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To most people, crocodilians are large -bodied carnivores that have been unchanged since the age of the dinosaurs. However, during their 230 million -year history, modern crocodilians and their extinct relatives evolved a stunning diversity of body plans, with many looking very different from those alive today (crocodi les, alligators, caimans and gharials).
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Title: Fossil Focus: Thalattosuchia
Author(s): Mark T. Young, Sven Sachs and Pascal Abel
Volume: 8
Article: 5
Page(s): 1-13
Published Date: 01/05/2018
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Young, M.T., Sachs, S. & Abel, P. Fossil Focus: Thalattosuchia. Palaeontology Online, Volume 8,
Article 5, 1-13.
Published on: 01/05/2018| Published by: Palaeontology [online] |Page 1
Published by: Palaeontology [online]
Fossil Focus: Thalattosuchia
by Mark T. Young*1, Sven Sachs2 & Pascal Abel3
To most people, crocodilians are large-bodied carnivores that have been unchanged since the age of
the dinosaurs. However, during their 230 million-year history, modern crocodilians and their extinct
relatives evolved a stunning diversity of body plans, with many looking very different from those alive
today (crocodiles, alligators, caimans and gharials).
The first crocodylomorphs (the term used for living crocs and various fossil groups) are known from the
Late Triassic Period, approximately 235 million to 237 million years ago. These animals lived on land
and looked much more like a greyhound than a crocodile, with long legs and a skull that was deep like
that of a meat-eating dinosaur, rather than flattened like that of a living crocodile. In fact, many of the
fragmentary fossils of these first crocodylomorphs have been confused with dinosaur fossils. For their
first 30 or so million years, crocodylomorphs lived only on land. This didn’t change until the
mass extinction event that separates the Triassic from the Jurassic Period (approximately 201 million
years ago), and which wiped out the terrestrial, semi-aquatic and marine reptiles that had been
dominant during the Late Triassic. This extinction event not only radically altered crocodylomorph
evolution, but ensured that dinosaurs became the dominant group of terrestrial vertebrates during the
Jurassic and Cretaceous Periods.
After the TriassicJurassic extinction event, the first semi-aquatic crocodylomorphs, which lived partly
on land and partly in water, appeared. These were followed soon after by the first marine
crocodylomorphs, the thalattosuchians. Thalattosuchia was the pinnacle of marine specialization
among crocodylomorphs. First appearing within 10 million years of the TriassicJurassic extinction
event, they continued well into the Early Cretaceous, with the most recent known thalattosuchian
fossil approximately 125 million years old. The early thalattosuchians looked similar to living Indian
gharials: they were semi-aquatic, long-snouted and probably fed on small-bodied fish. These forms
would ultimately give rise to species that could swim in the open ocean and looked more like today’s
dolphins and killer whales. This article provides an introduction to this extraordinary group, including
an overview of their diversity, fossil discoveries and what we can infer about their biology.
Thalattosuchian biodiversity:
Thalattosuchians are divided into two groups: Teleosauroidea and Metriorhynchoidea (Figs. 1, 2). We
know that these two groups diverged during the Early Jurassic, by at least the early Toarcian age
(approximately 182 million years ago). The fossil record reveals a high diversity of teleosauroids
(Steneosaurus bollensis, Steneosaurus brevior, Steneosaurus gracilirostris and Platysuchus
multiscrobiculatus; Fig. 3) and one metriorhynchoid (Pelagosaurus typus) from across Europe at this
time. Fossils are known from the Holzmaden area of Germany, Normandy in France, and Whitby and |Page 2
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Figure 1 A simplified look at the evolutionary relationships of Thalattosuchia. Top: how the two
major subgroups of thalattosuchians relate to living crocodilians. Bottom: the two subgroups of
metriorhynchids. The crocodyliform skull is a Yacare caiman (Caiman yacare), the teleosauroid
skull is Steneosaurus leedsi and the metriorhynchoid skull is Dakosaurus maximus. The geosaurine
skull is ‘Metriorhynchuscasamiquelai and the metriorhynchine skull is Cricosaurus araucanensis.
