GEOL 431 Vertebrate Paleobiology

Spring Semester 2025

The Reptilian Stem - A Work in Progress

Synapsida is only half of Amniota. The other half - "reptiles" - contains living turtles, squamates, Sphenodon, crocodylians, and birds, and their fossil relatives. Traditionally, and for most of the age of cladistics, most of its diversity was taken up by Diapsida, characterized by creatures with classic diapsid temporal fenestration or its derivatives. But things have changed. Alas, we have no consensus on:

• The exact shape of its cladogram
• Exactly who is a member
• Proper taxonomic nomenclature for this part of the tree, especially its more basal part.

The cladogram at right is not the one used in GEOL431, but it represents an honest consensus of 21st century work. Fortunately, researchers seem to be converging on a new consensus. This year, we present simplified version of Jenkins et al., in press. - a hopeful preprint.
The last decade has seen no consensus. Consequently the 2023 strict consensus cladogram above is actually less resolved than our naive view from 2015. And yet it is not so bad. By definition, amniotes belong to one of two total-group clades:

• Synapsida
• Sauropsida (as the term has come to be used.)

What has changed since 2023:

• Parareptilia - the repository for many Paleozoic sauropods since the early days of cladistics (see may Gauthier et al., 1988) has been challenged as paraphyletic (Ford and Benson, 2019), supported as monophyletic (Simões et al., 2022) but in the "last word" for this semester by Jenkins et al., in press. finds them to be polyphyletic. (Yikes!)
• Captorhinidae, Hylomonus, and Paleothyris - long regarded as stem sauropsids - have been recovered by several research groups as (Simões et al., 2022, Jenkins et al., in press.) as stem amniotes!
• Even more disruptively, Simões et al., 2022 recover Araeoscelidia as stem amniotes! Especially problematic as these are the earliest creatures to display classic diapsid fenestration. Most cladistics definitions of Diapsida are hung off of them. More recently, Jenkins et al., 2025 restore them to Sauropsida.

Reptilia or Sauropsida?

Both terms have been used for the sister-taxon of Synapsida and neither has a "clean title" to the clade because of issues that haunt us from the early, revolutionary days of cladistics when:

• Systematists were overconfident in the permanence of the phylogenetic hypotheses they developed
• And did not feel obliged to respect rules of priority

Ford and Benson favor Reptilia, however this name has at least four phylogenetic definitions in the literature (Modesto and Anderson, 2004.) Respecting priority, GEOL431 goes with the definition of Gauthier et al., 1988 which is:

"The most recent common ancestor of extant turtles and saurians, and all its descendents."
AKA: Min ∇ (Testudines, Sauria)

Given Gauthier et al.'s convictions that:

• Turtles belonged in Parareptilia
• Saurians belonged in Diapsida

...this yielded a nice amniote dichotomy between Synapsida and Reptilia. Alas, the position of turtles in the amniote tree has become one of the most contentious issues in vertebrate phylogeny, with very disparate competing hypotheses, each implying its own roster for Reptilia or making it redundant with other common group names like "Sauria." Moreover, it is laden with cultural associations to pre-cladistic taxonomy and common usage (by which a "bird" is not a "reptile.")

Sauropsida is defined by Laurin and Reisz, 1995 as:

"The last common ancestor of mesosaurs, testudines and diapsids, and all its descendents."
AKA: Min ∇ (Mesosaurus, Testudines, Diapsida)

This anchors the definition on Mesosauridae - a basal group never appearing on the synapsid side. But alas:

• Ford and Benson place Mesosauridae, along with other members of Parareptilia, far up the tree
• Simões et al. move Araeoscelidia - classic "diapsids" outside Amniota. All of which makes Laurin and Reisz' definition useless to us as well.

So we punt!

In common usage, "Sauropsida" has come to be treated as the total-group sister-taxon of Synapsida. This is a cultural response to a need that is useful, even if it doesn't respect priority. For now and until a proper definition appears, we use this name that is at least free of cultural association with traditional definitions of "reptiles." (We address the issue of turtles in a later lecture.) Live with it.

