Palæos:		Unit 160: Temnospondyli
The Vertebrates		500: Trematosauria (2)

Temnospondyli: Trematosauria (2)

Metaposauroids, Plagiosauroids, Rhytidosteids & Brachyopoids

TETRAPODA
|--+--LEPOSPONDYLI
|  `--REPTILOMORPHA
|
Temnospondyli
|--Edopoidea
`--+--Euskelia
   |  `--LISSAMPHIBIA
   `--Limnarchia
      |--Dvinosauria
      `--Stereospondyli
         |--Rhinesuchidae
         `--+--Capitosauria
            `--Trematosauria
               |--Trematosauroidea
               `--+--Metoposauroidea
                  |  `--Inflectosaurus  
                  `--+--Plagiosauroidea
                     |  |--Plagiosauridae
                     |  |  |--Plagiosaurinae
                     |  |  `--Plagiosterninae
                     |  `--Laidleria
                     `--+--Rhytidosteidae
                        `--Brachyopoidea 
                           |--Chigutisauridae
                           `--Brachyopidae 

1. Brachyopidae X
2. Brachyopoidea X
3. Chigutisauridae X
4. Inflectosaurus X
5. Laidleria X
6. Metoposauroidea X
7. Plagiosauridae X
8. PlagiosaurinaeX
9. Plagiosauroidea X
10. PlagiosterninaeX
11. Rhytidosteidae X

The Weirdness of Plagiosaurs

Plagiosaurs are unreasonably difficult beasts to research. Despite the fact that Gerrothorax is one of the world's most-illustrated stem tetrapod, articles on the taxon are not easy to find. I am very grateful to Prof. Anne Warren of La Trobe University who has recently (too recently to be used in this version of this Note) supplied me with copies of many of the key articles in the field.

This note will largely consist of a question without even speculative answers. For example, Laidleria, the sister of the Plagiosauridae, has an almost flat, sharply triangular skull, with relatively close-set, dorsal eyes. The plagiosaurs are much bigger, a bit thicker, but generally adhere to the same plan. We are frequently told that this betokens a bottom - dwelling ambush predator like a ray. But rays have a unique ambush style which is completely different from anything a temnospondyl could manage. 

Granted, lifting the flat head off the bottom quickly could generate the same kind of suction that assists the ray, but the plagiosaur has no obvious means of managing that trick against the considerable resistance of the water column. The neural spines of plagiosaurs are strong, but low -- not the sort of arrangement one might associate with strong muscles and powerful tendons which could rapidly raise the head. further, the vertebral column (to extrapolate from Warren (1985) and Warren (1998)) seems designed to inhibit arching of the back. Figure 2 is not to be taken too seriously, but represents an attempt to reconstruct the vertebral articulations, based on Laidleria.

Suppose this obstacle is overcome in some way, then is the plagiosaur to leap lithely forward at the wildly gyrating prey and snap its jaws over the struggling victim? But the plagiosaurs had doubled occipital condyles. These were combined, to be sure, with a relatively advanced atlas-axis complex. Still, quick turns to the side were no easy matter for plagiosaurs. To make matters worse, many plagiosaurs were fairly heavily armored, making quick dashes more difficult.

Likewise, a quick snap of the jaws seems impractical. The very flatness of the head leaves little room for large jaw adductors, as the small (or even absent) adductor chambers (post- or sub- temporal fenestrae) demonstrate. Nor are there tabular horns which might signal a posterior extension of the adductors. In any case, there is almost no conceivable place one could put the adductors which would give them a reasonable mechanical advantage in a flat head.

So how did creatures like plagiosaurs, Diplocaulus or, for that matter, galeaspids, manage to get by? It is hard to imagine an entirely obsolete ecological guild, but it is notable that there are no vertebrates today with wide, flat, relatively inflexible and immobile skulls, dorsal orbits, and terminal or subterminal mouths. Yet this was a fairly common design in Paleozoic times. We may well be dealing with a way of life that has no easy parallels in today's world. ATW010422.

Descriptions

Metoposauroidea: Almasaurus, Buettneria. Definition: Almasaurus + Metoposaurus. Yates & Warren (2000).