Credit: M. Young, S. Sachs, P. Abel and NHM image resources.
Somerset in England. Thus, at least in the group of islands that made up what is now Europe 182
million years ago, the teleosauroids were the largest-bodied and most dominant crocodylomorph
History of study:
Thalattosuchians were among the first group of fossil reptiles to be named and described in scientific
journals. In 1758, two scientific papers were written about the discovery of a partial thalattosuchian
skeleton near Whitby. In the first, Captain William Chapman stated that the skeleton had been found in
January 1758; the second paper, by Charles Morton and John Wooller, claimed the skeleton had
actually been discovered about ten years previously. This skeleton also caused other disagreements
among eighteenth-century naturalists and anatomists. Some considered it to be closely related to living |Page 3
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Figure 2 Life reconstructions of two thalattosuchians. Top: the teleosauroid Machimosaurus
hugii (by Dmitry Bogdanov). Bottom: the metriorhynchid Metriorhynchus superciliosus (by Dmitry
Figure 3 Skeleton of Steneosaurus bollensis, Lower Jurassic (early Toarcian, Posidonienschiefer
Formation) of Holzmaden (Germany). Length ~ 4 m. On display at the Urweltmuseum Hauff in
Holzmaden (Germany). Credit: S. Sachs. |Page 4
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crocodilians, whereas others thought it was a whale or dolphin. Recent re-study of this materials
strongly suggests that the specimen is a large teleosauroid, belonging to the species Steneosaurus
bollensis. To the best of our knowledge, this specimen was the first fossil crocodylomorph described in
a scientific paper. After its discovery, it formed part of the collection of the Royal Society, a scientific
society in London, but it was transferred to the British Museum in 1781. Today, the specimen is held in
the Natural History Museum in London.
During the latter half of the eighteenth century, more thalattosuchian fossils were discovered across
Europe, particularly in England, France, Germany and Italy. The first scientific paper that named a
thalattosuchian was by Samuel Thomas von Sömmerring in 1814 (Fig. 4). This was the
teleosauroid Steneosaurus priscus. The skeleton was discovered in a quarry in Daiting in Bavaria,
Southern Germany. The first description of a metriorhynchoid was also by von Sömmerring, two years
later, in 1816. This species, Geosaurus giganteus, was discovered in the same quarry as Steneosaurus
priscus, albeit at twice the depth. This means that the first species in both major thalattosuchian
subgroups had already been described and named before William Buckland named the first dinosaur in
1824. Thalattosuchians continued to be widely studied during the nineteenth century in Europe: there
was something of a ‘gold rush’ in naming new species, or giving already-described species new names,
between the 1820s and the 1850s. It was not until the seminal work of the French father-and-son team
Jacques and Eugène Eudes-Deslongchamps (Fig. 4) in the 1860s that the classification of
thalattosuchians began to stabilize.
Unfortunately, the study of thalattosuchians largely fell into obscurity during much of the twentieth
century in Europe (although there were certain decades in which thalattosuchian research advanced in
France, in particular the 1950s and 1990s). The longest and most detailed descriptions of
thalattosuchians were written between 1901 and 1913, including seminal papers on German
specimens by Eberhard Fraas (Figs. 4, 5) in 1901 and 1902 (where the term Thalattosuchia was first
coined and defined) and the descriptive catalogue of the British Oxford Clay marine reptiles held in the
Figure 4 Photographs of famous naturalists and scientists who have conducted research into
thalattosuchians. Left: The German anatomist Samuel Thomas von Sömmerring (28 January 1755
2 March 1830). Source: Wikimedia Commons. Middle: The French naturalist Eugène Eudes-
Deslongchamps (10 March 1830 21 December 1889). Source: Wikimedia Commons. Right: The
German scientist Eberhard Fraas (26 June 1862 6 March 1915). Source: Wikimedia Commons. |Page 5
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Figure 5 Cricosaurus suevicus from the Upper Jurassic (late Kimmeridgian) of Nusplingen
(Germany). Length ~ 2 m. Top: Photograph of the skeleton described by Eberhard Fraas in 1901,
1902. Middle: The life reconstruction appearing in Fraas’ work. Bottom: A modern life
reconstruction by Joschua Knüppe. Skeleton on display in the Staatliches Museum für Naturkunde
Stuttgart. Credit: M. B. Andrade.
