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AMNIOTA |--SYNAPSIDA `--SAUROPSIDA |--ANAPSIDA `--EUREPTILIA `--o DIAPSIDA |==Araeoscelida `--o Neodiapsida |=="Younginiformes" `--+--Claudiosaurus |--Weigeltisauridae `--+--+--+?--Thalattosauria | | `?--Saurosphargidae | `--ICHTHYOSAURIA |?--Omphalosaurus |?--Hupehsuchia `--o Sauria |?--CHORISTODERA |--LEPIDOSAUROMORPHA `--ARCHOSAUROMORPHA
The Eureptilia would have been a footnote in the history of turtles and mammals but for the evolution of the diapsids. The bulk of reptiles fall into the Diapsida. The Diapsida ("two arches") are so called because they have two (di-) skull openings on either side of the head, for attachment of temporal muscles (to work the jaw). However it is now known that some reptiles lost one or even both pairs of openings, so the presence or absence of this characteristic is not a completely reliable guide.
The Diapsida are divided into two important groups, the lepidosaurs (Lepidosauromorpha) and the archosaurs (Archosauromorpha). There are also a number of different lineages of early, mostly small lizard-like insectivorous forms that lived during the late Carboniferous, Permian, and Triassic periods. These have been traditionally grouped under the "Eosuchia," but it is now known that this is an artificial taxon, a "waste basket" category for any diapsid that is neither lepidosaur nor archosaur. Nevertheless the term retains some use as a synonym for "basal diapsid." MAK991026.
Depending on which phylogenetic and methodological paradigm you prefer, the Diapsida are either the single largest and most diverse subclass of class Reptilia (see Classification page), or the single largest clade of amniotes (see Dendrogram page). Either way, they are pretty important. There is even a pretty good basic phylogentic consensus, of sorts, regarding where the different groups go, which forms the backbone of the present unit. Thiis includes not only the usual terrestrial and semi-aquatic lizard-like forms, like Acerosodontosaurus, illustrated above, but gliders like Coelurosauravus, the first vertebrates to take to the air.
It is quite frustrating then, in putting up this unit dedicated to primitive (basal) diapsids, to find, once we go beyond this central and safe axis, so many small oddball , mostly aquatic, Triassic lineages here. The whole abbreviated dendrogram and table of contents ends up looking like one of those really annoying dendrograms (annoying because they contain so little phylogenetic information) which begin with an enormous list of incertae sedis (taxa of uncertain position) about which all that can be said is that they belong somewhre in that particular clade.
Well, that's all that can be said here. There is no doubt that thalattosaurs are neodiapsids (and they are among the more important lineages of the Triassic marine reptiles. at that) but are they more closely related to Ichthyosaurs (Bickelmann et al. 2009) or to Sauropterygians (Li et al. 2011)? Or both (assuming ichthyosausr and plesiosaurs are related) or neither?
The same can be said, and asked, of Hupehsuchus. It's a diapsid, but where on the diapsid tree does he belong? Near primitive ichthyosaurs? Or Sauropterygia? Or simply an independent line of be basal neodiapsids?
Or the mollusc (or seaweed?) eating Omphalosaurus. An ichthyosaur? Or (perhaps like the hupehsuchians) ichythosaur mimic? Or, if we really want to talk about odd balls, Sinosaurosphargis, a creature that resembled a turtle or a placodont, but about which all we can say for sure is that it wasn't a turtle or a placodont.
For that matter, where exactly do the ichthyosaurs and the sauropterygians - the two groups that between them pretty much embody the 19th century "antedeluvian monsters" of the Mesozoic seas? How much progress has been made in almost two centuries. We used to be pretty sure where the ichtyosaurs and the plesiosaurs (& co., i.e. the sauropterygia) went, now we aren't quite so sure.
The problem is that all these strange groups were so highly specialised, and so different from their obviously more conventional ancestors, that cladistics simply isn't any help. Without shared characteristics with which to compare, or transitional forms that had such characteristics, we simply cannot determine where a creature belongs on the cladogram, the abstract evolutionary tree. It's the reason for the uncertainty of placement of groups like mesoasurs and turtles; both of which may be anapsids or diapsids, but both are so specialised it's impossible to tell.
Here there is the choice between the fool and the coward. The fool's option is to go out on a limb and say "this group belongs with X and is related to Y." To follow this course at least gives us one assurance: that we will almost certainly be shown to be totally wrong when new discoveries turn up and new phylogenetic analyses are made. Whereas the coward's option is to say "I don't have any idea", which at least is being honest and truthful, but it makes for dendrograms that really, really, suck.
