Archosauromorpha: Prolacertiformes

Taxa on This Page

1. Dinocephalosaurus X
2. Prolacerta X
3. Prolacertiformes X
4. Tanystropheidae X
5. Malerisaurus X
6. Macrocnemus X

Life restoration of Malerisaurus robinsonae, a prolacertiform from the Late Triassic of India. The long neck and long hind limbs (for bursts of bipedal running) are primitive archosauromorph features. Illustration by Nobu Tamura (Wikipedia)

The Prolacertiformes - an assemblage of early archosauromorphs

Prolacertiformes or Protorosaurs are an assemblage of mostly lizard like archosauromorphs that, with the exception of the Permian Protorosaurus, were limited to the Triassic Period, although during this time they were diverse and widespread. There is a close similarity between Prolacerta and Proterosuchus (Chasmatosaurus), and it generally accepted that Prolacertiformes (sensu stricta) and Archosauriformes are sister-groups, or in other words that Archosauriformes ("thecodonts" in older books) evolved from a Archosauriforme-like ancestor. Rhynchosaurs and Trilophosaurs seem to be less closely related.

Prolacertiformes such as Protorosaurus, Prolacerta, Macrocnemus, and Tanystropheus, share numerous characteristics and are usually considerd a monophyletic clade (Chaterjee, 1980; Benton 1984; Benton 1985, Chaterjee, 1986, Jalil 1997).

Benton 1984 and Benton 1985 present the following, which was the first cladistic study of basal Diapsida.

Apart from the Pterosauria, which are now considered Ornithodira rather than basal archosauromorphs, this defined the basic approach to the Prolacertiformes. Chaterjee, 1986 who discovered and desceribed Malerisaurus, offers a basically similar phylogeny (Protorosauria = Prolacertiformes only):

This phylogeny gives a deeper division between Protorosauridae (= Prolacertidae) and Tanystropheidae, as Protorosaurus has been moved up from the base of the tree.

A later study by Jalil 1997, including an analysis of more characters and more taxa, including the newly described Jesairosaurus, gives the following, quite different cladogram:

Here the Trilophosaurs and Rhynchosaurs play musical chairs. Also, in contrast to Benton 1984 and 1985 and Chaterjee, 1986, Prolacertiformes are not divided into Prolacertidae/Protorosauridae and Tanystropheidae, but represent instead a single series.

But that is not the end of the matter. Through analysis of a newly described genus, Czatkowiella, Borsuk-Białynicka & Evans, 2009 broke with the paradigm of a monophyletic Prolacertiformes by defining Protorosaurus as a more basal form, and making the Prolacertiformes in general as a grade of basal archosauromorphs rather than a clade .

The following cladogram is from Borsuk-Białynicka & Evans, 2009, using the data matrix of Müller 2004, with Czatkowiella added and Protorosaurus updated:

Here traditional Prolacertiformes have been shown in bold. This phylogeny is interesting because it includes some of the various marine Diapsids that seem to appear out of nowhere in the course of the Triassic. The Testudines (turtles) are here shown as basal archosauromorphs, in keeping with molecular phylogeny. Coelurosauravus however is more often understood as a basal Neodiaspid. Macrocnemus and Tanystropheus, rather than representing highly derived Prolacertiformes, constitute distinct parallel lineages. Also, Prolacerta is the most derived frather than a fairly basal Prolacertiform (Malerisaurus and similar forms are not considered but may perhaps go here as well). Rhynchosaurs and Trilophosaurs are here shown as more derived than "Prolacertiformes", whereas in other cladograms beginning with Benton 1985 the inverse is almost always the case. Quite possibly, Rhynchosaurs, Trilophosaurs, and "Prolacertiformes" represent a general assemblage of early archosauromorphs, which differ only in that the Rhynchosaurs and Trilophosaurs have become more specialised as regards a herbivorous diet, whereas the "Prolacertiformes" have retained their plesiomorphic (primitive) generalised lizard-like shape.

It is somewhat unsettling that, in considering the above cladograms, there does not appear to be a single clear consensus among them all in the series that the various taxxa appear, beyond Protorosaurus occupying a basal position, and the close relation between the two specialised and long-necked forms: Tanystropheus and Tanytrachelos. Were these poorly known and fragmentary forms one could understand, but these are well known animals pof which a number of taxa are known from complete skulls and skeletons.

