Palaeos:		Temnospondyli
The Vertebrates		Rhinesuchids  & other primitive Stereospondyls

Rhinesuchids & other primitive Stereospondyls

TETRAPODA
|--+--LEPOSPONDYLI
|  `--REPTILIOMORPHA
|
TEMNOSPONDYLI
|--Edopoidea
`--+--Dvinosauria
   `--+--Euskelia
      `--Stereospondyli 
         |--Lapillopsidae
         |==Rhinesuchidae
         |  |--Rhinesuchus
         |  |--Laccocephalus
         |  |--Uranocentradon 
         |  `--Broomistega
         `--+--Lydekkerinidae
            |--+--Plagiosauroidea
            |   `--+--Rhytidosteidae
            |       `--Brachyopoidea
             `--+--Capitosauria
                 |  `--+--Mastodonsaurus
                 |     `--Capitosauridae
                 `--Trematosauria
                    |--Trematosauroidea
                    `--+--Metoposauroidea

1. Broomistega X
2. Laccocephalus X
3. Lapillopsidae X
4. Lydekkerinidae X
5. Rhinesuchidae X
6. Rhinesuchus X
7. Stereospondyli X
8. Uranocentradon X

Evolving from archegosaur ancestors, the Stereospondyls were the last of the groups of great classic Palaeozoic amphibians, yet also among the most diverse and successful. Following the trend among dvinosaurs and advanced Euskelia, they abandoned the terrestrial and semi-aquatic lifestyle of the eryopoids in favour of a totally aquatic existence. The backbones became simplified and weakened, the legs small and vestigal, the heads huge and flattened. Some reached gigantic size, four meters or more in length, with heads alone a meter or more in length. Such animals were clearly efficient aquatic ambush preditors on fish and smaller amphibians and reptiles. One highly specialised group, the meter-long Plagiosaurs, were neotenic, retaining their larval gills in adulthood.

As with temnospondyls as a whole, the phylogeny of the Stereospondyls is highly controversial, with different cladistic analysies coming up with quite different results. There seem to be a few points of agreement though. The large Rhinesuchids and small Lapillopsids are the most primitive (basal) and constitute the general ancestral condition. At some point, these ancestral forms diverged into three or four subsequent evolutionary branches: the short-headed brachyopoids and plagiosaurs (which may or may not be related); the large, flat headed capitosaurs; and the longer headed, marine trematosaurs and their cousins the short-lived but abundant late Triassic metaposaurs which mimicked the capitosaurs and could only be distinguished by details of the skull (such as the eyes being forwad rather than placed in the middle of the head). Another grup, the rather unspecilaised Rhytidosteids may be early stereospondyls or more advanced forms on the brachyopid stem

The heyday of the Stereospondyls was during the Triassic, although a few stragglers continued through to the later Mesozoic, relicts in a world inhabited by lissamphibians and dinosaurs. MAK111115

Descriptions

Stereospondyli: 

Range: Late Permian to middle Cretaceous.

Phylogeny: Stereospondylomorpha: Archegosauroidea + *: Lapillopsidae + (Rhinesuchidae + (Capitosauria + Trematosauria))).

Characters: $ Lacrimals excluded from both orbit & nares [Y99]; $ pterygoid, palatine ramus posteriorly retracted; $ pterygoid with flat, broad internal process articulating with most of lateral edge of parasphenoid plate; $ pterygoid sutured to parasphenoid early in ontogeny; [Y99] $ pterygoid ornamentated on ventral surface; $ mandible with distinct post-glenoid region [Y99].

Links: Stereospondyli; Phylogeny and Apomorphies of Temnospondyls; The Lapillopsidae; Axial Skeleton; Untitled Document. 

References: Yates (1999) [Y99]. 

Lapillopsidae: Lapillopsis, Rotaurisaurus

Range: Early Triassic of Australia. 

Phylogeny: Stereospondyli: (Rhinesuchidae + (Capitosauria + Trematosauria))) + *.

Characters: small, semi-terrestrial; $ narrow pterygoid-parasphenoid articulation.

References: Yates (1999) [Y99] ATW030112. 

Comment: combination of primitive and advanced features make taxonomic and phylogenetic placement diofficult (Carroll 2009, p.214) MAK111115

Rhinesuchidae: 

Range: Late Permian to Early Triassic of South Africa.

Phylogeny: Stereospondyli:: (Capitosauria + Trematosauria) + *: Rhinesuchus + Laccocephalus + Uranocentradon + Broomistega.

Introduction: The Rhinesuchids are relatively massive temnospondyls known, as yet, only from the later Permian and earliest Triassic of South Africa. They were large to very large (200-400 cm) flat-headed, semi- or perhaps completely aquatic tetrapods, transitional between the eryopids and the Capitosaurs, and fulfilling the same ecological role of large semi- to fully-aquatic predator. The eyes are small and face upwards, located towards the rear of the skull. The pubis is ossified, but the wrist and ankles only seem to be partially so. We may imagine that the animal would lie motionless on a pond bottom, then lunch at fish or smaller tetrapods that would swim past or above it. Carroll (1988) lists six genera, which may include junior synonyms.  MAK

As the illustration from Müller (1968) shows, the Rhinesuchids are morphologically transitional between early Permian eryopids and Triassic Capitosaurs and mastodonsaurs. Note the sequence from the short, heavy skull of the semi-terrestrial Eryops (left) to the large flat light (notice the large vacuities or gaps in the bone of the roof of the mouth) late Triassic Cyclotosaurus.  MAK 010423, ATW020723.