No doubt in these pages, in trying to avoid this conundrum, we have been foolish where discretion would be the better part of valour, and cowardly where there was the need to bodly state new hypotheses. MAK111108
In the 19th century, the evolution of the reptiles was mostly interpreted in terms of current groups. The Diapsida as a distinct taxon were only named in 1903 by Henry Fairfield Osborn, who distinguished between two arched (Diapsida) and single arched (Synapsida) reptiles. His system was very different to that used today; the Pelycosaurs for example were not Synapsids but Diapsids! So were the Procolophonia (perhaps because Procolophon sometimes displays temporal openings), mesosaurs, and ichthyosaurs). His ideas were further taken up by Watson (1917, 1957), Williston (1925), and Broom (1925), among others. While all agreed on the monophyly of the Diapsida, they argued over the relationships of the various groups, as well as the nacetsors of extant taxa like Squamata (see Benton (1985) and Evans (1984) for more on this). It was at this time that classification in terms of temporal fenestrae was developed, abd this remained standard until the cladistic revolution of the 70s and 80s. Meanwhile, back in the 1930s, Alfred Sherwood Romer, the most influential vertebrate paleontologist of his time (and perhaps of the 20th century) rejected the idea of a monophyletic Diapsida. He considered that the lepidosaurs evolved from cotylosaurs (anapsids, captorhinomorphs) via the Eosuchia/Younginiformes, and the Archosaurs likewise via the Thecodonts. Such was his influence that it was not until the 1970s that a monophyletic Diapsida was once again accepted as paleontological canon, although other authors like Colbert (1969) retained a monophyletic Diapsida with Romer's Archosaurs and Lepidosaurs as infraclasses. Interestingly, early cladists like Lovtrop 1977 and Gardiner 1982 similarily rejected a monophyletic Diapsida (on morphological grounds) in favour of a crocodile-turtle clade, ironic in favour of recent molecular studies, although Benton 1985pp.104-6 rejects most of their claims. Today, despite the almost complete consensus of molecular studies which say that turtles are crown group archosaurs, no cladist accepts such a topology, favouring instead the anapsid or the lepidosauromorph hypothesis
Romer's objection was that there wasn't any suitable ancestral form with characteristics common to both lepidosaurs and archosaurs. This changed with restudy of Petrolacosaurus by Robert Reisz (1977, 1981), as well as studies of various eosuchians by Carroll, Gow, and others (summarised in Benton (1982)).
With the Diapsida firmly established as a valid taxon, it seems that our understanding of reptile evolution would go smoothly from thsi point on. Unfortunately, science being science, and phylogeny being phylogeny, this was only the beginning of further controverises, explained in the following section, courtesy of the Tree of Life web project and open source science (yayy!). MAK111119
Michel Laurin and Jacques A. Gauthier
Even though diapsid phylogeny has been intensively studied, relatively few computer-assisted studies of early diapsids have been published. The large-scale phylogenetic analyses of Benton (1985), Evans (1988), and Gauthier et al. (1988) focused on lepidosauromorphs and archosauromorphs. More recently, phylogenies based on data matrices incorporating several diapsid terminal taxa (more than 20) were presented. Therefore, only simplified versions of these phylogenies are presented here (the node names are modified to be consistent with the recommended usage and do not necessarily follow the published versions).
This page deals mostly with the phylogeny between the various stem-diapsids, Lepidosauromorpha and Archosauromorpha. However, some taxa, such as ichthyosaurs and sauropterygians, have been variously thought to be stem-diapsids, lepidosauromorphs, or archosauromorphs, so their affinities are briefly discussed below. Turtles are usually not considered diapsids, but a few paleontological studies have suggested that they were closely related to sauropterygians (and to lepidosauromorphs). The results of these studies are also briefly exposed below.
Benton (1985) presented the following phylogeny:
===================================== Araeoscelidia | | === Younginiformes =====| ==Lepidosauromorpha=| ==Neodiapsida=| === Lepidosauria | ======================= Archosauromorpha
Evans (1988) studied more stem-diapsids, but her main conclusions are similar to those of Benton (1985) and Gauthier et al. (1988):
============================================ Araeoscelidia | | ========================================= Coelurosauravus | | | | ====================================== Sauropterygia | | | | | |===================================== Heleosaurus | | | =====| | |===================================== Galesphyrus ===| | ===| === Heleosuchus | ===| | ==Lepidosauromorpha=| === Younginiformes ====Sauria==| | | ====== Lepidosauria | ========================== Archosauromorpha
Both of these studies, as well as Gauthier et al. (1988) suggested that younginiforms were lepidosauromorphs. Laurin (1991) argued that younginiforms were not saurians, and this conclusion has been accepted by most scientists (Rieppel, 1993, 1994; Gauthier, 1994). The phylogeny suggested by Laurin (1991) is as follows:
================================ Araeoscelidia | | ================== Coelurosauravus | | | | =============== Apsisaurus =====| | | ==Neodiapsida=| | ============ Younginiformes ===| | ===| === Lepidosauromorpha ==Sauria=| === Archosauromorpha
Rieppel (1994), Rieppel and deBraga (1996), and deBraga and Rieppel (1997) argued that turtles (Testudines) are closely related to sauropterygians (a group of aquatic diapsids from the Mesozoic), and that both groups are lepidosauromorphs. This phylogeny suggests that turtles are actually diapsids that have lost their temporal fenestrae. While this conclusion remains controversial, it deserves to be presented:
==================================================== Araeoscelidia | | ====================================== Claudiosaurus | | | | =================================== Younginiformes | | | ====| | | === Sauropterygia ==Neodiapsida=| | ===| ===| ==Lepidosauromorpha=| === Testudines ==Sauria=| | | ====== Lepidosauria | ========================== Archosauromorpha
Gauthier (1994) reviewed amniote phylogeny.
