The Marine (Aquatic) Younginiformes

Whenever one thinks of marine reptiles,
the giant mosasaurs, ichthyosaurs and plesiosaurs immediately come to mind. Dig a little deeper and the placodonts, mesosaurs and thalattosaurs pop up. Basal to all these taxa are the pachypleurosaurs. Basal to the pachypleurosaurs are the marine younginiformes (Fig. 1), beginning with Galesphyrus. The odd saurosphargids are newcomers to this list (Fig. 2) nesting between basal younginiforms and pachypleurosaurs.

Yesterday we looked at the outgroup taxon to the younginiformes, Spinoaequalis. Today we’ll discuss the basal marine younginiformes beginning with Galesphyrus (Fig. 1).

Figure 3. Spinoaequalis and descendant marine younginiformes.

Figure 3. Spinoaequalis and descendant marine younginiformes. These give rise to plesiosaurs, placodonts, mesosaurs, ichthyosaurs and thalattosuchians. Click to enlarge. Note hoe hew taxa adequately preserve the skull. The flattening of the pectoral girdle is notable here.

1. Galesphyrus (Carroll 1976; Late Permian ~260 mya) This headless articulated partial skeleton had no obvious aquatic adaptations, other than, perhaps, those big, broad feet (and also note that wider tarsus and more widely separated tibia and fibula). The hourglass-shaped proximal carpal is a trait shared with basal diapsids, like AraeoscelisGalesphyrus had more robust limbs and relatively larger feet than Spinoaequalis. Most of the tail is unknown.

2. Youngina capensis? BPI 3859 (Broom 1922; Late Permian ~260 mya). The genus Youngina was once considered basal to both lepidosaurs and archosaurs. The BPI 3859 specimen does not nest with the holotype. So the BPI 3859 specimen is not a Youngina. The BPI 3859 specimen has a taller scapula than Galesphyrus. Few other traits are preserved in common.

3. Acerosodontosaurus piveteaui (Currie 1980; Bickelmann, Müller and Reisz 2009; Late Permian ~260 mya). Acerosodontosaurus descended from a sister to Galesphyrus was a sister to the BPI 3859 specimen attributed to Youngina (see below) and phylogenetically preceded Hovasaurus, Claudiosaurus and Thadeosaurus and Tangasaurus.

4. Thadeosaurus colcanapi (Carroll 1981; Late Permian, ~260 mya), nests between Acerosodontosaurus and ClaudiosaurusTangasaurus and Hovasaurus are sister taxa. The scapula and coracoid were fused only in adults. Many specimens are known including several juveniles. None preserve the skull very well. A juvenile skull gives provides the most data.

5. Hovasaurus boulei (Piveteau 1926, Currie 1981; Late Permian to Early Triassic ~250mya ) was originally considered a tangasaurid. Here Hovasaurus nests as a sister to Tangasaurus and Thadeosaurus. Distinct from Thadeosaurus, the cervicals were more robust. The torso was shorter and deeper with longer dorsal ribs. The presacral number was 26. Accessory articulations were present on the vertebra. The tail had higher neural spines and deeper chevrons. The chevrons were broader distally. The scapulocoracoid was larger and the scapula was part of the chest shield. The metarsals were shorter. Pedal digit 5 was longer relative to digit 4.This genus is known from several specimens varying in size. Gravel was found in the belly of several specimens, likely used for ballast or digestion.

6. Tangasaurus mennelli (Haughton 1924; Late Permian) was known from only two specimens collected in 1922. Later, Currie (1982) reported over 300 partial specimens were attributed to Tangasaurus, but reattributed most of them elsewhere. Haughton (1924) described a long, powerful, flattened tail and presumed an aquatic existence. The great size of the transverse processes at the base of the tail are notable. So is their anterior curvature. These reflect the size of the caudofemoralis muscles driving the large hind limbs. Note the large coracoid, central sternum, short ribs, massive humerus (especialy distally) and high caudal spines creating a sculling tail ideal for swimming.