Natural History Museum, London, by Charles Andrews in 1913. However, there was extensive research
into thalattosuchians in South America, starting during the 1970s and continuing to the present day.
New species were described from beautifully preserved 3D skulls (Fig. 6). These exquisite specimens
helped show just how diverse thalattosuchians were becoming during the Jurassic. |Page 6
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Figure 6 Skulls of the South American metriorhynchids (seen in dorsal view). a, Cricosaurus
araucanensis from the Upper Jurassic (early Tithonian) of Argentina. b,
Metriorhynchuscasamiquelai from the Middle Jurassic (middle Callovian) of Chile. c, Dakosaurus
andiniensis from the Upper Jurassic (late Tithonian) of Argentina. d, Purranisaurus potens from the
Upper Jurassic (late Tithonian) of Argentina. Credit: Z. Gasparini and S. Sachs.
Since the turn of the millennium, there has been a renaissance in thalattosuchian research. More new
species have been named in the past 18 years than in the previous 100. Moreover, their evolutionary
relationships are being investigated, and insights are being made into how they fed, moved and
reproduced. Today, research into thalattosuchians is at the cutting edge of science, with CT
scanning being used to peer inside their skulls to investigate what happened to their brains and sensory
systems during the land-to-sea transition.
During the Jurassic, teleosauroids spread out across the world, and today fossils are found in Africa
(Ethiopia, Madagascar, Morocco and Tunisia), Asia (China, India and Thailand) and Europe (France,
Germany, Luxembourg, Poland, Portugal, Spain, Slovakia, Switzerland and the United Kingdom). Their
fossils are mostly known from lagoons and coastal marine environments, but some fossils are also
known from estuaries and freshwater ecosystems. At the same time as they spread out, they became
highly diverse. In most ecosystems, there were three ‘ecomorphotypes’: (1) long-snouted forms that
looked like a living Indian gharial, with many small, pointed teeth well suited to grasping small fish; (2)
long-snouted forms that had a shorter snout and fewer teeth than ‘ecomorphoype’ 1, although the
teeth were bigger and would have helped tackle larger prey; and (3) short-snouted forms that had even
fewer teeth, but in which the teeth were very robust, with numerous ridges on the enamel. These
three ecomorphotypes occurred in several ecosystems where multiple species of teleosauroids are
known, and it seems that teleosauroids maintained their high species diversity by feeding on different
types of prey.
By the end of the Middle Jurassic, a new type of teleosauroid had evolved: the Machimosaurini. This
group includes the genera Lemmysuchus and Machimosaurus (Fig. 7). These were the giants of the |Page 7
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Figure 7 Skulls of machimosaurin teleosauroids in dorsal view. Top: Machimosaurus buffetauti,
Upper Jurassic (early Kimmeridgian, Lacunosamergel Formation) of Neuffen (Germany).
