hearts and warm blood like mammals.

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. 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.
In 1968, Romer wrote that ‘‘Very probably the saurischians arose in mildly polyphyletic fashion from two or several pseudosuchian forms.’’

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 (Romer 1971, 1972; Bonaparte 1975) induced a change in the views of dinosaurs origins. Also from South America came a variety of new dinosaurs, including the basal dinosaurs Herrerasaurus and Ischisaurus 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 the a profound shift in thinking on nearly all aspects of dinosaur evolution, biology and ecology. This process was called the dinosaur renaissance.

In 1974 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”.

Gauthier, in 1986, showed that Dinosauria was cladistically monophyletic and that birds were hierarchically included in Saurischia and Theropoda.

A meeting of vertebrate paleontologists (1968). From left to right: Romer, Bonaparte, W. Sill, R. Casamiquela, R. Pascual and O. Reig. (From F. Novas, 2009)

As pointed out by Steve Brusatte: “The evolutionary radiation of dinosaurs did not follow a simple pattern, but by the Early Jurassic, the Age of Dinosaur dominance was in full swing.”

References:

Padian K 2013. 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

Agostino Scilla and the true nature and origin of fossils.

Agostino Scilla (1629-1700)

At the beginning of the sixteenth century and throughout the seventeenth century a great debate about the true nature of and the origin of fossils started in Italy, the cradle of Leonardo and Aldrovandi. There was two hypothesis in dispute: the first one postulated an inorganic origin for the fossils (directly formed within rocks) and the second, which contemplated an organic origin.

The most strong supporter for the organic origin of fossils was the italian painter Agostino Scilla. He was born in Messina on August 10, 1629. He was  a disciple of Antonio Barbalunga in Messina and Andrea Sacchi in Rome, and later became a member of the Academy of Messina ‘della Fucina.

Original frontispiece of ‘La vana
speculazione disingannata dal senso by A. Scilla

He published only one scientific treatise: La vana speculazione disingannata dal senso, lettera risponsiva Circa i Corpi Marini, che Petrificati si trouano in vari luoghi terrestri (The vain speculation disillusioned by the sense, response letter concerning the marine remains, which are found petrified in various terrestrial places). The aim of the work was the demonstration that fossils, which are found embedded in sediments on mountains and hills, represent the remains of lithified organisms, which at one time lived in the marine environment. The text was later translated to Latin and it was written as a response to a letter sent to him by Giovanni Francesco Buonamico, a doctor from Malta.

Scilla  applied a method of analysis that we today define, in all respects, empirical and scientific and his observations constitute the seeds for the emergence of taphonomy and paleontology in general. The central theme of Scilla’s work was the demonstration that the ‘Glossopetrae’ (shark teeth) and other petrified objects resembling living animals actually represent the remains of organisms that once lived in the sea. He thought that the correspondence of so many parts, well structured and coincident in the fossil, cannot be due to a coincidence or  a freak of nature.

Several shark teeth
(Glossopetrae) (II and III in the original plate). From Scilla, 1670, plate VII.

Like Aldrovandi, he thought that the natural world must first be analysed in the field by direct observation. He went to fossiliferous sites himself, especially a locality name Musorrima, near the city of Reggio Calabria.

He was also a pioneer in the field of taphonomy. For example, starting from the original shape that characterises echinoids in life, Scilla  infers that many shells had been compressed and crushed by the weight of the sediment after burial, and he pointed out that individual shells were deformed in different ways, in relation to how they were oriented in the sediment and thus in relation to the direction of the compressive force.

Illustration of sea urchins compressed and fractured in different ways. From Scilla, 1670, plate XXVI.

Scilla’s writing includes some fundamental concepts for paleontology and the geological sciences like: actualism, taphonomy, preservation through the process of internal/external moulding and basic sedimentology.

