l'aventure des crocodyliformes par un paléontologue
Crocos !
Natural History Museum, Londres, 2014
Titre du projet
Morphologie contre moléculaire : les faux et vrai gavials sont-ils étroitement apparentés ?
Financement
Le financement de projets SYNTHESYS permet aux scientifiques basés dans les pays européens membres et les états associés, d’entreprendre des visites courtes pour l’utilisation des infrastructures (collections, expertise et installations) dans l’une des 16 institutions partenaires dans le cadre de leurs recherches.
Photos
Project objectives
The project focuses on the Gavialis-Tomistoma debate, and conflict between molecular and morphological phylogenetic relationships of these two genera and their relatives. Until now, molecularists consider Gavialis as closely related to Tomistoma, while morphologists consider it as basal crocodylian. Recent works suggest that outgroups could be problematic in morphological analyses. I plane 1/ review of basal tomistomines and gavialoids, 2/ review of alligatoroids recently considered as closely related to closest outgroup to Crocodylia, 3/ resampling of neosuchians included as outgroups, 4/ finally, result of this expanded analysis will help to test the reliability and robustness of the morphological phylogenetic analysis.
Traduction
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Divergences between molecular (left) and morphological (right) phylogenetics analyses. From Brochu (1997).
Project summary
The relationships of Gavialis relative to Tomistoma and other crocodylian genera are strongly conflicted between molecular and morphological datasets. When the tub-snouted Gavialis is considered as basal crocodylian in morphological analyses, molecularists find it as much more inclusive, closely related to another tub-snouted crocodylian, the false gharial Tomistoma. Both species are the unique living representative of their respective family. In this context, analyses of new fossils of basal tomistomines and reevaluation of basal gavialoids in light of basal crocodylians, gavialoids, and tomistomines preserved in the Natural History Museum of United Kingdom (NHMUK) are thus primordial.
Several recently described species (Acynodon and Massaliasuchus) have been considered as alligatoroids, but recent analyses suggest that they could be basal eusuchians, and probably Hylaeochampsidae. These hylaeochampsids are the main outgroups used to test phylogenetic relationships of Crocodylia. They strongly resemble alligatoroids, and retain characters previously considered as alligatoroid synapomorphies. Consequently, if these character states are plesiomorphic for Crocodylia, this could have consequences on relationships of basal crocodylians, in particular within gavialoids. The basal gavialoid affinities of thoracosaurs have been recently questioned. If thoracosaurs are not gavialoids, all remaining gavialoids are post-Lutetian in age, congruent with the divergence time estimates based on molecular analyses. Do these new considerations could change the gavialoid relationships to make them more congruent with molecular analyses?
Acynodon iberoccitanus
from the Late Cretaceous of Southern france.
©S.Jouve
These problems requires resampling of the taxa included in the analyses, and to expand the traditional matrices used to larger range of neosuchians such as goniopholidids and atoposaurids, to explore the new optimisation of character states provided by introduction of more hylaeochampsids.
Scientific content and project methodology
NHMUK is one of the wealthiest European Museum for crocodyliform fossil collections. It is thus particularly difficult to work, describe, and study morphology and phylogenetic relationships of crocodylians without compare new material with those housed in this museum. Moreover, the gavialoid-tomistomine debate requires reevaluation of basal gavialoids and tomistomines. Most of them, and the best preserved, are housed in the NHMUK.
Part of the Gavialis-Tomistoma problem could thus be outgroups dependent. Clear analysis of the group most closely related to Crocodylia, such as the hylaeochampsids, as addition of more neosuchians as outgroups, is thus essential. Most of them are housed in the NHMUK. The two best preserved hylaeochampsids are preserved in the NHMUK with Hylaeochampsa vectiana. Anatomical study of both species is required for clear determination of hylaeochampsid characters, and comparison and reevaluation of possible other hylaeochampsids from South France previously described as alligatoroids.
This project requires a detail study and analysis of many neosuchians and eusuchians, for comparison with basal and derived gavialoids and their introduction in matrices including at least 240 characters. The anatomical study and introduction of neosuchians in large matrix requires at least 2.5 days. The goniopholidids specimens are numerous (4 species and more than 30 specimens), and the best preserved atoposaurid is housed in the NHMUK, with Theriosuchus pusillus (at least 4 specimens). This later is probably the most important species as it seems to be one of the most closely related outgroup to the eusuchians.
Of course, basal eusuchians such as Hylaeochampsa vectiana (PV R177) need a particular attention for coding and anatomical comparison with recently described alligatoroids from southern France that are suspected to be hylaeochampsids. It is these reevaluations that change some character optimisations from alligatoroid synapomorpies to crocodylian plesiomorphies. This requires at least an half of a day in the NHMUK.
The type specimen of the main thoracosaur species previously considered as lost has been recently find again and a reevaluation seems to demonstrate more gavialoid characters than previously supposed. As the gavialoid affinities of thoracosaurs are problematic compared to molecular phylogenies, comparisons with several basal gavialoids from the NHMUK such as Thoracosaurus (4 spécimens), Eogavialis africanus (at least 10 specimens) and Eosuchus (1 specimen) is fundamental, and requires 1 day.
Basal tomistomines preserved in NHMUK should be studied for comparison with other basal tomistomines currently under study from Spain and Southern France. Their Early Eocene age is particularly important in the Gavialis-Tomistoma debate. 2 days is required for the study of 3 species (Kentisuchus (6 specimens), Rhamphosuchus (3 specimens) and T. dowsoni (2 specimens), and 2 species of basal allogatoroids (Borealosuchus canadensis (1 specimen), Diplocynodon hastingsiae (2 specimens).
Hylaeochampsa vectiana
from the Barremian of Isle of Wight.
©S.Jouve
Theriosuchus pusillus
from the Berriasian of England
©S.Jouve
Theriosuchus pusillus
(From Owen, 1849-1884)
Kentisuchus spenceri
Holotype, from the Early Eocene of England
©S.Jouve
Gain expected from the visit
Trying to solve the Gavialis-Tomistoma conflict requires first a clear understanding of the tomistomines and gavialoids and their respective phylogenetic relationships, particularly of basal forms. So, at first, the study and comparison of new species with those preserved in the NHMUK would allow to increase the reliability and robustness of their respective relationships.
Second, it is hope that with the comparison and reanalysis of the “alligatoroids” from Southern France in light of species preserved in NHMUK, it will be possible to set their alligatoroid or hylaeochampsid affinities.
Finally, introduction of new outgroups (goniopholidids, atoposaurids, hylaeochampsids from both Museum collections) is also primordial for analysis of the character optimisations in crocodylian phylogenetic relationships. This final work would provide more robust and reliable phylogenetic analysis of crocodylians, and help to determine if morphological phylogenetical analysis more congruent with molecular result is possible, and if previous result was related to erroneous optimisation related to outgroup choices.
With this project I plane to produce at least 7 papers directly related to the material studied in the NHMUK during the 3 next years.
Phylogenetic relationships of tomistomines is until now particularly unclear. Study of new material from Spain and France that seems to be closely related to Kentisuchus housed in the NHMUK and the revision of Tomistoma dowsoni (material housed in MNHN) will allow to produce at least 3 papers. At least 3 more papers will be required for the study of Thoracosaurus and of its gavialoid affinities, and reevaluation of the Frenchs Massaliasuchus, Allodaposuchus and Acynodon.
When phylogenetic relationships of basal tomistomines and gavialoides will have been cleaned, at least one article will attempt to analyse the Gavialis-Tomistoma problem, in light of expended matrix and taxa included.