The Reptilian Stem: Sauropsida, Eureptilia, Diapsida
- Systematists were overconfident in the permanence of the phylogenetic hypotheses they developed
- And did not feel obliged to respect rules of priority
"The most recent common ancestor of extant turtles and saurians, and all its descendents."
Given their convictions that:
- Turtles belonged in Parareptilia
- Saurians belonged in Diapsida
Sauropsida is defined by Laurin and Reisz, 1995 as:
"The last common ancestor of mesosaurs, testudines and diapsids, and all its descendents."
This anchors the definition on Mesosauridae - a basal group yet never appearing on the synapsid side. This makes it both stable and free of cultural association with traditional definitions of "reptiles."
Palatal views of pareiasaurs Deltavjatia and Scutosaurus from the U. C. Berkeley History of Life.
Sauropsid synapomorphies include:
- Suborbital opening (foramen (right) or fenestra) in palate beneath orbits. Compare with synapsids.
- Single coronoid in jaw. (Link to image.)
- Tabulars reduced or absent
- Color vision.
- Dry aglandular skin with a new kind of scale made of keratin-B (similar to the keratin-A of your hair and fingernails - NOT homologous to the scales of fish or temnospondyls.)
The basal sauropsid Mesosaurus from Wikipedia.de
- Broad, paddle-shaped limbs adapted for swimming.
- Blade of scapula shortened, as often seen in secondarily aquatic tetrapods
- Ribs pachyostotic thickened with dense bone tissue acting as ballast
- A "snout-hunter" - Elongate rostrum with long slender teeth for capturing prey with rapid sideways motion of head. (Living analogs include: gars, gavials).
- Viviparity: Piñeiro et al., 2012b report the presence of late stage embryos in the abdominal cavities of mesosaurs. Viviparity is a common adaptation to marine life in amniotes.
- For a century, the presence or absence of an infratemporal fenestra has been debated. Piñeiro et al., 2012 seems to have resolved this in favor of its being present. Whether this autapomorphic or characteristic of a larger group (possibly even Amniota) depends on the resolution of...
- The phylogenetic position of mesosaurs, which have been regarded as:
- Basal members of Parareptilia (Gauthier et al., 1988)
- Stem sauropsids (Laurin and Reisz, 1995)
- Stem amniotes (Hill, 2005)
University of California Museum of Paleontology
(Early Permian - Late Triassic or Quaternary, depending) This group has historically included reptilian-grade organisms closer to Eureptilia that did not have living members. The arrival of phylogenetic systematics allowed it to be phylogenetically defined as monophyletic, however its precise membership has been fuzzy because of uncertainties about the position of turtles.
- Lacrimal does not contact naris
- Caniniform teeth (synapomorphy of Amniota) are lost
- A median embayment on the posterior margin of the skull roof (in dorsal view.)
- Absence of a subtemporal process of the jugal.
- Absence of a supraglenoid foramen (typically between quadrate and quadratojugal in other aimals.)
Millerina rubidgei from Paleofile
- Embayment of squamosal and quadratojugal forms recess for tympanum
- Small infratemporal fenestra frequently present between postorbital, squamorsl, and jugal. (Presumed convergent with those of synapsids and diapsids.)
Acleistorhinus an ankyramorph, from U. C. Berkeley Museum of Paleontology
- dorsal process of premaxilla narrow
- anterodorsal process of maxilla high and extending to the dorsal limit of the external naris
- posterior process of postorbital nearly as wide as long
- heavily sculptured dermal bones of skull roof
- base of quadrate ramus of pterygoid deeply excavated posteriorly
- ectopterygoid contributes to outer-most border of transverse flange of palate if present (See Lanthanosuchus)
- parasphenoid rostrum shorter than body
- paroccipital process antero-posteriorly expanded
- posterior process of postorbital nearly as wide as long
- Surangular and prearticular short - do not extend anterior to coronoid process (See Owenetta.)
- Trunk neural arches swollen
- Anchor-shaped interclavicle.