Other Triassic diapsids that have been associated with the Prolacertiformes over the last three decades or so, but only because there is nowhere else to put them, and making it a sort of "wastebasket taxon". These include a number of unusual arboreal forms, as enigmatic, specialised, and difficult to place in their own way as the various Triassic marine forms. They include the glider Sharovipteryx, the chameleon-like drepanosaurs, and Longisquama, a small animal with long feather-like scales that would obviously have been used for intra-specific display or perhaps to frighten away predators (by making the animal look larger). While all of these forms are almost certainly archosauromorphs, their exact relationships remain unclear. And with the Prolacertiformes possibly losing their monophyletic status, the way is open for new interpretations and rearrangements of these various taxa. MAK101012

Descriptions

Dinocephalosaurus, traced from the in situ fossil. Note how different the neck is from that of Tanystropheus. Although both were near-shore marine archosauromorphs, their lifestyle, environment, and food sources would have differed. Original art by David Peters also on Wikipedia

Dinocephalosaurus: D. orientalis Li, 2003 . 

Comments: Although considered a Prolacertiforme, trhere is no reason why Dinocephalosaurus should be specifically related to that group, beyond sharing plesiomorphic archosauromorph chracatreistics. One area in which it is very unlike other Prolacertiformes is as regards the 25 cervical vertebrae, which means the neck is very long but also flexible like a plesiosaur (overall length of animal about 2.7 meters, of which the neck makes up 1.7 m), in in contrast to the long but stiff (few vertebrae) neck of Tanystropheus. Although similar in appearance, and both marine, the long necks of both taxa evolved independently. The limbs are also more paddle like than those of Tanystropheus, and carpal and especially tarsal bones resemble those of nothosaurs as simple rounded bones. It may quite likely represent simply one more aquatic diapsid lineage.

Links: Wikipedia, Dinosaur Mailing List, BBC news, Pharyngula, photos MAK101012

Skeleton of Tanystropheus longobardicus, from the Grenzbitumenzone (middle Triassic, Anisian/Ladinian boundary) of Monte San Giorgio, Switzerland. The extraordinarily long neck consists of 12 elongate cervical vertebrae. Photo by Ghedoghedo, Wikipedia

Prolacertiformes: Prolacerta, Malerisaurus, Tanystropheus.

Links: tanyst; Nuova pagina 1. wikipedia

Macrocnemus basanii; skull, in dorsal and lateral view, from Benton 1985

Macrocnemus: Macrocnemus basanii (Nopcsa, 1930).

Phylogeny: Archosauromorpha/"Prolacertiformes" ::: *.

Comments: shares characteristics both with Prolacerta and Tanystropheus, may be closer to one or the other, or alternatively represent a third lineage of "Prolacertiformes".

Links: Wikipedia MAK101012

Malerisaurus: M. langstoni Chatterjee, 1986; M. robinsonae Chatterjee, 1980.

Phylogeny: "Prolacertiformes" : Prolacerta + *.

Comments: A lightly built lizard-like form with long hind limbs. Originally placed in the Protorosauridae with Protorosaurus by Chaterjee, 1980 on the basis of its apparently closed lower temporal bar and other features. With further material it was realised that the temporal bar was open as with Prolacerta which is seen to be a closely related form Chaterjee, 1986

References: Chaterjee, 1980 Chaterjee, 1986.

Links: Wikipedia MAK101012

Prolacerta broomi; skull, in dorsal, palatal, and lateral views, from Benton 1985	Prolacerta broomi; skeleton, length of original about 60 cm, from Benton 1984

Prolacerta: Prolacerta broomi Parrington, 1935.

Comments: Previously regarded as an ancestral lizard, an eosuchian and a proto-thecodont.

Links: Wikipedia MAK101007

Tanystropheus longobardicus; skull, in dorsal, palatal, and lateral views, from Benton 1985.	Tanystropheus longobardicus; skeleton, length of original upto 6 meters, from Benton 1984. Apart from the unusual neck, the proportions are very similar to Prolacerta

Tanystropheidae: Tanystropheus, Tanytrachelos.

Phylogeny: Archosauromorpha/"Prolacertiformes" ::: *.

Comments: These are fresh-water to fully marine forms, characterised by the elongation of the neck through 12 elongate cervical vertebrae. This reaches an extremne form in Tanystropheus longobardicus. The neck was stiffened by long, slender cervical ribs. Less derived other "Protorosaurs" and Prolacertiformes (including Protorosaurus, Prolacerta, Kadimakara, Macrocnemus, Malerisaurus, etc) have a moderately long neck, with 8 vertebrae. Tanystropheus was classified as a euryapsid by Romer, but is now placed in the Prolacertiformes. Tanystropheus was a large animal, which grew upto 5 to 6 m long. Although Tanytrachelos is much smaller (20 cm long), and has procoelous vertebrae, the two share a number of unique characteristics

References: Benton 1985, Chaterjee, 1986 .

Links: Wikipedia MAK101012