Bottom: Lemmysuchus obtusidens, Middle Jurassic (middle Callovian, Oxford Clay Formation) of
Peterborough (England). Credit: R. Schoch and NHM image resources.
teleosauroid group, reaching between 5 metres and 7.2 metres in length, and they were the largest
crocodylomorphs of the Jurassic. Machimosaurins started out similar to the third ecomorphotype
described in the previous paragraph, with short snouts and robust teeth. During the Late Jurassic and
Early Cretaceous, they evolved blunt teeth with serrated edges, even shorter snouts and huge jaw-
closing muscles, and they are thought to have fed on sea turtles. Marine turtle shells from the Late
Jurassic and Early Cretaceous of Europe and Africa are frequently found with Machimosaurus teeth
stuck in their shells, or with deep circular bite-marks matching Machimosaurus teeth (Fig. 8). |Page 8
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Figure 8 Marine turtle shell fragments from the Solothurn Turtle Limestone of Switzerland
(Upper Jurassic, late Kimmeridgian). Left and middle: Unidentified turtle shell with
round Machimosaurus bite marks. Right: Grooves on a Plesiochelys carapace caused by the back
teeth of Machimosaurus. Credit: S. Thüring.
The second group of thalattosuchians, metriorhynchoids, can be split into two subgroups: (1) true
metriorhynchids that evolved into dolphin-like forms with a tail fin, flippers and loss of bony armour;
and (2) ‘basal metriorhynchoids’ that are intermediate between the gharial-like teleosauroids and the
dolphin-like metriorhynchids (Fig. 9). These basal metriorhynchoids are a poorly understood series of
species that are largely known from broken fossils, mostly incomplete skulls. However, they must have
been successful, because from a 15-million-year span of time in the Early and Middle Jurassic they are
known from the United Kingdom, France, Germany, Portugal, Hungary, China, Chile and Oregon in the
United States. Each of these species can provide some insights into how the metriorhynchid body plan
evolved. Unfortunately, because all but one of these species are known only from fragmentary fossils,
it’s not clear what skeletal changes occurred as thalattosuchians evolved from gharial-like to dolphin-
like forms.
The oldest known basal metriorhynchoid is Pelagosaurus typus (Fig. 10). Pelagosaurus is well known
and lived across Europe during the Early Jurassic (approximately 182 million years ago), alongside other
species of teleosauroids. It looked similar to teleosauroids, but had numerous features that show it is
more metriorhynchid-like: eyes facing outwards and slightly forwards (not upwards as in living
crocodilians and teleosauroids); a bony ring to support the eyeball; enlarged openings for carotid
arteries at the back of the skull; and long foot bones (metatarsals), making the feet more paddle-like.
The fossil record shows that the species of basal metriorhynchoids known from fragmentary fossils
became increasingly metriorhynchid-like through time.
True metriorhynchids are known from the Middle Jurassic to the Early Cretaceous (approximately 168
million to 125 million years ago; Fig. 11). They evolved numerous adaptations to living in a marine
environment, including flippers and a tail fin (Fig. 12). Oddly, however, the oldest known |Page 9
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Figure 9 Life reconstructions of two metriorhynchoids. Top: The ‘basal
metriorhynchoid’ Pelagosaurus typus (by Nobu Tamora). Bottom: The metriorhynchid Cricosaurus
suevicus (by Dmitry Bogdanov).
Figure 10 Skeleton of Pelagosaurus typus, Lower Jurassic (early Toarcian, Posidonienschiefer
Formation) of Dotternhausen (Germany) with a pathological lower jaw. Length ~ 2 m. On display
at the Werksforum Dotternhausen in Dotternhausen (Germany). Credit: S. Sachs. |Page 10
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Figure 11 Skeleton of Metriorhynchus superciliosus, Middle Jurassic (middle Callovian, Oxford
Clay Formation) of Peterborough (England). Length ~ 3 m. On display in the Paläontologische
Sammlung der Universität Tübingen (Germany). Credit: S. Sachs.
metriorhynchids are too advanced to have been the ancestors of this unique group. Analyses of the
evolutionary relationships of thalattosuchians place the oldest known metriorhynchid, Neptunidraco
ammoniticus from the Middle Jurassic of Italy, as a member of the group Geosaurinae. This confirms
that metriorhynchids can be split into two major subgroups: (1) the metriorhynchines, which include
the forms that were best adapted to a marine existence (large tail fins and nostrils that were not on the
tip of the snout, but closer to the eyes) and are largely long-snouted species that probably fed on small
fish or squid; and (2) the geosaurines, which include the largest-bodied metriorhynchids
(Plesiosuchus and the species closely related to it), forms such as Dakosaurus that were similar to living
false killer whales, and Geosaurus, which was similar to living barracudas. However, no one has yet
found any fossil relatives of these two groups, which would represent the earliest metriorhynchids, and
therefore we still have no answers to key questions such as when metriorhynchids evolved, where they
evolved and what underpinned their sudden marine diversification.
