Plagiosauridae is a clade of temnospondyl amphibians of the Middle to Late Triassic. Deposits of the group are most commonly found in non-marine aquatic depositional environments from central Europe and Greenland, but other remains have been found in Russia, Scandinavia, and possibly Thailand. The majority of plagiosaurid remains are of the genus Gerrothorax , which have been recovered from the Fleming Fjord Formation of Jameson Land, East Greenland  , and from many localities in southern Germany   . All of this material is currently assigned to a single species, pulcherrimus .
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Evolutionary stasis long-term stability of morphology in an evolving lineage is a pattern for which explanations are usually elusive. This evolutionary stasis coincides with the occurrence of this species in a wide range of habitats and environmental conditions.
By the combination of palaeoecological and palaeohistological analyses, we found great ecological flexibility in G. We conclude that G. This would have been made possible by a unique life history strategy that involved a wide reaction norm, permitting adjustment to fluctuating conditions such as salinity and level of nutrients. Growth rate, duration of juvenile period, age at maturity, and life span were all subject to broad variation within specimens of G. In addition to providing a better understanding of fossil ecosystems, this study shows the potential of such a methodology to encourage palaeobiologists and evolutionary biologists to consider the mechanisms of variation in extant and fossil organisms by using a similar time-scope reference.
Understanding the processes that influence the survival and extinction of evolutionary lineages is a major task of palaeo- biology. Many studies revealing the causes of mass extinction events have been proposed not only by palaeontologists e. Conversely, long-term survival of taxa has fascinated evolutionists for decades e. Most interesting are the cases for which the phenotypic homeostasis can be analysed in environmental, developmental and genetic contexts.
Firstly, organism-wide morphological stasis was thought to reflect canalisation and developmental homeostasis Eldredge and Gould ; Waddington , but many studies have since shown that genotype—phenotype, development, and environment relationships are far more complex e.
Checking therefore the genetic, developmental, and environmental variability in homeostatic taxa is of major interest for a better understanding of evolutionary stasis. From a time-scale point of view, the most obvious study cases of evolutionary stasis usually come from the fossil record. Based on ontogenetic, metabolic, and palaeoecological data, plausible scenarios can then be proposed for understanding the long-term evolutionary success of fossil populations and taxa.
Here, we report on such a case from a long-extinct tetrapod, the temnospondyl Gerrothorax pulcherrimus. This plagiosaurid, a flat-bodied aquatic form with extensive dermal armour, is interpreted as a suction feeder at the bottom of Triassic lakes and streams Hellrung ; Jenkins et al.
Gerrothorax was an obligatorily aquatic taxon, which retained lateral line organs and gills in adults Schoch and Witzmann Recently, Schoch and Witzmann studied the ontogeny and stratigraphic distribution of G. This pattern is exceptional among fossil amphibians in two respects: first, the extent and duration of evolutionary stasis; and second, the apparent ecological flexibility through time and space. The causes of both patterns form the focus of the present analysis.
The objective of the current study first is to analyze growth rates, individual ages, timing of sexual maturity, and metabolic traits in two samples of G.
Secondary, these data will be compared to another group from one of these localities Plagiosuchus pustuliferus and then integrated with rich data on the palaeoenvironments.
The final aim is to match the inferred life history traits with environmental and ecological parameters, in order to understand the evolutionary strategy of Gerrothorax.
The material studied here was collected in two different German localities: Kupferzell and Vellberg. The time span covered by these localities is maximally a few ten thousand years. The existence time for each lake was much less, supposedly a few hundred years only.
The Kupferzell locality is constituted of a basal green calcareous mudstone horizon i. It is a now-overbuilt road-cut. The richest bed, a grey mudstone bed that formed in a freshwater lake, preserves a temnospondyl fauna consisting of Mastodonsaurus , Kupferzellia , Trematolestes , Callistomordax , and Gerrothorax. The studied limb bones are assignable to G. Regarding G. In total, twenty-five long bones were sectioned at mid-shaft. Regarding P. All elements sectioned and studied here were compared in detail to those of articulated specimens from the same localities and horizons.
Taxonomical confusion can be ruled out because the anatomy of G. Both the humerus and femur of Plagiosuchus are readily distinguishable from those of Gerrothorax by their much stouter appearance, even at small ontogenetic stages, and the characteristic coarse surface at the articular ends of the long bones. In Plagiosuchus , the humerus lacks a supinator process, and the femur has a much shallower crest than in Gerrothorax Hellrung A further consistent difference is that the limb elements of Plagiosuchus have a more rudimentary appearance than those of Gerrothorax , particularly in the extent of bone in the proximal and distal ends of the humerus; only the largest specimens of Plagiosuchus reach the morphological condition that is typical of Gerrothorax.