Range: Middle Triassic (Ladinian) to Late Triassic (Norian). Probably cosmopolitan.

Phylogeny: Trematosauria:: (Plagiosauroidea + (Rhytidosteidae + Brachyopoidea))) + *: Inflectosaurus. 

Introduction: A group of large flat-headed aquatic tetrapods.  Although in appearance very like the capitosaurids in size and body proportions, the metoposaurs belong to a distinct lineage of temnospondyls.   The most noticeable distinction (apart from various details of the skeleton) are the more forward position of the eyes.  The metoposaurs  were large mostly aquatic predators that fed on fish and small animals.  As with many Paleozoic tetrapods the head was large and flat, with the eyes looking upwards.  The creature probably spent a great deal of time submerged and motionless, waiting for an unwary fish, smaller tetrapod or reptile to swim past.  The jaw was lined with teeth, and there were large teeth on the palate.  Metoposaurs were strong swimmers, but would have been very clumsy on land, and it is likely that they ventured from water rarely, if at all.

Large numbers of fossil specimens have been found crowded together, dying when the ponds and lakes in which they lived dried up, and preserved when the mud that covered them hardened into rock. (MAK 980114)

Characters: $ infra-orbital sulcus with a step-like flexure between orbit & naris [this appears to mean that the sensory line canal passing below the orbit makes a sharp medial or dorsal turn just anterior to the orbit, then an opposite turn anteriorly, to pass just outside each naris. However, this interpretation may be erroneous, since Batrachosuchus (=Batrachosaurus) (Plagiosauroidea) and even Neldasaurus (Dvinosauria) seem to have the same feature]; $ length of posterior skull table >90% of width; $ "gutter" bordered by ridges at margin of otic notch; exoccipital-pterygoid suture visible in ventral view; paired anterior palatal fossae; $ keeled lateral margin of clavicle. 

Links: Paleontology and Geology Glossary: M. 

References: Yates & Warren (2000) [comparative remarks based on figures in Carroll (1988) from Watson (1956) and from Chase (1965), vide Carroll (1988)]. (ATW 000213)

Inflectosaurus: Shishkin 1960.  S. amplus Shishkin 1960.

Range: Early Triassic of Russia, Yarenskian Gorizont (Astrkhan')

Phylogeny: Metoposauroidea: *. [S02]

Characters: skull up to 70 cm [S+00]; orbits small [S+00]; pineal foramen close to middle of orbit - occiput distance [S+00]; quadrates posterior to occipital condyles [S+00]; preotic pterygoid ascending ramus strongly developed [S+00].

References: Shishkin et al. (2000) [S+00]; Steyer (2002) [S02]. ATW020721.

Plagiosauroidea: Laidleria + Plagiosaurus. 

Range: Early Triassic to Late Triassic 

Phylogeny: Trematosauria::: (Rhytidosteidae + Brachyopoidea) + *: Plagiosauridae + Laidleria. 

Introduction: These strange aquatic forms had a very short, wide skull with pustular ornamentation, and upwardly facing eyes in the middle of the head.  Many forms had external gills, indicating a return to a fully aquatic environment.  So specialized and distinct are they that have often been placed in their own suborder. MAK010417.

Characters: Very short, wide skull with pustular ornamentation; orbits closely spaced and dorsal; pterygoid denticles absent; orbits near midline; $ frontal participates in orbit; quadratojugal dorsal to quadrate and overhangs quadrate; subtemporal fenestra reduced or absent; $ otic notch absent; tabular horns absent; neural spines with lateral buttresses; neural spines relatively short; $ pleurocentra extremely reduced or perhaps absent; $ tessellated & ornamented dorsal osteoderms; some (juvenile forms?) with external gills. 

Note: for image, see Laidleria. 

Links: Plagiosauroidea (Mikko's Phylogeny).  

References: Warren (1998); Yates & Warren (2000).  ATW010725.

Plagiosauridae: Gerrothorax, Plagiosternum, Plagiosaurus, Plagioscutum, Plagiobatrachus. 

Range: Early Triassic of Australia; Middle Triassic of Germany, and Kazakhstan; Late Triassic of Germany and France, Sweden, Spitzberg, Greenland, and Thailand

Phylogeny: Plagiosauroidea: Laidleria + *:Plagiosaurinae + Plagiosterninae. 