Diapsida

(Late Carboniferous to Quaternary.) Apomorphy based - descendants of first taxon with supratemporal fenestra. An interesting consequence of the banishment of previous stem-sauropsids (Recumbirostra, Captorhinidae, Protorothyrididae includine Paleothyris and Hylonomus) to the amniote stem is the emergence of this group as the largest node-based clade within Sauropsida.

Diapsids are 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 :

• Birds
• Crocodylians
• Squamates (lizards and snakes)
• Sphenodon, the New Zealand tuatara.

Their fossil members have included the largest/scariest land animals ever, however at their root, they were adapted to life as small, fast-moving predators of small prey items.

Synapomorphies are visible in Petrolacosaurus (Right - Late Carboniferous) the earliest and among the most primitive diapsids:

• 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.)

One feature commonly associated with Diapsida but absent from our list is the supratemporal fenestra. According to Jenkins et al., in press., this feature has arisen at least three times independently in Diapsida, first in Araeoscelidia.

There are two general patterns here:

• 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.
  Arguably connected with the presence of temporal fenestrae. Abel et al., 2022 used Captorhinus to model the distribution of stress generated by biting across the elements of the skull roof. Their model identifies the location of the infratemporal and supratemporal fenestra as zones of weakness where there may be selective pressure not to deposit bone.
• Lengthening of the limbs (especially of the distal elements) suggest adaptations for faster locomotion.

Diapsid diversity: Diapsids, as generally conceived, are an apomorphy-based group - the sauropsids that ancestrally possess the diapsid pattern of temporal fenestration, They were among the first groups to be studied with the methods of cladistics (See Gauthier, 1984 and Benton 1985).

• Sauria: The crown-group of living diapsids - the last common ancestors of lizards and birds and all of its descendants will be covered later

Today we focus on the stem.

Araeoscelidia: (Late Carboniferous - Early Permian). Recognize since the beginnings of cladistics as the basal clade of Diapsida, including Late Carboniferous Petrolacosaurus - the oldest known sauropsid. Small slender animals characterized by:

• Relatively long neck and cervical series (eight cervical vertebrate).
• Long slender limbs

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

At least one member, Araeoscelis secondarily closes the infratemporal fenestra, prefiguring the widespread modifications to the diapsid pattern seen in Diapsida. Indeed, although araeoscelids possess the supratemporal fenestra that characterizes diapsids, its presence or absence going up the stem is highly variable.

Parareptilia - in memorium:

Before proceeding, let's pause to note the passing of Parareptilia - a proposed clade originally anchored on turtles (Gauthier et al., 1988) but containing many Paleozoic sauropsids. Now dispersed across the saurian stem. Will it rise from the grave in later analyses? GORK. But it's labile history certainly does not inspire confidence.

Let's look at its constituents.

Bolosauridae: (Late Carboniferous - Middle Permian) Small (20 - 40 cm.) probably ecological generalists to herbivores with complex teeth. Although bolosaurids had been known through the 20th century, it was Berman et. al, 2000 who demonstrated that they were monophyletic phylogenetically. Recent illumination has come from Jenkins et al.'s, 2024 redescription of the skull of Bolosaurus major using CT data. This revealed that although there is no margination to support a tympanum, the stapes was long and slender. Was this, perhaps, a prerequisite to the evolution of a proper impedance-matching ear?

Synapomorphies:

• Teeth with cusps and ridges
• Extremely tall coronoid process
• The transverse flange of the pterygoid and ectopterygoid form a load-bearing buttress against the medial surface of the maxilla.

Although not quite a synapomorphy, note that the supratemporal fenestra is absent. Hard, at this point, to say whether that is a plesiomorphy or a reversal. In all, bolosaurid features suggest strong bites and the ability to process tough food - presumably plant material. This makes them contenders, along with diadectids and basal synapsids, for the earliest herbivores.

Bolosauridae contains the earliest known facultative biped - Eudibamus cursorius. Eudibamus was redescribed by Berman et al., 2021 as actually being capable of moving its limbs in an upright "parasagittal" digitigrade posture, a definite first for the Early Permian.

Neoreptilia

(Early Permian - recent)

Min ∇ (Ankyramorpha, Sauria).