Samples studied. Ontogenetic series of humeri and femora midshaft thin sections of Gerrothorax pulcherrimus showing the different histotypes. Specimen numbers written in red refer to specimens coming from the Kupferzell locality and specimen numbers written in blue refer to specimens coming from the Vellberg locality.
The current study focusses on limb bone histology. This latter is usually very informative, especially within the framework of growth series. The long-bone cortex is centrifugally appositionally deposited following the regularity of yearly biological cycles Castanet ; Castanet et al. The nature of the successive bone deposits therefore reflects the global metabolic activity of an organism Castanet and Baez ; Montes et al.
Because stylopodial bones are informative archives for skeletochronological studies in small-to-medium sized temnospondyls Sanchez et al. This is the ossification centre, i. Long bones were embedded in a polyester resin and then sectioned. Thin sections were observed under natural and polarized light through an optical microscope Nikon Eclipse 80i. Assessment of age and ontogenetic staging is based on skeletochronology Castanet ; Castanet et al. Bone palaeohistology, as a complementary tool to anatomical and environmental analyses, can provide further information about the palaeobiology and palaeophysiology of extinct taxa Castanet et al.
In order to avoid any misinterpretation of the bone microstructure variation, which could be related to a phylogenetical signal Cubo et al. In order to set our hypotheses in an evolutionary framework, the fossil bone samples were extracted from sedimentary beds representing different palaeoecological niches of varied ages. They are discriminated on the basis of differential: cortical compactness; vascular mesh organisations, densities and canal size; remodelling stage; quantities of calcified cartilage; patterns of lines of arrested growth; bone matrix types; bone-cell distributions, size and shape.
All these histological parameters are significantly related to metabolic, physiological and biological specific functions e. Because the sectioned material consists of isolated bones, we describe these histotypes for humeri and femora separately.
Humeral histotypes were therefore numbered as 1 and 2, femoral histotypes as A, B, C and D. Histotype 1—humerus, Gerrothorax pulcherrimus. Mid-shaft transversal thin section made in the humerus a show a rather compact cortex c, a delimited from a loose central spongiosa s, a. In the humerus b , the spongiosa greatly extends towards the external surface of the bone. This spongiosa is already obviously remodelled, as suggested by the presence of secondary-bone sb, c deposit in the youngest specimen Numerous primary osteons po, d are longitudinally oriented and concentrically aligned in both specimens.
Seven lines of arrested growth black arrows , c regularly cross the mid shaft of this specimen and get closer to each other towards the periphery. Radial extrinsic fibres ref, e — f can be observed in both specimens e — f. Scale bars for the images d , e and f : 0. Photos a , b , c taken under natural light; photos d , e , f taken under polarized light. The mid-shaft section of the humerus of this specimen is elongated and triangular Fig.
It has a relatively well-preserved peripheral primary cortex 1. This indicates that the medullary cavity must have been substantially eroded and the trabecular bony structure of the medulla relatively weak. The inner part of the primary cortical surface is not visible. This completes the previous observation: the inner part of this cortex must have been eroded as well in continuation of the medullary cavity, forming an extended spongiosa.
The structure of the trabeculae reveals that the uncrushed part of the spongiosa is mainly secondary sb, Fig. There is no remnant of calcified cartilage. The remaining cortical bone is substantially remodelled, including numerous primary osteons po, Fig. These osteons are longitudinally oriented, i. Up to six series of osteon rows can be counted. The bone matrix forms a fibro-lamellar system.
Radial extrinsic fibres are present in certain areas, especially the periphery of the cortex ref, Fig. Seven lines of arrested growth LAGs were identified black arrows, Fig. In the secondary bone deposits, bone cells are bigger This whole section is relatively well preserved in three dimensions, except for crushing in the central most part Fig. The cross-section is elongated and triangular. The outer surface is irregular and damaged, indicating weathering or transport before final burial, which are usual phenomena at Kupferzell.
As preserved, the average cortical thickness is 2. The vascular canals within the primary bone matrix of the cortex are more eroded than those of SMNS Fig. They form large primary osteons, that overprint the original organization in rows. The osteons are oriented longitudinally. The primary bone matrix is fibrous, and the spongiosa is made of remodelled trabeculae that are oriented in all directions.
There is no remnant of calcified cartilage Sanchez et al. The primary bone tissue includes radial extrinsic fibres Fig. Two LAGs were identified in the peripheral cortex, but no LAG pattern can be obviously recognized from the lamellae in the innermost part of the cortex. The distance between the peripheral LAGs is 0. They are round in most cases even if few lacunae are slightly flattened.
They are aligned in circles around the blood vessels. The mid-shaft section of this bone is elongated but crushed in its central part. The medullary cavity and the innermost part of the primary cortical bone must have therefore been substantially eroded, thereby forming an extended secondary spongiosa.
The remaining cortical bone is substantially remodelled, including numerous primary osteons. They are longitudinal or radial and concentrically aligned.