Another important question related to the biology of metriorhynchids is whether they gave birth to live
young (like some sea snakes), or came back onto land to lay eggs (like marine turtles). Unfortunately,
no fossilized eggs or embryos have been discovered yet. However, the unusual shape of their pelvis
could be informative: it has a large diameter, which could be a sign that they gave birth to live young.
Only the discovery of new fossils will answer this tantalizing question.
Exactly when the thalattosuchians became extinct is currently unclear. New discoveries from closer to
the equator are pushing the fossil record of teleosauroids and metriorhynchoids deeper into the
Cretaceous. The 7-metre giant Machimosaurus rex from Tunisia is currently the most recent known
teleosauroid (from around 130 million years ago), and the most recent known metriorhynchid fossil is |Page 11
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Figure 12 Modifications of the limbs and tail in Thalattosuchia, showing the evolution of
flippers and a tail fin, and loss of bony armour. Top: The forelimb (left) and hindlimb (right) of the
teleosauroid Platysuchus multiscrobiculatus, Lower Jurassic (early Tithonian, Posidonienschiefer
Formation) of Holzmaden (Germany). Middle: The forelimb (left) and hindlimb (right) of the
metriorhynchid Dakosaurus maximus, Upper Jurassic (late Kimmeridgian, Torleite Formation) of
Painten (Germany). Bottom: End of the tail of the teleosauroid Steneosaurus bollensis (left) and
the metriorhynchid Cricosaurus sp. (right). Credit: M. Young and S. Sachs. |Page 12
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an isolated tooth crown from the Aptian Age (about 125 million years ago) of Sicily. This tooth crown
has a unique shape, cutting edge and serration, which suggests that it belonged to a close relative of
the largest known metriorhynchid, Plesiosuchus (and not a plesiosaur, as suggested by some
Future work:
The biggest obstacle to our understanding of thalattosuchian evolution comes from finding and
describing new fossils, in particular those from outside Western Europe. The amazing Argentinean and
Chilean fossils have transformed our understanding of thalattosuchian diversity (in particular that
of Dakosaurus andiniensis; Fig. 6). New discoveries of teleosauroid fossils from Thailand hint that some
thalattosuchians might have lived in freshwater ecosystems, and the recent discovery of the
giant Machimosaurus rex from the Early Cretaceous of Tunisia has overturned our ideas that
teleosauroids went extinct at the end of the Jurassic. These findings suggest there is still plenty left to
discover and learn.
We would like to thank the EU SYNTHESYS ( Project (and a Leverhulme
Trust Research Project Grant RPG-2017-167 to Mark Young) for funding our research. Furthermore, we
thank Nobu Tamura, Dmity Bogdanov and Joschua Knüppe who kindly allowed us to use their artwork,
and the curators of the collections we visited to study thalattosuchian specimens.
Suggestions for further reading:
Fernández, M. S. & Gasparini, Z. Salt glands in the Jurassic metriorhynchid Geosaurus: implications for
the evolution of osmoregulation in Mesozoic crocodyliforms. Naturwissenschaften 95, 7984 (2008).
DOI: 10.1007/s00114-007-0296-1
Herrera, Y., Fernández, M. S., Lamas, S. G., Campos, L., Talevi, M. & Gasparini, Z. Morphology of the
sacral region and reproductive strategy of Metriorhynchidae: a counter-inductive approach. Earth and
Environmental Science Transactions of the Royal Society of Edinburgh 106, 247255 (2017).
