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Sep 10, 20217 min read

The evolutionary history of Crocodylia and the solution to the gharial problem

Updated: Oct 13, 2021

Irena Raselli | 10.09.2021

Paper in a nutshell


Original title:

"Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem"


Publication date: September 6, 2021

Authors of the article:

Jonathan P. Rio, Philip D. Mannion​

Article in: PeerJ ; Page count: 156 ;

DOI: 10.7717/peerj.12094

(Figure from article: "Fig.12: Time-calibrated

phylogeny of Alligatoroidea from Analysis 1.3.") Link to the article: https://peerj.com/articles/12094/

 

Get to know Crocodylia

The today-living crocodiles, alligators, caimans and gharials are extant represen-tatives of the clade Crocodylia. The re-cently recognized 25+ living species are semi-aquatic ambush predators and pisci-vores, inhabiting freshwater and eustarine environments. Ancestors of this clade date back until the Late Cretaceous, counting 140 extinct species. The taxonomic diversity as well as the morphological disparity of these extinct species clearly overshadow the recent taxa. The contrast becomes even more impressive if we look on a bigger scale, where we find fully marine, fossorial and herbivore Crocodylomorpha.


The evolutionary history of Crocodylia has gained much attention in the last few decades and as more and more data is gained and analytical techniques advance, our understanding of this clade improves and allows to ask even more specific research questions. One of the main challenges however, remains in resolving the crocodylian phylogeny. Quite often morphological and molecular datasets lead to partially contradicting results.The most prominent of which are the phylogenetic affinities of the extant gharial, Gavialis gangeticus and the false gharial, Tomistoma schlegelii (see figure 1); but other systematic problems in the clade of Crocodylia exist and are recently being studied as well.


1. Figure from article: "Figure 5: Contrasting topologies of molecular (A) and morphological (B) phylogenies illustrating differences in the divergence of principal crocodylian clades.

Molecular divergences based on Oaks (2011) and morphological divergences based on the stratigraphically oldest fossils assigned to each clade prior to the current study."


For a better understanding of all the phylogenetic relations between crocodylian taxa and especially to unravel enigmatic affinities, a coherent reevaluation and compilation of former studies and poorly described fossil material is required.


...and so Rio and Mannion did just that and generated a new morphological dataset for Crocodylia. It is the most taxonomically comprehensive dataset for this clade known to date, including 144 taxa and 330 characters, of which 46 are new. The main results of their study are summarized here as follows.



The troubling relation between Gavialis and Tomistoma finally resolved

It is for the first time that a morphology-only dataset recovers Gavialis gangeticus as the closest living relative to Tomistoma schlegelii. This result doesn't seem to be related to convergence in long-snouted taxa, but rather due to changes of character scores and construction, as well as taxon sampling.

From these changes it can be concluded that:

  • several gavialoid species lack plesiomorphic features that formerly drew them towards the stem of Crocodylia

  • more widespread similarities are present in between taxa that are traditionally classified into tomistomines and gavialoids. These similarities are interpreted in this study as homology rather than homoplasy.

The resulting topology is in agreement with molecular studies and such based on the combination of molecular and morphological data.



Temporal incongruence in Gavialoidea

Although the newly obtained, morphology-based topology is congruent with molecular data, a temporal incongruence remains in Gavialoidea. This might be due to the fact, that some enigmatic taxa, such as Portugalosuchus from the early Late Cretaceous and 'thoracosaurus' are being included in this clade (see figure below).


add. Figure 1: left: Portugalosuchus azenhae, holotype (ML1818) ; (from Mateus et al., 2018)

right: Thoracosaurus isorhynchus, lectotype (MNHN.F.MTA61); ( from Brignon, 2017)


The inclusion of Portugalosuchus and 'thoracosaurus' drags the base of Gavialoidea far down and so inevitably extends the ghost lineages by over 30 million years (see 2. figure from article). The main reason why these taxa fall into the clade Gavialoidea is due to the convergence in snout lenght. However, recent studies based on updated anatomical data, suggest the exclusion of Portugalosuchus and some members of 'thoracosaurus' from Crocodylia. These taxa may belong to a yet unrecognized clade of non-crocodylian eusuchians and should be further reevaluated in future studies.