Lanthanosuchus from U. C. Berkeley Museum of Paleontology
- Lanthanosuchidae: with flat, heavily sculpted skulls. (E.G. Lanthanosuchus right.)
- Acleistorhinidae: with short deep skulls (right). Acleistorhinus, itself, is the oldest known parareptile.
- Elongate basipterygoid process
- Small but distinct infratemporal fenestra low on the cheek, enclosed by jugal, squamosal, and quadratojugal. (Again presumed convergent.)
Bolosaurus from Wikimedia Commons
Hypsignathus from Stuart Sumida's BIOL622 - California State University San Bernardino
- Owenettidae: (Late Permian)
- Procolophonidae: (Late Permian - Late Triassic).
- Naris circular or dorsoventrally expanded
- Maxillary depression present (cheeks?)
- Three - four premaxillary teeth
- Maxillary teeth with trnasversely expanded bases
- Ten - twelve maxillary teeth
Bashkyroleter mesensis (a) and Macroeter poezicus (b) from Wikipedia.
- distinct emargination at the posterolateral edge of the skull concave
- smooth depression in the temporal region extending across most of the squamosal and large parts of the quadratojugal
- remaining area of the temporal region is characterized by distinctive dermal sculpturing
- a well developed, laterally protruding rim at its dorsal margin of the otic notch formed by the overhanging supratemporal and postorbital.
Scutosaurus karpinskii from Mathematical.com
- Proportions: Medium to large herbivorous reptiles. Short, deep and wide bodies, presumably with large digestive systems. Probably similar ecologically and metabolically to living giant tortoises.
- Skull: Short and wide, characterized by flaring armored cheeks, bumps and horns. E.G.: Bunostegos. There is no temporal fenestration or anything analogous to it.
- Teeth: Teeth are phyllodont, similar to those of large herbivorous iguanas.
- Armor: Pareiasaurs have extensive dermal armor. In some, the armor is interlocking.
- A ventral flange of the quadratojugal
- A posterior extension of the squamosal that covers the area occupied by the quadrate emargination in other parareptiles.
- A large boss on the supratemporal
- A large ventral process on the angular.
The Early Permian captorhinid Eocaptorhinus laticeps from Paleocritti
(Late Carboniferous - Quaternary) This group includes Hylonomus lyelli, a Joggins tree-stump victim and the earliest well-known amniote, along with all living diapsids.
- Supratemporals reduced so as not to contact postorbitals. This creates a broad contact between parietals and squamosals.
- Anterior process of quadrate is short. (Compare Eocaptorhinus to the synapsid Dimetrodon)
Diversity: Diapsida comprises the majority of Eureptilia, but we should note some stem-diapsids.
- Captorhinidae: (Late Carboniferous - Late Permian) Eureptiles characterized by adaptations to a strong, slow bite with trends toward rounded crushing teeth in multiple tooth-rows. Including Eocaptorhinus (right) but also larger forms such as Labidosaurikos (~1.5 m).
Paleothyris acadiana from Carroll, 2009
- "Protorothyridids": (Late Carboniferous - Early Permian) Eureptiles with incipient adaptations to a weaker, quick bite. These include:
- Paleothyris acadiana (right)
- Hylonomus lyelli, from Joggins, NS. Arguably the earliest known amniote, but co-occurs with the fragmentary Protoclepsydrops, which might be the earliest synapsid.
Paleothyris acadiana from Carroll, 2009
- The snout is longer than the temporal region
- Cervical vertebrae are keeled ventrally.
- Metatarsals overlap proximally.
- The stapes is robust.
- There is no suggestion of an otic notch.
Diapsida: (Late Carboniferous to Quaternary.) Diapsids re among the first amniotes of the Late Carboniferous, however during the Paleozoic they were a minor component of the terrestrial fauna. That changed during the Mesozoic, when they achieved ecological dominance. Modern diapsids include :
Petrolacosaurus kansensis from Reisz, 1981
- Infratemporal and supratemporal fenestrae.