Caldwell (1996) did one of the first computer-assisted phylogenetic analyses of diapsids incorporating ichthyosaurs. He found that if turtles and ichthyosaurs were included in his matrix, ichthyosaurs were the sister-group of sauropterygians, and this large clade of aquatic diapsids was close to the base of Archosauromorpha, while turtles appeared to be lepidosauromorphs.
======================== Araeoscelidia | =====| ===================== Claudiosaurus | | ===| ================== Younginiformes | | ===| ====== Lepidosauria | ===| | | ====== Testudines ==Sauria=| | === Sauropterygia | ===| ===| === Ichthyosauria | ====== Archosauria
Without ichthyosaurs, the same matrix resulted in a topology in which turtles were the sister-group of sauropterygians, and in which this clade was part of Lepidosauromorpha (sauropterygians shifted from archosauromorphs to lepidosauromorphs).
======================== Araeoscelidia | =====| ===================== Claudiosaurus | | ===| ================== Younginiformes | | ===| ====== Lepidosauria | ===| | | | === Sauropterygia ==Sauria=| ===| | === Testudines | ========= Archosauromorpha
Finally, without turtles, ichthyosaurs were the sister-group of sauropterygians, and that clade was located outside Sauria (Sauria is identified with the wrong clade in Caldwell's figure).
===================== Araeoscelidia | =====| ================== Claudiosaurus | | ===| =============== Younginiformes | | ===| ========= Ichthyosauria | ===| | | ========= Sauropterygia ===| | === Lepidosauromorpha ==Sauria=| === Archosauromorpha
Motani et al. (1998) reassessed the affinities of ichthyosaurs using the data matrices of Rieppel and deBraga (1996) and of Caldwell (1996) by adding ichthyosaurs (in the first case) or recoding them by incorporating data that they had recently obtained about the early (Lower Triassic) ichthyosaur Utatsusaurus hataii. Using the matrix of Rieppel and deBraga (1996), they obtained the following topology:
======================== Araeoscelidia | =====| ===================== Claudiosaurus | | ===| ================== Younginiformes | | ===| =============== Ichthyosauria | | ===| === Lepidosauria | ===| ==Sauria=| === Sauropterygia | ====== Archosauromorpha
In that tree, turtles (not shown) were located outside diapsids, in their traditional location (along with other parareptiles). According to that tree, ichthyosaurs are stem- diapsids, and sauropterygians are lepidosauromorphs.
Using the data matrix of Caldwell (1996), Motani et al. (1998) obtained the following topology:
=========================== Araeoscelidia | =====| ======================== Claudiosaurus | | ===| ===================== Younginiformes | | ===| ================== Ichthyosauria | | ===| ====== Lepidosauria | ===| | | | === Testudines ==Sauria=| ===| | === Sauropterygia | ========= Archosauromorpha
In this tree, the position of ichthyosaurs and sauropterygians remains the same, but turtles move into lepidosauromorphs, as the sister-group of sauropterygians.
Finally, Merck (1997b) suggests that ichthyosaurs are the sister-group of sauropterygians, and that this clade is part of Archosauromorpha.
The conflicting results obtained from various authors, and sometimes, by a single author (or group of authors) using different taxa suggest that more work is needed to assess the affinities of ichthyosaurs, sauropterygians, and turtles.
Range: from the Late Carboniferous.
Phylogeny: Eureptilia : Protorothyrididae + * : Araeoscelida + Neodiapsida.
Characters: Upper (triradiate squamosal & triradiate post-orbital) [R89] and lower temporal fenstrae; posttemporal fenestra present, bordered by narrow occipital flange of squamosal, tabular (if present), supraoccipital & paroccipital process [R89]; suborbital fenestra consisting of a relatively large hole in the palate located between palatine, ectopterygoid, and maxilla [Other taxa may have foramen in this region, but there is usually no fenestra] [R89]; ossified sternum; $ (primitively) radius long, measuring 70 - 90% of humerus length; ridge-and-groove ankle joint between tibia and astragalus.
Links: Temporal Fenestration and the Classification of Amniotes; Introduction to the Diapsids; Reptilian Systematics; Introduction to the Diapsids; Diapsida; Basal Diapsida; Reptiles - Subclass Diapsida.
Note: Domed skull of Amniotes permits muscles to run vertically. Vertical muscles allow strong static pressure when jaws are closed or almost closed. Increase in muscle bulk would strain periosteal covering of bone. Fenestrae may have evolved as unossified attachment points. Later, evolved as openings which "bulging" of these jaw muscles. Note no obvious change in jaw musculature in primitive forms.
References: Rieppel (1989) [R89]. ATW070113.
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