2. Figure from article: "Figure 17: Time-calibrated phylogeny of Gavialoidea.

Ages = Ma. Support values: Bootstrap/Jackknife/Bremer. Abbreviations: Af, Africa; As, Asia; CA, Central America; Eur, Europe; NA, North America."



Implications for the biogeographic history of Gavialidae

Based on the phylogenetic relationships within Gavialidae their biogeographical history can be interpreted as follows:

"(1) Neotropical gavialids are descended from African ancestors and their interrelationships imply more than one instance of trans-Atlantic oceanic dispersal.

(2) Gavialis is more closely related to South American than African taxa."

Therefore, ancestors of Gavialis must have originated in the Neotropics and most likely crossed the Atlantic by the late Miocene, before adapting to freshwater environments.


3. Figure from article: "Figure 41: The distribution of named Gavialis species, illustrating alternative biogeographic routes suggested from a close relationship between South American and Asian gavialids. Global palaeogeographical reconstruction at 2 Ma from Fossilworks (Alroy, 2013) based on data in the Paleobiology Database."



The taxonomic content of Alligatoroidea

Similar to some other recent studies, an early diverging clade from North America is recovered within Caimaninae, including the genera Ceratosuchus, Wannaganosuchus, Stangerochampsa and Brachychampsa (see 4. & 5. figure from article). It should therefore be considered that Caimaninae originated in North America rather than South America. This would also mean that Alligatoroidea began to diversify before the K/Pg mass extinction, rather than after.

4. Figure from article: "Figure 12: Time-calibrated phylogeny of Alligatoroidea from Analysis 1.3. Ages = Ma. Support values: Bootstrap/Jackknife/Bremer. Abbreviations: As, Asia; Eur, Europe; NA, North America; SA, South America."


Eocaiman, a South American taxon from the early Paleogene, is recovered outside of Caimaninae. This phylogenetic placement would make it the only non-caimanine member of Alligatoroidea ever known from this continent (see figure below). This placement would imply a second dispersal of alligatoroids around the K/Pg boundary from North to South America. However, this hypothesis requires further investigation.


5. Figure from article: "Figure 28: The distribution of named alligatoroids in the Paleocene.

Global palaeogeographical reconstruction at 60 Ma from Fossilworks (Alroy, 2013) based on data in the Paleobiology Database."



Tomistoma schlegelii stands with Gavialoidea, rather than Crocodyloidea

The phylogenetic placement of species formerly assigned to tomistomines, including Tomistoma schlegelii, within Gavialoidea rather than Crocodyloidea, introduces several

changes to the topology of the clade Crocodyloidea.

add. Figure 2: Tomistoma schlegelii.

Image source: Zoo Leipzig


As a result, the early Paleogene European taxa Asiatosuchus depressifrons and - germanicus, as well as the North American

species ‘Crocodylus’ affinis can be interpreted as 'stem' crocodyloids. Following this rational, the primarily Australasian clade Mekosuchinae probably should be placed outside of the crown group Crocodylidae. In contrary to other studies, Asiatosuchus nanlingensis from Asia is included in Mekosuchinae, while Australosuchus clarkae and Quinkana from Australasia are recovered outside of this clade. The latter result in combination with the close relationship of Jiangxisuchus to Mekosuchinae and Crocodylidae, suggests an Asian origin for Mekosuchinae. The migration of Mekosuchinae's ancestors to Australia during the early Paleogene was most likely achieved by the crossing of the Eastern Tethys or alternatively by non-oceanic way as shown in the figure below.