Characters: Body short & broad; skull flattened ventrally, broad, short and parabolic; maxilla has some, but reduced, participation in choana; dentition with simple infolding of dentine; teeth on medial margin of choana; 4-6 palatine teeth; usually a row of larger, marginal teeth with inner row of smaller teeth & no tusks; width of interpterygoid vacuity pair more than 90% of length; lateral margin of pterygoid on subtemporal vacuity (adductor chamber?) straight in ventral view; subtemporal fenestra reduced or absent; nares closely spaced; orbits anterior to mid-length of skull; lacrimal present; nasal participates in orbital margin; prefrontal absent (?); pineal foramen anterior to center of radiation of parietal; postparietal pair large and more than 4 times wider than long; exoccipital - pterygoid suture present and visible in ventral view; ascending process of pterygoid absent; pterygoid articulates with parasphenoid & exoccipital; "palatal ramus of entopterygoid" (= pterygoid?) does not reach vomer; well-developed osseous bar formed by parotic processes of tabular & exoccipital; otic notch reduced or absent; stapes massive; occipital condyles strongly projecting; braincase and gill arches have relatively high degree of ossification; ceratobranchials ossified; possible external gills in adults; gill rakers (!?); atlas highly elongated; vertebrae platycoelous, with equally developed anterior and posterior parapophyses; adjacent vertebral (inter)centra "share" neural arches, so that each centrum has two arches; centra elongate & solid; hemal arches reduced or absent; ribs articulate with 2 consecutive arches (Gerrothorax); normal temnospondyl posterior extension of interclavicle absent; cleithrum very elaborate with 3 lamina, almost completely ornamented on exterior face; dermal armor including equivalent of ossified ventral ribs present (Gerrothorax); ornamentation reticulate or of regularly spaced pustules; frequently found with scutes and complex armor, sometimes ornamented.  ATW

Comments: These specialised short-headed amphibians were probably bottom-living suction gulpers, adapted to a totally aquatic existence, as indicated by the presence of gills. They reach a maximum of 2.5 meters, although most were under a meter in length. Plagiosaurus and Gerrothorax were medium-large (around a meter long) of the clade Plagiosauria that lived alongside Cyclotosaurs and phytosaurs. They are best known from the Ladinian to Rhaetian of Europe, but have also been found in Greenland, Spitzberg, the Huai Hin Lat Formation (Norian) of Thailand, and the Arcadia Formation (Olenekian - Early Triassic) of Australia (Plagiobatrachus australis [Warren 1985]). The latter is the only record of the family from Gondwana. Plagiosaurs have been found in association with Mastodonsaurs, Capitosaurs, and Metoposaurs, but - except for the Australian record - never with Brachyopids. It can be assumed that the two groups required the same ecological niche, and hence would outcompete each other. MAK020305

Links: sh: Prehistoric Animals; Plagiosauroidea; Gerrothorax Printout- Enchanted Learning Software; gerrothorax (Swedish?); Paléontologie, licence, chapitre 7; Dinosaurios, Tutorial interactivo; amph.htm.

References: Milner 1994; Nilsson (1946); Warren (1985); Warren (1995); Warren (1998); Warren & Davey (1992); Warren & Marsicano (2000); Yates & Warren (2000). 

Image: Gerrothorax, approx 100cm. 

Note: [1] In temnospondyls, the intercentrum dominates over the pleurocentrum. In most other tetrapods, the reverse is true. In fact, the intercentrum disappears in amniotes. I assume that the presence of two pairs of parapophyses is related to this condition. [2] The relationship with Laidleria is not completely secure. ATW020902.

Gerrothorax pulcherrimus, from the Stubenstein (Middle Norian) of Stuttgart, SW Germany; Fleming Fjord Formation of Greenland illustration from the Staatliches Museum für Naturkunde

Plagiosaurinae: Gerrothorax, Plagiosaurus

Range:

Range: Middle to Late Triassic (Ladinian to Rhaetian) of Germany, Greenland, and ?Thailand

Phylogeny: Plagiosauridae: Plagiosterninae + *. 