- Suborbital opening in the palate appears as a broad suborbital fenestra.
- Limbs long and slender, emphasizing zeugopodium and autopodium. Typically, radius is at least 70% length of humerus. (Compare Claudiosaurus with non-diapsid Captorhinus.)
- Complex joint between tibia and astragalus that creates a relatively solid immobile articulation between the two. (In extreme cases, as with birds, these elements fuse into a tibiotarsus.)
- Metatarsal IV at least twice the length of metatarsal I. (Link to pes of Petrolacosaurus.)
Petrolacosaurus kansensis from Reisz, 1981
- The jaws are long and slender, and the leverage of their adductor muscles limited by the shortening of the temporal region, resulting in a quick but weak bite, suitable for hunting the insects that were diversifying during the Carboniferous.
- Lengthening of the limbs (especially of the distal elements) suggest adaptations for faster locomotion.
- Sauria: The crown-group of living diapsids - the last common ancestors of lizards and birds and all of its descendants.
- Araeoscelidia: The monophyletic group at the base of Diapsida
Araeoscelidia: (Late Carboniferous - Early Permian). Small slender animals characterized by:
- Relatively long neck and cervical series (eight cervical vertebrate).
Araeoscelidians are specialized either as arboreal or aquatic animals. Remarkable more for their plesiomorphies, including retention of:
- caniniform teeth
- a lacrimal that extends, unrestricted, to the naris
- the postsplenial of the jaw
- the posterior coracoid
- the cleithrum
Orovenator mayorum (Early Permian) from Reisz et al, 2011
- Caniniform teeth not lost or undifferentiated
- Lacrimal tapers anteriorly (the beginning of a trend)
- Infratemporal fenestra approaches cheek margin ventrally.
- Posterior coracoid lost.
We hang all of this on the front 2/3 of the skull roof of a single tiny critter, but that is typical for the study of Paleozoic diapsids, where we must dine on scraps.
Lanthanolania ivakhnenkoi (Middle Permian) from Modesto and Reisz, 2003
- Excluding the lacrimal from the naris altogether.
- Reducing the posterior process of the jugal, opening the question of whether the lower temporal bar might be breached by the infratemporal fenestra.
Youngina capensis (Late Permian) from Wikipedia
Note: It's not clear that all members of this grade had complete lower temporal cheek bars.
Potential synapomorphy of Younginiformes:
- The postorbital posterior process extends past the end of the supratemporal fenestra.
Potential synapomorphies of "younginiformes" and Sauria:
- The postfrontal forms part of the anterior margin of the supratemporal fenestra.
- The femur is sigmoid in shape. (Link to comparison.)
Claudiosaurus germaini (Late Permian) from Paleofile
Claudiosaurus was a limb-propelled swimmer with a relatively long neck. When first described it was called a "plesiosaur ancestor." That's probably wrong, but it demonstrates that even in the Permian, diapsids displayed a tendency opportunistically to evolve aquatic forms. They were not, however, marine.
Megalancosaurus preonensis (Late Triassic) from Vertebrate Paleontology at Insubria University
Drepanosauridae: the climbers:
- Megalancosaurus (Late Triassic - right) Link to reconstruction. With:
- prehensile tail
- opposable digits on manus and pes
- long neck
- barrel chested with ribs fused to vertebrae
- thoracic vertebrae fused into notarium
- pointed snout
- Drepanosaurus (Late Triassic) Link to description. Like a large, headless Megalancosaurus, but with strangely developed forelimbs.
Monophyletic? Senter, 2004 found these critters and others like them to form a monophyletic group that he called Avicephala. Synapomorphies include:
- Jaw articulation translated anteriorly to beneath the orbits
- Skull broad posteriorly and pointed anteriorly.
Sauria:(Late Permian - Rec.) The most recent common ancestor of living lizards, Sphenodon, crocodylians, and birds.