6. Figure from article: "Figure 35: The distribution and biogeographic history of Mekosuchinae. (A) Global palaeogeographical reconstruction at 50 Ma from Fossilworks (Alroy, 2013) with arrows showing alternative dispersal routes for an Asian origin of Mekosuchinae. (B) the distribution of named mekosuchines in eastern Australia and South Pacific islands from the Neogene and Quaternary. Based on data in the Paleobiology Database."


Conclusion

Rio and Mannion conclude their work and methodological approach with the following words: "Our study has numerous additional ramifications for the evolutionary and biogeographical history of Crocodylia, and our new dataset provides a platform for future studies to evaluate macroevolutionary patterns in this clade. Furthermore, we demonstrate that the application of extended implied weighting with higher k-values produces optimal phylogenetic results with regards to stratigraphic congruence and internal accuracy, at least for crocodylians. Extended implied weighting tends to override any differences in the treatment of quantitative data, i.e. whether continuously coded, discretised, or excluded. However, the main results of our study are consistent across analyses, regardless of treatment of quantitative characters or weighting strategies. This suggests that such methods can be applied to improve resolution in morphological datasets, without producing spurious results."


 

References in this blog post

 

What’s more in the paper?

Complete content list with the article’s original chapter titles (taxa marked in bold letters):


Introduction

Previous studies of crocodylian interrelationships

The gharial problem

Other taxonomic problems

Methods

Taxon sampling

Character list construction

Character construction

Discrete morphological characters

Continuous morphological characters

Re-discretised morphological characters

Extended implied weighting

Phylogenetic analysis

Measures of phylogenetic support

Stratigraphic congruence

Constrained searches

Categorization of synapomorphies

Results

Overall topological results

Phylogenetic support

Stratigraphic congruence

Detailed topological results

Overall topology

Non-crocodylian eusuchians

Alligatoridea

Crocodyloidea

Gavialoidea

Discussion

The impact of quantitative character treatment

The use of extended implied weighting

Anatomical support and implications for the systematics of non-crocodylian eusuchians

Isisfordia

Hyaelochampsidae and Allodaposuchidae

Borealosuchus

Emended diagnosis of Crocodylia

Anatomical support and implications for the systematics of Alligatoridea

Modifications to the diagnosis of Alligatoroidea

The phylogenetic relationships and monophyly of Diplocynodon

The taxonomic content of Caimaninae

The phylogenetic affinities of Eocaiman

The phylogenetic affinities of Protocaiman peligrensis

The phylogenetic affinities of Protocaiman, Eocaiman, Neocrosuchus and Bottosaurus

The taxonomic content of Jacarea

The phylogenetic affinities of Caiman lutescens (MACN PV 13551)

The phylogenetic affinities of UCMP 39978

Anatomical support and implications for the systematics of Planocraniidae

Anatomical support and implications for the systematics of Gavialoidea

Is the recovery of Gavialidae due to convergence in long-snouted taxa?

What are the reasons for the recovery of Gavialidae?

Atavistic characters

Character support for Gavialoidea

Character support for unnamed gavialoid clade 1

Character support for Gavialidae

Character support for unnamed gavialoid clade 2

Character support for unnamed gavialoid clade 3

Character support for unnamed gavialoid clade 4

Summary of character support for Gavialoidea and Gavialidea

Are ‘thoracosaurus’ gavialoids?

Anatomical support and implications for the systematics of Crocodyloidea

Asiatosuchus germanicus, ‘Crocodylus’ affinis and ‘Asiatosuchus’ depressifrons – basal crocodyloids or stem longirostrines?

Phylogenetic affinities and taxonomic content of Mekosuchinae

Phylogenetic intrarelationships of Mekosuchinae

Osteolaeminae

Crocodylinae

Implications for the evolutionary and biogeographic history of Crocodylia

Eusuchia and the origin of Crocodylia

Alligatoroidea

Planocraniidae

Crocodyloidea

Gavialoidea

Conclusions

Comments

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