References:

Romano, Marco, Historical Biology (2013): ‘The vain speculation disillusioned by the sense’: the Italian painter Agostino Scilla (1629–1700) called ‘The Discoloured’, and the correct interpretation of fossils as ‘lithified organisms’ that once lived in the sea, Historical Biology: An International Journal of Paleobiology, DOI: 10.1080/08912963.2013.825257

To see the world in a grain of sand: Planktonic foraminifera and Evolution.

Planktonic foraminifera. (Credit: Paul Pearson, Cardiff University)

“To see the world in a grain of sand…”, this is the first line of William Blake´s poem “Auguries of Innocence” which describe a series of paradoxes about innocence, evil and corruption. But in a biological sense, this line can also describe how “a grain of sand” could gives a glimpse of how evolution works using the remains of planktonic foraminifera which resemble grains of sand to the naked eye and date back hundreds of millions of years.

Foraminifera are an important group of single celled protozoa with shells of different composition and granuloreticulose pseudopodia.  The first record of the group is from the Early Cambrian and extend to the present day. Their size range is from about 100 micrometers to almost 20 centimeters long.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Planktonic foraminifera are ideal subjects for testing how species evolve over time. They are a diverse extant clade that have an exceptional fossil record, due to extremely large population sizes and widespread species distributions. They also can record the climate and environmental conditions on their calcium carbonate shells.

It seems that gradual morphological trends do not strictly reflect the rate of speciation or its mode within the clade. In a paper published in 1998, Kucera & Malmgren,  showed that gradual change in the Cretaceous planktonic foraminifera Contusotruncana fornicata probably resulted in a shift in the relative proportion of high conical to low conical forms through time, yet isotopic data indicated a rapid separation of the population.

Globigerina bulloides and Globoconella
inflata, two examples of planktonic foramininfiera.

Using stratigraphic, phylogenetic and ecological data from the exceptional fossil record of Cenozoic macroperforate planktonic foraminifera, Dr Thomas Ezard from the  University of Southampton, explains how the fossil record contains signals of biological processes that drive genetic evolution. He used a complete phylogeny of those Cenozoic foraminifera to provide palaeontologically calibrated ages for every divergence within the clade that are independent of molecular data. Their  hypothesis is that speciation provokes a burst of rapid genetic change, giving molecular evolution a punctuational component. This rapid burst helps isolate the new species from its ancestor.

The study shows how the fossil record contains signals of biological processes that drive genetic evolution and promotes the importance of using fossil records in conjunction with the molecular models.

References:

Ezard, T. H. G., Thomas, G. H., Purvis, A. (2013), Inclusion of a near-complete fossil record reveals speciation-related molecular evolution. Methods in Ecology and Evolution, 4: 745–753. doi: 10.1111/2041-210X.12089

The legacy of Ulisse Aldrovandi.

Ulisse Aldrovandi (1522-1605).

Ulisse Aldrovandi was born  in Bologna  to a noble family on September 11, 1522. He  studied humanities, law, mathematics, medicine and philosophy at the university of Bologna where became the first professor of natural sciences in 1561. He was arrested for heresy in 1549 and remained in custody or house arrest till he was absolved in April 1550. During this time he coined the term geology and focused on Zoology and Botany.

He is considered one of the foremost biologists of the Renaissance and in 1568 founded the Bologna City Gardens. Monstruorum Historia contains some of the most impressive illustrations of Aldrovandi’s work.

Like da Vinci and Bauhin, some of the most emblematic figures of the Renaissance, Aldrovandi was a pioneer of ichnology. He described several trace fossils in his work Musaeum Metallicum. Like most of Aldrovandi’s works it was published posthumously. The book was entitled originally De Fossilibus but it was changed by Bartolomeo Ambrosini, the book’s editor.

Gastrochaenolites, as figured in Aldrovandi’s Musaeum Metallicum and Gastrochaenolites in a coral. From Wikimedia Commons.