DOI: 10.1017/S1755691016000165
Hua, S. & Buffrénil, V. Bone histology as a clue in the interpretation of functional adaptations in the
Thalattosuchia (Reptilia, Crocodylia). Journal of Vertebrae Paleontology 16, 703717 (1996).
DOI: 10.1080/02724634.1996.10011359
Johnson, M. M., Young, M. T., Steel, L., Foffa, D., Smith, A. S., Hua, S., Havlik, P., Howlett, E. A. & Dyke,
G. Re-description of ‘Steneosaurus’ obtusidens Andrews, 1909, an unusual macrophagous teleosaurid
crocodylomorph from the Middle Jurassic of England. Zoological Journal of the Linnean Society 182,
385418 (2017). DOI: 10.1093/zoolinnean/zlx035 |Page 13
Published by: Palaeontology [online]
Wilberg, E. W. A new metriorhynchoid (Crocodylomorpha, Thalattosuchia) from the Middle Jurassic of
Oregon and the evolutionary timing of marine adaptations in thalattosuchian crocodylomorphs. Journal
of Vertebrate Paleontology 35, e902846 (2015). DOI: 10.1080/02724634.2014.902846
Young, M. T., Brusatte, S. L., Ruta, M. & Andrade, M. B. The evolution of Metriorhynchoidea
(Mesoeucrocodylia, Thalattosuchia): an integrated approach using geometrics morphometrics, analysis
of disparity and biomechanics. Zoological Journal of the Linnean Society 158, 801859 (2010).
DOI: 10.1111/j.1096-3642.2009.00571.x
Young, M. T., Rabi, M., Bell, M. A., Steel, L., Foffa, D., Sachs, S. & Peyer, K. Big-headed marine
crocodyliforms, and why we must be cautious when using extant species as body length proxies for
long extinct relatives. Palaeontologia Electronica 19.3.30A, 114 (2016). DOI: 10.26879/648
1Grant Institute, School of Geosciences, The King’s Buildings, University of Edinburgh, James Hutton
Road, Edinburgh, EH9 3FE, UK.
2Naturkunde-Museum Bielefeld, Abteilung Geowissenschaften, Adenauerplatz 2, 33602 Bielefeld,
3University of Erlangen-Nuremberg, Geozentrum Nordbayern, Paläontologisches Institut,
Loewenichstraße 28, 91054 Erlangen, Germany.
ResearchGate has not been able to resolve any citations for this publication.
Full-text available
Teleosaurids were a clade of crocodylomorphs that attained near-global distribution during the Jurassic Period. Within Teleosauridae, one particular sub-clade of durophagous/macrophagous taxa achieved large body sizes and were apex predators in shallow marine environments during the Late Jurassic and Early Cretaceous in Europe and around the coast of the Tethys Seaway. Unfortunately, the origins of this clade are still poorly understood. 'Steneosaurus' obtusidens is a little-studied macrophagous species from the Oxford Clay Formation (Callovian, Middle Jurassic) of the UK and near Migné-les-Lourdines (Middle Callovian) in France. Despite being considered a sister taxon of the Late Jurassic taxon Machimosaurus, the taxonomy of 'S.' obtusidens remains unclear. Although three different synonymies have been proposed (variously a subjective synonym of other taxa), these taxonomic hypotheses have not been based on detailed anatomical comparisons and thus have not been tested. Here, we re-describe the holotype of 'S.' obtusidens, demonstrate that it is indeed a valid taxon, restrict the referred specimens to a fragmentary skeleton, nearly complete skull, and partial rostrum, and establish a new monotypic genus, Lemmysuchus. Our re-description reveals five autapomorphies for Lemmysuchus obtusidens and nine apomorphic characters that support the tribe Machimosaurini (Lemmysuchus + Machimosaurus).