Synapomorphies:

• Lacrimal excluded from margin of naris
• Caniniform teeth (synapomorphy of Amniota + Protorothyrididae) are lost

Ankyramorpha

Previously the monophyletic core of "Parareptilia," this clade encompasses important land animals of the Early Permian to Triassic, some of whom were conspicuous parts of land fauna in the Permian. Our cladogram is informed by Jenkins et al., in press. for the position and overall structure of Ankyramorpha, and by Cisneros et al., 2021 for internal details.

Ankyramorphs are comprised of creatures with relatively short but broad heads whose dermal bones show various degrees of ornamentation. Synapomorphies include:

• 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
• 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
• Surangular and prearticular short - do not extend anterior to coronoid process (See Owenetta.)
• Anchor-shaped interclavicle.

Acleistorhinidae: (Early Permian) Small (20 - 40 cm.) probably ecological generalists with simple peg-like teeth and short deep skulls (right). Acleistorhinus, itself, is the oldest known ankyramorph. The only other acleistorhinid, Colobomycter is known only from a cranium that sports elongate premaxillary teeth. Synapomorphies:

• Elongate basipterygoid process

The small but distinct infratemporal fenestra low on the cheek, enclosed by jugal, squamosal, and quadratojugal is likely plesiomorphic.

Procolophonoidea: (Late Permian - Late Triassic) Small (20 - 40 cm.) probably ecological generalists to herbivores with bulbous blunt teeth. Typically stocky with short tails, Many members sport cranial ornamentation and armor of dermal scutes. Contains:

• Owenettidae: (Late Permian). (Link to Owenetta) Relatively plesiomorphic, but distinguished by the presence of an infra temporal embayment rather than a fully enclosed fenestra - a derived feature that we will see independently derived in many other sauropsids.
• Procolophonidae: (Late Permian - Late Triassic).

Synapomorphies:

• Naris circular or dorsoventrally expanded
• Maxillary depression present (cheeks?)
• Three - four premaxillary teeth
• Maxillary teeth with transversely expanded bases
• Ten - twelve maxillary teeth

In addition, procolophonoids tend to expand the orbit posteriorly into the temporal region, however in contrast to many more basal parareptiles, there is no temporal fenestration. In most, the cheeks are broadly flaring. In some, the teeth are complex and closely interlocking. (Meade et al., 2024) Were they the ecological replacements for earlier bolosaurids? Procolophonoids are the only ankyramorphs to survive through to the end of the Triassic.

Pareiasauromorpha

The "crown" of the ankyramorph tree contains three disparate groups from the Permian.

Lanthanosuchoidea: (Early - Late Permian) Small (20 - 40 cm.) with flat, heavily sculpted skulls. (E.G. Lanthanosuchus right.) Probably ecological generalists with simple peg-like teeth.

The infra temporal fenestra remains large.

Nycteroleteridae: (Middle Permian - Late Permian) Small (20 - 40 cm.) Mostly known from skulls, however Emeroleter shows a large skull on a relatively gracile bodies and limbs. These include the ankyramorphs with the most extensive embayments of the posterior cheek margin and small slender stapes - strongly indicative of impedance-matching ears. Their phylogeny is poorly resolved but Tsuji et al. 2012 find Macroleter to be the most basal. This tracks with its retention of a small infra temporal fenestra, otherwise absent in this group.

Synapomorphies:

• 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.

Pareiasauria: (Late Permian). Medium - big, squat, ugly, bumpy. These were major herbivores in the Late Permian world. They competed ecologically with therapsid dicynodonts, and were hunted by therapsid gorgonopsians. Indeed, to judge from paleo-art, provisioning gorgonopsians was their only task.

General trends:

• 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.

Thus, everything points to these creatures having been large, slow moving armored herbivores with extensive digestive systems. To judge from paleoart, their only purpose was to provision therapsid gorgonopsians.

Synapomorphies:

• 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.

Now we return to the diapsid stem:

Mesosauridae: (Early Permian) Mesosaurus, Braziliosaurus, and Stereosternum. Inhabiting the incipient basin of the South Atlantic, their fossils are from southern South America and Africa. Mesosaur distribution was among the data of Alfred Wegener supporting continental motion. The first amniotes with clear adaptations to aquatic life. Known from many well-preserved specimens, yet phylogenetically enigmatic. Mesosaurs are known from good growth series, with adults typically approaching 35 cm in snout-vent length, however larger specimens with overall length of 2.5 m are known. (Piñeiro et al., 2025)

Characteristics:

• 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.

