- Paleontology, Evolution, Phylogenetics, Systematics (Taxonomy), Comparative Anatomy, Geometric Morphometrics, and 12 moreCrocodylians, Crocodyliformes, Crocodylomorpha, Morphology, Vertebrate Paleontology, Earth Sciences, Evolutionary Biology, Systematics, Phylogeny/phylogenetics, Vertebrate Palaeontology, Crocodilian Biology, and Cladisticsedit
- Christopher A. Brochuedit
Outgroup sampling is a central issue in phylogenetic analysis. However, good justification is rarely given for outgroup selection in published analyses. Recent advances in our understanding of archosaur phylogeny suggest that many... more
Outgroup sampling is a central issue in phylogenetic analysis. However, good justification is rarely given for
outgroup selection in published analyses. Recent advances in our understanding of archosaur phylogeny suggest that
many previous studies of crocodylomorph and crocodyliform relationships have rooted trees on outgroup taxa that are
only very distantly related to the ingroup (e.g., Gracilisuchus stipanicicorum), or might actually belong within the ingroup.
Thalattosuchia, a group of Mesozoic marine crocodylomorphs, has a controversial phylogenetic position—they are recovered
as either the sister group to Crocodyliformes, in a basal position within Crocodyliformes, or nested high in the crocodyliform
tree. Thalattosuchians lack several crocodyliformapomorphies, but share several character states with derived long-snouted
forms with a similar ecological habit, suggesting their derived position may be the result of convergent evolution. Several of
these “shared” charactersmay result from ambiguouslyworded character state definitions—structures that are superficially
similar but anatomically different in detail are identically coded. A new analysis of crocodylomorphs with increased
outgroup sampling recovers Thalattosuchia as the sister group to Crocodyliformes, distantly related to long-snouted
crocodyliforms. I also demonstrate that expanding the outgroup sampling of previously published matrices results in
the recovery of thalattosuchians as sister to Crocodyliformes. The exclusion of thalattosuchians from Crocodyliformes
has numerous implications for large-scale evolutionary trends within the group, including extensive convergence in
the evolution of the secondary palate characteristic of the group. These results demonstrate the importance of careful
outgroup sampling and character construction, and their profound effect on the position of labile clades.
outgroup selection in published analyses. Recent advances in our understanding of archosaur phylogeny suggest that
many previous studies of crocodylomorph and crocodyliform relationships have rooted trees on outgroup taxa that are
only very distantly related to the ingroup (e.g., Gracilisuchus stipanicicorum), or might actually belong within the ingroup.
Thalattosuchia, a group of Mesozoic marine crocodylomorphs, has a controversial phylogenetic position—they are recovered
as either the sister group to Crocodyliformes, in a basal position within Crocodyliformes, or nested high in the crocodyliform
tree. Thalattosuchians lack several crocodyliformapomorphies, but share several character states with derived long-snouted
forms with a similar ecological habit, suggesting their derived position may be the result of convergent evolution. Several of
these “shared” charactersmay result from ambiguouslyworded character state definitions—structures that are superficially
similar but anatomically different in detail are identically coded. A new analysis of crocodylomorphs with increased
outgroup sampling recovers Thalattosuchia as the sister group to Crocodyliformes, distantly related to long-snouted
crocodyliforms. I also demonstrate that expanding the outgroup sampling of previously published matrices results in
the recovery of thalattosuchians as sister to Crocodyliformes. The exclusion of thalattosuchians from Crocodyliformes
has numerous implications for large-scale evolutionary trends within the group, including extensive convergence in
the evolution of the secondary palate characteristic of the group. These results demonstrate the importance of careful
outgroup sampling and character construction, and their profound effect on the position of labile clades.
Research Interests:
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... more
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.
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.