Here, for once, in a pleasing cladogram is the arrangement of Gauthier, 1984, in which two crown-groups were recognized in Sauria:
- Archosauria: The most recent common ancestor of crocodylians and birds and all descendants.
- Lepidosauria: The most recent common ancestor of squamates and Sphenodon and all descendants.
- Archosauromorpha: All organisms more closely related to archosaurs than to lepidosaurs.
- Lepidosauromorpha: All organisms more closely related to lepidosaurs than to archosaurs.
- Of the skull:
- Prominent retroarticular projection of jaw: An extension to the jaw that projects behind the jaw joint.
- Quadrate is embayed posteriorly:
- The stapes is slender: These three features together strongly suggest the convergent evolution of an impedance-matching ear.
- The adductor muscles of the jaw originate on the dorsal surface of the skull table.
- The paroccipital process of the opisthotic firmly sutured to the skull roof.
- Of the appendicular skeleton:
- Michael Benton, 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84, 97-164.
- David Berman, Robert Reisz, Diane Scott, Amy Henrici, Stuart Sumida, and Thomas Martens, 2000. Early Permian bipedal reptile. Science. 290 (5493). 969-972.
- Robert Carroll, 2009. The Rise of Amphibians: 365 Million Years of Evolution. Johns Hopkins University Press, Baltimore. 360 pp.
- Susan Evans, 1980. The skull of a new eosuchian reptile from the Lower Jurassic of South Wales. Zoological Journal of the Linnean Society 70: 203Ð264.
- Susan Evans, 1991. A new lizard-like reptile (Diapsida: Lepidosauromorpha) from the Middle Jurassic of England. Zoological Journal of the Linnean Society 103(4) 391Ð412.
- Jacques Gauthier, 1984. A cladistic analysis of the higher systematic categories of the Diapsida. [PhD dissertation]. Available from University Microfilms International, Ann Arbor, #85-12825, vii + 564 pp.
- Jacques Gauthier, Arnold Kluge, and Timothy Rowe, 1988. The early evolution of the Amniota. In: M. Benton Ed. The phylogeny and classification of tetrapods, Vol I: Amphibians, reptiles, birds. Clarendon Press, Oxford. Pp. 103-155.
- Robert Hill, 2005. Integration of Morphological Data Sets for Phylogenetic Analysis of Amniota: The Importance of Integumentary Characters and Increased Taxonomic Sampling. Systematic Biology. 54(4): 530-547.
- Michel Laurin and Robert Reisz, 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society. 113:165-223.
- Sean Modesto and Robert Reisz, 2003. An enigmatic new diapsid reptile from the Upper Permian of Eastern Europe. Journal of Vertebrate Paleontology 22(4):851-855.
- Sean Modesto and Jason Anderson, 2004. The Phylogenetic Definition of Reptilia. Systematic Biology, 53(5):815-821.
- Graciela Piñeiro, Jorge Ferigolo, Alejandro Ramos, and Michel Laurin, 2012. Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed. Comptes Rendus Palevol, 11(5), Pp 379-391
- Graciela Piñeiro, Jorge Ferigolo, Melitta Meneghel, and Michel Laurin, 2012. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology, iFirst 2012, Pp 1-11.
- Robert Reisz, 1981. A diapsid reptile from the Pennsylvanian of Kansas. Special Publication of the Museum of Natural History, University of Kansas 7: 1-74.
- Robert Reisz, Sean Modesto, and Diane Scott. 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731Ð3737.
- Silvio Renesto, Justin Spielmann, Spencer Lucas, and Giorgio Spagnoli. 2010. The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepanosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha). New Mexico Museum of Natural History and Science Bulletin. 46:1Ð81.
- Phil Senter. 2004. Phylogeny of Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology 2 (3): 257-268.
- Linda Tsuji, Johannes Müller, and Robert Reisz, 2012. Anatomy of Emeroleter levis and the phylogeny of the nycteroleter parareptiles. Journal of Vertebrate Paleontology 32(1): 45-67.