Mesosaur issues:

• Caudal autotomy: MacDougal et al., 2020 report planes of caudal autotomy in the mesosaur Stereosternum but speculate that they represent a seldom-used plesiomorphy, as other aspects of the anatomy, including webbed limbs, indicate an aquatic life style for which an intact tail would have been important. We have seen evidence of caudal autotomy as far down as captorhinids (leBlanc et al., 2018.), so not implausible.
• 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)
  However the weight of recent opinion (E.G. Tsuji et al., 2012) favors the basal parareptile position, in which case...

Moving upward,

Orovenator mayor: (Early Permian). Known only form the anterior skull of a single specimen with diapsid fenestration - a second origin of the supratemporal fenestra (Jenkins et al., in press.). Unremarkable except that the overall skull shape - with a long snout and slender temporal bars separating fenestrae begin more closely to resemble what we see in creatures farther up the tree.

Parapleurota

New for 2025, Jenkins et al., in press. recover a significant new diapsid clade - Parapleurota (Middle Permian to Rec.)Its diversity:

• Millerettidae: Another ex-parareptile, previously regarded as a basal parareptile despite its relatively young age, invoking an awkward ghost lineage. (Ford and Benson, 2019. Now bumped up the tree into a more stratigraphically congruent position.
• Neodiapsida: Includes Sauria - the diapsid crown group, and its closest relatives, among whom the classic diapsid condition stabilizes.

Synapomorphies primarily pertain to evolution of the impedance-matching ear.

• Dorsal process of the stapes absent. This frees the stapes from its role as a structural component, repurposing it exclusively for conducting sound.
• Embayment for a tympanum formed by the squamosal, quadratojugal, and quadrate.
• Lower infratemporal bar is absent, causing infratemporal fenestra to appear as a broad open embayment

Millerettidae: (Permian)

Small (20 - 40 cm.) probably ecological generalists with simple peg-like teeth. Their synapomorphies are highly technical. But note:

• Phylogenetically basal forms like Lanthanolania have open infra temporal embayments. But this is reduced in more derived forms, yielding a reversal to an enclosed fenestra in Milleretta.
• No millerettid for whom the temporal skull roof is preserved shows a supratemporal fenestra, however Milleretta's parietal is laterally expanded in a way that suggests that it may be present in young but is closed developmentally.

Neodiapsida: (Early Permian - Quaternary) First coined in the 1980s to encompass diapsids closer to Sauria than to Araeoscelidia, its definition has been contentious. The last word belongs to Ford and Benson, 2019, who apply it to the sister taxon of Parareptilia. Jenkins et al., in press. have sunk Parareptilia, however Neodiapsida continues to occur as the sister taxon of Millerettidae, the most phylogenetically crown ward es-parareptiles. Thus, Max ∇ (Sauria ~ Millerettidae). In Neodiapsida, the diapsid morphotype and fenestration pattern stabilizes around something like the traditional concept of a "diapsid."

Synapomorphies:

• Descending flange of parietal adjacent to supratemporal fenestra, resulting in adductor muscles originating, in part, from dorsal surface of parietal.
• Postfrontal participates in margin of supratemporal fenestra.
• Quadrate is exposed laterally by reduction of squamosal and quadratojugal
• Parasphenoid teeth reduced or absent
• Femur is slender and sigmoid

"Younginiformes" (Late Permian - Early Triassic) Considered a clade during the 20th century (Gauthier et al., 1988), this "group" has emerged as a grade of basal neodiapsids in 21st century analyses, such as Hunt et al., 2023's description of CT scans of Youngina. This grade group encompasses fully terrestrial members (Youngina) and semiaquatic ones (Hovasaurus) Their skulls are relatively uniform, with longish low snouts and homodont dentition.

• In Youngina (right), the infratemporal fenestra is fully enclosed - a reversal.
• In Acerosodontosaurus, it is open ventrally (plesiomorphic)

Youngina retains plesiomorphies like parasphenoid teeth that make it one of the earliest-branching neodimapsids. It is interesting in showing evidence for communal nesting.