In his Musaeum Metallicum Aldrovandi correctly interpreted bioerosional traces and the corresponding illustration reveals the ichnogenus Gastrochaenolites, a bioerosional trace commonly produced by bivalves. The specimen is presented as “Silicem dactylitem” and is described as a rock presenting “hollows” of varied diameter. He describes the “hollows” as “resembling the cavities in which some lithophagous bivalves seek shelter”.

a- Cosmorhaphe, described by Aldrovandi as snake-like structure. b. Detail of Cosmorhaphe.

He also  describes Cosmorhaphe as a natural curiosity resembling the sinuous curves of a snake. Unlike da Vinci, Aldrovandi argues for an inorganic origin of traces and believes that are  formed by fluids circulating within rocks or natural curiosities -for instance, ammonites are named Ophiomorphites or “snake-shaped stones”- , but he often compares them to existing animals.

Aldrovandi’s Musaeum Metallicum,
1648.

Aldrovandi’s work represents a major step in the history of Ichnology because  includes one of the first examples of a scientific approach to trace fossils and includes some of the earliest artistic representations of invertebrate trace fossils.

References:

Baucon, A. (2010). Leonardo da Vinci, The Founding Fatheer of Ichnology,  PALAIOS, 25 (6), 361-367 DOI: 10.2110/palo.2009.p09-049r

Baucon, A. (2008). Italy, the Cradle of Ichnology: the legacy of Aldrovandi and Leonardo, Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 15-29

MICROFOSSILS AND THE OCEAN HISTORY.

Forams from deep-sea. Credit: Miriam Katz, Rensselaer Polytechnic Institute. (Originally published by Micropress.)

Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments.

The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contains clues about the temperatures in which they lived.

Following this pioneering work, Schott working on sediments of the METEOR Expedition (1925-1927) introduced quantitative counting of species within the fossil assemblages on the sea floor and realized that surface water temperature changed as the climate fluctuated between glacial and interglacial conditions.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

In 1955, Emiliani, who was then a student of Harold Urey at the University of Chicago,  published a paper entitled “Pleistocene temperatures” where introduced isotope stratigraphy to paleoceanography. He used the density of a heavy oxygen isotope in planktonic foraminifera from deep sea cores to outline oxygen isotope stages for the Quaternary, believing these would reflect surface temperature changes and the ice volume changes.  He concluded that the last glacial cycled had ended about 16,000 years ago, and found that temperature increased steadily between that time and about 6000 years ago. Many of Emiliani’s findings are still valid today, however in 1970 several improvements to Emiliani’s work were made, such as a revision of the temperature scale.

Oxygen isotope records have also been obtained from well-preserved microfossil materials in the Late Cretaceous  when bottom waters appear to have been much warmer than at present.

This concepts of paleotemperature reconstruction, as first developed for planktic foraminifera, apply to other groups of microfossils. Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antartic Oceans, especially where calcareous fossils are less abundant. Diatom assemblage are also used in reconstructions of paleoproductivity.

Climatic modes and sea-level fluctuations indicated by calcareous nannofossils of the Oligocene deposits from the Romanian Carpathians. (Melinte, 2004)

The calcareous nannoplankton represents good proxy for the sea-level fluctuations. The group exhibit  a clear latitudinal distribution pattern, for instance, the presence of mixed nannofloral assemblages (taxa of low-middle latitudes together with high ones) are indicative of the sea-level rise,  while endemic assemblages characterize periods of low sea-level.

By studying cores from those ocean sediments, its possible determine the ages of the rocks, the ocean environment and some atmospheric conditions using the information  provided by the microfossils present in that core, as well as stable isotope analysis and magnetic stratigraphy.

Each layer of the core recorded the geological history of the ocean basins, changing climates, evolving biota and the events that could altered the course of Earth history.

References:

Armstrong, Howard A. and Martin D. Brasier.  Microfossils.  Blackwell Publishing, 2005.

Berger, W. H., Sea level in the late Quaternary: patterns of variation and implications, Int J Earth Sci (Geol Rundsch) (2008) 97:1143–1150