Characters: Pustulate dermal ornament

Comments: these were the common plagiosaurs of the late Triassic of Europe. They flourished rioght up untiol; the end of the Triassic

Some representative species:

References: Milner 1994

MAK020305

Plagiosterninae:  Plagiosternum

Range: Middle to Late Triassic (Ladinian to Carnian) of Germany and Spitzberg

Phylogeny: Plagiosauridae: Plagiosaurinae + *. 

Characters: No ornament

References: Milner 1994

MAK020305

Laidleria: 

Range: Early Triassic of South Africa. 

Phylogeny: Plagiosauroidea: Plagiosauridae + *. 

Characters: 30-40 cm long. Extremely thin, flat skull forming an equilateral triangle; dentary teeth larger than corresponding maxillary teeth; dentary tusks present; maxilla excluded from choana by sutural articulation of palatine & vomer; more than 8 palatine teeth; no denticles on parasphenoid; ventral surface of pterygoid ornamented; ascending ramus of pterygoid does not contact squamosal; prefrontal and jugal in sutural contact; orbits behind mid-length of skull; quadratojugal with lateral projection; stapes robust, terminating under a solid section of skull roof, with no possible contact with a tympanum; uniform ornamentation of pits surrounded by ridges; strongly armored and "turtle-like;" no specialized scutes associated with neural spines.

Links: Plagiosauroidea. 

References: Warren (1998); Yates & Warren (2000). ATW010421.

Rhytidosteidae: Acerastia, Acadia, Deltasaurus, Derwentia,  Nanolania, Peltostega, Pneumatostega, Rewana, Rhytidosteus. 

Range: Late Permian to Early Triassic worldwide. 

Phylogeny: Trematosauria:::: Brachyopoidea + *. 

Characters: Skull triangular, with straight sides; lacrimal absent; $ transversely short, triangular condyles on quadrate without median sulcus [Y00]; otic notch shallow; broad cultriform process; quadrate ramus of pterygoid not "twisted"; $ body of pterygoid, palatine ramus of pterygoid, vomers, cultriform process & anterior 2/3 of parasphenoid covered with dense field of denticles [Y00]; denticles lateral to dentary tooth row; teeth absent from coronoids; tusks on anterior ectopterygoid.  

Links: Temno.htm; Lydekkerinidae/ Rhytidosteidae; Untitled Document; amphibians; TRIÁSSICO INFERIOR. 

References: Yates (2000).  ATW020820.

Brachyopoidea: 

Range: Early Triassic to middle Cretaceous

Phylogeny: Trematosauria:::: Rhytidosteidae + *. Chigutisauridae + Brachyopidae. ATW000420.

These were short-headed semi-aquatic forms, they flourished during the Triassic.  A few survived into the Jurassic and Cretaceous and grew to quite large size.  MAK010417.

Chigutisauridae: Keratobrachyops, Koolasuchus, Siderops.  The last temnospondyls.

Range: Triassic? to middle Cretaceous.

Phylogeny: Brachyopoidea: Brachyopidae + *.

Brachyopidae: Banksiops, Batrachosaurus, Batrachosuchoides, Batrachosuchus, Blinasaurus, Brachyops, Gobiops, Notobrachyops, Platycepsion, Sinobrachyops, Vanastega, Vigilius, Xenobrachyops. 

Range: Triassic? to Late Jurassic.

Phylogeny: Brachyopoidea: Chigutisauridae + *.

The Brachyopids were a group medium-sized tetrapods characterized by short, broad flat skulls with large eyes situated far forward.  The legs are relatively small; the creature would have spent most of its life in streams and lakes, although it may have been quite capable of moving about on land.  The upper margin of the mouth was armed with large fangs, indicating fish-eating habits.  The different species are distinguished mainly by details of skull shape. MAK000208.

Links: A phylogeny of the Brachyopoidea (Temnospondyli, Stereospondyli) (abstract); amphibians (range data); JVP Content; Some Amphibian and Reptilian Remains (1859) (historical importance of Brachiops); Dinosaur museum (Sinobrachiops image and Chinese text); TRIÁSSICO INFERIOR.  ATW031120. 

checked ATW050530