Some "younginiforms" form a clade - Tangasauridae whose members show evidence of semi-aquatic adaptations, best seen in Hovasaurus.

Drepanosauromorpha: (Late Triassic) Odd arborealists - convergent on certain saurians, but lacking key saurian synapomorphies like evidence for an impedance-matching ear:

• 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.
• Avicranium (Late Triassic) The best known drepanosaurid cranium, described by Pritchard and Nesbitt, 2017 reveals that for all its outward similarity to a bird's, it's skull lacked saurian features like an embayment for a tympanum.

The oversized ungual phalanges of some drepanosaurids (e.g. Drepanosaurus) has invited speculation about their function. Jenkins et al., 2020 determined that larger unguals closely resemble those of "hook-and-pull" diggers like the living tamandua and silky anteater - creatures that combine arboreality with the ability to dig into ant and termite nests. Indeed, Renesto and Saller, 2023 describe the hindlimb of two species of Megalancosaurus as biomechanically distinct, suggesting niche partitioning:

• Megalancosaurus preonensis (pictured) has a robust hindlimb and clawless opposable hallux indicative of prehensile climbing on narrow branches
• Megalancosaurus edennae's hallux is not opposable and all digits have long slender claws, suggesting that they climbed on larger supports.

Wiegeltisauridae: (Late Permian - Early Triassic) The first gliders, employing a patagium supported by dermal elements of the trunk. Also the earliest obligate amniote arborealists.

• Coelurosauravus (Late Permian)Ornate triangular head with "casque" resulting from enlargement of the infratemporal fenestra. Most recently redescribed by Buffa et al., 2021.

  A gliding membrane supported on elongate dermal "palatial" elements that are not ribs (in contrast to the extant gliding squamate Draco Volans.) Buffa et al., 2022 propose that these could be elongate versions of lateral gastralia, as they appear to be continuous with the gastralia and originate low on the trunk. They also note that, like Draco, Coelurosauravus could have grabbed the leading edge of of its patagium with its manus to stabilize it.

• Possibly related, Longisquama (Middle-Late Triassic) Triangular head and elongate plume-like display (?) structures.

Phylogeny: The more derived stem saurians display considerable convergence on other forms, leading to a diverse range of phylogeny hypotheses. Some highlights:

• Senter, 2004 found drepanosaurids and wiegeltisaurids to form a monophyletic group that he called Avicephala.
• More recently, Renesto et al. 2010 and Buffa et al. 2024 found Avicephala to be polyphyletic, with the drepanosaurids as members of sauria.
• With the benefit of Avicranium, Pritchard and Nesbitt, 2017 and Pritchard et al., 2021 confirmed the monophyly of Avicephala. The polytomy we present here is, therfore a consensus.

Claudiosaurus: The lower temporal bar is definitely incomplete in Claudiosaurus (right). In life, its place would be occupied by an unossified ligament. Various diapsids would eventually reossify that ligament in various ways, but from this point forward, we deal with diapsids who are descended from creatures whose infratemporal fenestrae have been transformed to broad embayments in the lower margin of their cheeks.

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. Claudiosaurus was fresh-water aquatic, however, not marine.

Sauria:

(Late Permian - Rec.) Min ∇ (Lepidosauria, Archosauria)

Here, for once, in a pleasing cladogram is the arrangement of Gauthier, 1984, in which two crown-groups were recognized in Sauria:

• Archosauria: Min ∇ (Crocodylia, Aves)
• Lepidosauria: Min ∇ (Squamata, Sphenodon)

Anchored on these, he defined total-groups that contained them:

• Archosauromorpha: Max ∇ (Archosauria ~ Lepidosauria)
• Lepidosauromorpha: Max ∇ (Lepidosauria ~ Archosauria)

This setup was embraced so firmly that even though there were technical difficulties with his definition of Lepidosauromorpha, (ask me sometime) most systematists continue to use the term, and so shall we.

Synapomorphies:

• Of the skull:
  • Prominent retroarticular projection of jaw: An extension to the jaw that projects behind the jaw joint.
  • The paroccipital process of the opisthotic firmly sutured to the skull roof.
• Of the appendicular skeleton:
  • The cleithrum is lost.
  • The fifth distal tarsal is lost.
  • The fifth metatarsal is hooked.

Stay tuned.