Full-text available
Body size is commonly used as a key variable for estimating ecomorphological trends at a macroevolutionary scale, making reliable body length estimates of fossil taxa critically important. Crocodylomorphs (extant crocodylians and their extinct relatives) evolved numerous 'aberrant' body-plans during their ~230 million-year history, ranging from ‘hooved’ terrestrial species to dolphin-like pelagic species. Such clades evolved distinct cranial and femoral scaling ratios (compared to total body length), thereby making extant taxa unsuitable proxies for estimating their body lengths. Here we illustrate that the fossil clade Teleosauridae also fits into this category. Teleosaurids were a predominately shallow marine clade that had a global distribution during the Jurassic. Known to have evolved a wide range of body lengths (2–5 m based on complete skeletons), there is currently no way of reliably estimating the size of incomplete specimens. This is surprising, as some teleosaurids have been considered very large (9–10 m in total length), thus making Teleosauridae the largest bodied clade during the first 100 million years of crocodylomorph evolution. Our examination and regression analyses of the best preserved teleosaurid skeletons demonstrates that: they were smaller than previously thought, with no known specimen exceeding 7.2 m in length; and that they had proportionally large skulls, and proportionally short femora, when compared to body length. Therefore, while many teleosaurid species evolved a cranial length of ≥1 m, these taxa would not necessarily have been larger than species living today. We advise caution when estimating body length for extinct taxa, especially for those outside of the crown group.
Morphological and physiological features indicate Metriorhynchidae as the only group of crocodylomorphs with a pelagic lifestyle. Some of these features have evolved convergently in several clades of tetrapods secondarily adapted to aquatic life. One striking feature of metriorhynchids as compared to other crocodylomorphs is the morphology of the pelvic region (i.e., ventrally deflected sacral ribs and reduced pelvic girdle), which increases significantly the depth of this region. This morphology, as a whole, resembles that of other viviparous Mesozoic marine reptiles not phylogenetically related to metriorhynchids. We tested two alternative hypotheses of reproductive strategies in this clade: oviparity vs . viviparity. Given the lack of direct evidence supporting one or the other, we explored the use of evidence that may disconfirm either of these hypotheses. Using this counter-inductive approach, we found no cases contradicting viviparity in metriorhynchids, except for their phylogenetic position as archosaurs. A survey of reproductive modes amongst amniotes depicts the evolutionary plasticity of the transition to viviparity, and a widespread occurrence among tetrapods secondarily adapted to a marine life. Assuming oviparity for metriorhynchids implies egg-laying out of the water. However, their postcranial morphology (i.e., features of fore and hind limbs, pelvic girdle, and tail) contradicts this possibility. In this context, we rejected oviparity for metriorhynchids.
Metriorhynchid thalattosuchians represent the most extreme archosaurian adaptation to the marine realm. Metriorhynchids possess aquatic adaptations throughout the skeleton. These adaptations were so extensive that some have suggested that they lost the ability to move on land, yet their evolutionary timing remains unresolved. The closest relatives of the metriorhynchoids, the teleosauroids, lack these aquatic adaptations, and the earliest metriorhynchoids are known exclusively from cranial material. Here I describe a partial skull with associated forelimb elements of a new marine crocodylomorph, Zoneait nargorum, gen. et sp. nov., of Aalenian–Bajocian age from the Snowshoe Formation of east-central Oregon. Phylogenetic analysis identifies Zoneait as the sister taxon to Metriorhynchidae. It possesses a derived skull with orbits that are more laterally directed and prefrontals that are more expanded than in other basal metriorhynchoids. The preserved forelimb elements are less derived. The humerus is elongate in comparison with that of other metriorhynchoids. The ulna is slightly reduced in length and flattened but resembles the teleosauroid condition more so than the plate-like element of metriorhynchids. This suggests that marine adaptations in metriorhynchoids were acquired in mosaic fashion, with modifications of the skull preceding forelimb reduction, with this forelimb reduction occurring first in the zeugopodial elements, prior to reduction of the humerus. This evolutionary timing has important implications for the transition from nearshore ambush predation to pelagic open-marine predation in Thalattosuchia, suggesting that adaptations related to prey detection and capture preceded the locomotor adaptations that allowed these organisms to fully invade the oceans. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at
The histological study of various bones in the families Teleosauridae and Metriorhynchidae reveals common, but also contrasting structural features of the skeleton. Both display a zonal pattern of bone tissue, suggesting a cyclic growth and an ecto-poikilothermic physiology, quite similar to those of recent crocodiles. However, the Teleosauridae exhibit no peculiar skeletal specializations related to marine life, which suggests that they had an amphibious, rather than a truly marine habitat. Conversely, the skeleton of the Metriorhynchidae displays a certain degree of structural lightening, especially obvious in their skull, but also present in their femora and ribs. This structural specialization of the skeleton, together with the supposed physiological regime of the Metriorhynchidae, had definite bearings on their body trim in water, locomotor capabilities, and activity cycles. These various topics are discussed with reference to the ecological and eco-physiological adaptations of the Thalattosuchia.