7. Claudiosaurus germaini (Carroll 1981; Late Permian ~260 mya) was originally described as a close relative of Thadeosaurus, and indeed it is. Claudiosaurus also nests with Adelosaurus and Atopodentatus. The skulls of predecesor taxa, like Hovasaurus, are poorly known, so distinct from Spinoaequalis, the reduced skull of Claudiosaurus had a premaxilla enlarged to a third or more of the rostral length. The premaxilla ascending process split the nasals. The naris is elongated horizontally  perhaps just contacting the lacrimal. The jugal was gracile. The supratemporal was a small oval.  A quadratojugal was present contacting both the jugal and the squamosal + quadrate, but other workers have not recognized that loose bone as the quadratojugal. A retroarticular process was present. The number of cervicals increased to at least nine and they decreased in size cranially. The posterior cervicals were as tall as the little skull. The cervical neural spines were taller than each centrum. Intercentra were absent. The pre sacral number of vertebrae dropped to 24.  Metacarpals 3 and 4 were subequal.

8. Adelosaurus huxleyi (Hancock and Howse 1870, Evans 1988) was originally considered to be a small and distinct species of Protorosaurus. Here, derived from a sister to Claudiosaurus, Adelosaurus was basal to, Atopodentatus and the rest of the marine younginiformes and enaliosaurs. Smaller than Claudiosaurus, Adelosaurus had more robust ribs. The humerus did not have an expanded distal end. The hind limb was more gracile. The proximal metatarsals were all subequal in width, except perhaps, metatarsal 5. Adelosaurus was one of the most terrestrial of the known enaliosaurs, showing few aquatic characters, but the disc-like shape of the scapulocoracoid is a trait that was retained. Evans (1988) considered the incomplete ossification of joint surfaces as evidence for immaturity or an aquatic lifestyle. Most taxa around this node have been considered immature for the same reasons.

Figure 3. Basal marine younginiformes, including Galesphyrus, Tangasaurus, Claudiosaurus and others. This is a subset of the large reptile tree.

Figure 3. Basal marine younginiformes, including Galesphyrus, Tangasaurus, Claudiosaurus and others. This is a subset of the large reptile tree. Spinoaequalis is also basal to the terrestrial younginiformes. 

In future posts
we’ll look at the terrestrial younginiformes that ultimately gave rise to the Archosauriformes. New data has clarified relationships at those nodes (Fig. 2).

References
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.
Broom, R. 1922. An imperfect skeleton of Youngina capensis, Broom, in the collecton of the Transvaal Museum. Annals of the Transvaal Museum 8:273–277.
Carroll RL 1976. Galesphyrus capensis, a younginid eosuchian from South Africa. Annals of the South African Museum 72(4):59-68.
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383.
Currie PJ 1980. A new younginid (Reptilia: Eosuchia) from the Upper Permian of Madagascar. Canadian Journal of Earth Sciences 17(4):500-51.
Currie PJ 1981. Hovasaurus bolei, an aquatic eosuchian from the Upper Permian of Madagascar. Palaeontologica Africana, 24: 99-163.
Evans 1988. The Upper Permian reptile Adelosaurus from Durham. Palaeontology 31(4): 957-964. online pdf
Gardner NM, Holliday CM and O’Keefe FR 2010. The braincase of Youngina capensis (Reptilia, Diapsida): New insights from high-resolution CT scanning of the holotype. Paleonotologica Electronica 13(3).
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Hancock A and Howse R 1870. On Protorosaurus speneri von Meyer, and a new species, Protorosaurus huxleyi, from the Marl Slate of Middridge, Durham. Quarterly Journal of the geological Society of London 26, 565-572.
Olsen EC 1936. Notes on the skull of Youngina capensis Broom. Journal of Geology, 44 (4): 523-533.
Piveteau, J. 1926. Paleontologie de Madagascar XIII. Amphibiens et reptiles permiens. Annls  Paleont. 15: 53-180.

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