Rhinesuchus from the Daptocephalus - Dicynodon Zone.  Image © Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Johannesburg.

Range: Late Permian of South Africa.

Phylogeny: Rhinesuchidae: Laccocephalus + Uranocentradon + Broomistega + *. 

Rhinesuchus whaitsi Broom 1908
Horizon: this and other species from the Tapinocephalus, Cistecephalus, and Daptocephalus zones, Lower and Middle Beaufort Beds, South Africa
Age: Capitanian to Late Wuchiapingian / Early Changhsingian (middle to late Permian)
Place: central Gondwana
Weight: 100 kg 

Comments: The mouth is armed with numerous tiny teeth on the palatine (roof of mouth), even smaller teeth on the pterygoid and parasphenoid bones of the hard palate. These animals presumably fed on fish and smaller tetrapods. Other similar species (perhaps transferred to other genera) include Rhinesuchus africanus Lydekker and R. nyasaensis Haughton (from the Karoo of Nyasaland). vonZittel (1932). Rhineceps from the Cistecephalus zone is a similar (or perhaps synonymous) genus. (MAK 010423).

Links: South African Museum - An Introduction to the Fossil Wealth of the Nuweveld Mountains; paleontology; fossils; South Africa; Fossil Picture Gallery. ATW020808.

Laccocephalus:

Range: Late Permian or Early Triassic of South Africa.

Phylogeny: Rhinesuchidae: Rhinesuchus + Uranocentradon + Broomistega + *. 

Laccocephalus Watson.
Horizon: Daptocephalus zone, Beaufort Beds, Orange Free State, South Africa
Age: ? Changhsingian
Place: central Gondwana

Comments: previously included with Uranocentradon in the family Uranocentrodontidae. Carroll lists this genus as early Triassic, but according to Anderson & Cruikshank (1978) it is late Permian. Perhaps it is from the early Changhsingian. (MAK 010423).

Uranocentradon:

Range: Late Permian or Early Triassic. 

Phylogeny: Rhinesuchidae: Rhinesuchus + Laccocephalus + Broomistega.+ *.

Uranocentradon senekalensis van Hoepen 1911
Horizon: Lower Lystrosaurus Zone, Beaufort Beds, Orange Free State, South Africa
Age: Latest Changhsingian or Induan
Place: central Gondwana
Known remains: several complete specimens
Length: skull about 50cm. Anderson & Anderson (1970). Overall length 375 cm. vonZittel (1932).

Comments: The skull is greatly flattened. The palatine (on the roof of the mouth) is equipped with a single row of large teeth. The pelvis is very like that of Eryops (indicating terrestrial ability), but the wrist and ankles are incompletely ossified (implying an aquatic existence). Presumably this animal was capable of crawling about on land when need be, but preferred to spend its time in water. vonZittel (1932). This was the last large Rhinesuchid. All the known specimens occur in a single locality and horizon, with specimens of Lystrosaurus occurring in horizons immediately above and below. Romer (1947) places Uranocentradon in its own family, the Uranocentrodontidae. Carroll and Winer (1977) place it in the Rhinesuchidae. The Fossil Record II retains the family Uranocentrodontidae.  MAK

Image: Uranocentrodon senekalensis from Late Permian of Malawi. Life Reconstruction by Dmitry Bogdanov, Wikipedia

References: Anderson & Anderson (1970); Romer (1947); vonZittel (1932).

Broomistega: 

Range: Early Triassic

Phylogeny: Rhinesuchidae: Rhinesuchus + Laccocephalus + Uranocentradon + *.

Broomistega putterilli (Broom 1930) synonym: Lydekkerina putterilli Broom 1930 
Horizon: Lystrosaurus Zone, Beaufort Beds, South Africa
Age: Induan
Place: central Gondwana
Length: small

Comments: Previously regarded as a species of Lydekkerina or a juvenile Uranocentrodon, this small aquatic tetrapod is now regarded as a paedomorphic rhinesuchid. This means that most of the Lystrosaurus Zone 'amphibians' are now paedomorphic miniature species, perhaps owing to the small size and shallowness of the ponds, lakes, and streams of this period.  MAK

Links: New rhynchosaur & temnospondyls

References: Shishkin & Rubidge (2000).

Lydekkerinidae: Chomatobatrachus, Lydekkerina

Range: Triassic of South Africa & Australia. 

Phylogeny: Capitosauria: (Mastodonsaurus + Capitosauridae) + *.

Comment: Lydekkerina's short anterior pterygoid and broader contact of pterygoid and parasphenoid are advanced features relative to the rhinesuchids, although the postcranial skeleton indicates a more terrestrial lifestyle (Carroll 2009, p.214). Highly uncertain phylogenetic possition: a basal capitosaur according to Yates & Warren 2000 and Ruta & Bolt 2008, basal rhytidosteid-brachyopoid in the Ruta et al 2003b and Ruta et al 2007 supertrees, and basal sterospondyl according to Pawley 2006, Schoch et al 2007 and Schoch 2008 MAK111115

Links: Wikipedia

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