Metriorhynchoid crocodylians represent the pinnacle of marine specialization within Archosauria. Not only were they a major component of the Middle Jurassic–Early Cretaceous marine ecosystems, but they provide further examples that extinct crocodilians did not all resemble their modern extant relatives. Here, we use a varied toolkit of techniques, including phylogenetic reconstruction, geometric morphometrics, diversity counts, discrete character disparity analysis, and biomechanical finite-element analysis (FEA), to examine the macroevolutionary history of this clade. All analyses demonstrate that this clade became more divergent, in terms of biodiversity, form, and function, up until the Jurassic–Cretaceous boundary, after which there is no evidence for recovery or further radiations. A clear evolutionary trend towards hypercarnivory in Dakosaurus is supported by phylogenetic character optimization, morphometrics, and FEA, which also support specialized piscivory within Rhacheosaurus and Cricosaurus. Within Metriorhynchoidea, there is a consistent trend towards increasing marine specialization, with the hypermarine Cricosaurus exhibiting numerous convergences with other Mesozoic marine reptiles (e.g. loss of the deltopectoral crest and retracted external nares). In addition, biomechanics, morphometrics, and character-disparity analyses consistently distinguish the two newly erected metriorhynchid subfamilies. This study illustrates that together with phylogeny, quantitative assessment of diversity, form, and function help elucidate the macroevolutionary pattern of fossil clades.© 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158, 801–859.
The presence of salt-excreting glands in extinct marine sauropsids has been long suspected based on skull morphology. Previously, we described for the first time the natural casts of salt-excreting glands in the head of the Jurassic metriorhynchid crocodyliform Geosaurus araucanensis from the Tithonian of the Vaca Muerta Formation in the Neuquén Basin (Argentina). In the present study, salt-excreting glands are identified in three new individuals (adult, a sub-adult and a juvenile) referable to the same species. New material provides significant information on the salt glands form and function and permit integration of evolutionary scenarios proposed on a physiological basis in extant taxa with evidence from the fossil record. G. araucanensis represents an advanced stage of the basic physiological model to marine adaptations in reptiles. G. araucanensis salt glands were hypertrophied. On this basis, it can be hypothesized that these glands had a high excretory capability. This stage implies that G. araucanensis (like extant pelagic reptiles, e.g. cheloniids) could have maintained constant plasma osmolality even when seawater or osmoconforming prey were ingested. A gradual model of marine adaptation in crocodyliforms based on physiology (freshwater to coastal/estuarine to estuarine /marine to pelagic life) is congruent with the phylogeny of crocodyliforms based on skeletal morphology. The fossil record suggests that the stage of marine pelagic adaptation was achieved by the Early Middle Jurassic. Salt gland size in the juvenile suggests that juveniles were, like adults, pelagic.