Palaeos Vertebrates 210.100 Eureptilia: Eureptilia

*Palæos:*		Unit 210: Eureptilia
*The* *Vertebrates*		100: Eureptilia

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Eureptilia

Captorhinids, Protorothyrids & Araeoscelidans

The Diapsids

The Eureptilia would have been a footnote in the history of turtles and mammals but for the evolution of the diapsids. The bulk of reptiles fall into the Diapsida. The Diapsida ("two arches") are so called because they have two (di-) skull openings on either side of the head, for attachment of temporal muscles (to work the jaw). However it is now known that some reptiles lost one or even both pairs of openings, so the presence or absence of this characteristic is not a completely reliable guide.

The Diapsida are divided into two important groups, the lepidosaurs (Lepidosauromorpha) and the archosaurs (Archosauromorpha). There are also a number of different lineages of early, mostly small lizard-like insectivorous forms that lived during the late Carboniferous, Permian, and Triassic periods. These have been traditionally grouped under the "Eosuchia," but it is now known that this is an artificial taxon, a "waste basket" category for any diapsid that is neither lepidosaur nor archosaur. Nevertheless the term retains some use as a synonym for "basal diapsid." MAK991026.

Descriptions

Characters: premaxilla without ventral curvature [MR05]; supratemporal small; parietal & squamosal broadly in contact, so that postorbital does not reach supratemporal; tabular not contact opisthotic; jugal anterior process sharply pointed [MR05]; jugal without medial process [MR05]; jugal with distinct anterior and posterior ventral margins [MR05]; stapes short [MR05]; parasphenoid denticulate [MR05]; lower jaw with dorsal ridge [MR05]; retroarticular process absent [MR05]; caniniforms absent [MR05].

Characters: Robust skull; premaxilla downturned, with massive ventral base [MR05]; maxilla with narrow lateral exposure [MR05]; lacrimal suture with nasal usually mucg shorter than frontal suture [MR05]; lacrimal suture with nasal as long as frontal suture [MR05]; prefrontal usually wedged into posterior section of nasal [MR05]; pineal foramen on anterior part of parietals [MR05$]; supratemporals reduced or absent; tabular absent [MR05$]; jugal anterior process broad & blunt [MR05]; jugal with medial process [MR05]; large medial process of the jugal; single (lower) temporal fenestra; braincase supported by supraoccipital process; middle ear region open at least laterally and ventrally; ectopterygoid absent [MR05$]; external surface of lower jaw usually ornamented [MR05]; flat teeth; most have multiple rows of marginal teeth; number of maxillary teeth <25 [MR05$].

Characters: premaxilla short, not overlapping lower jaw [MR05]; premaxilla without ventral curvature [MR05!]; premaxilla with 2-pronged nasal suture [MR05]; Type B septomaxilla, close to posterior border of naris [MR05]; maxilla slender & elongate [MR05]; maxilla dorsal lamina slight [MR05]; maxilla forming ventral border of naris, not extending to orbit [MR05$]; maxilla with well-developed horizontal lamina, expanded anteromedially [MR05]; nasals long, plate-like, rectangular, with slight expansion posteriorly [MR05]; lacrimal quite large, forming most of anterior & half of ventral orbital margin [MR05]; lacrimal dorsally expanded [MR05$]; dorsal & ventral lacrimal duct foramina on lacrimal in orbital margin [MR05]; lacrimal strongly interdigitating nasal suture [MR05]; lacrimal elongate contact with jugal [MR05]; prefrontal posteriorly elongate, almost reaching postfrontal & forming much of orbit dorsal margin [MR05]; prefrontal wide anterior to orbit, with sharp anterior tip wedged between nasal & lacrimal [MR05]; lacrimal suture with nasal as long as frontal suture [MR05]; prefrontal ventral lamina partially covered by lacrimal [MR05]; frontals rectangular, forming most of skull table above orbits [MR05]; frontals slightly expanded posteriorly, with serrated suture to parietals [MR05]; postfrontal triradiate, with sharp anterior point, and forming posterodorsdal orbit [MR05] [1]; postfrontal with posterior point inserted into parietal [MR05]; parietals short, much broader than frontals, & posteriorly embayed, creating 1 median and 2 lateral posterior processes [MR05!]; posterolateral process of parietal with slot for small, superficial supratemporal [MR05]; anterior of supratemporal on skull table, with ornament, but posterior occipital and unornamented [MR05]; parietal with extended occipital flange underlying paired, adjacent, semilunar postparietals on occiput [MR05]; tabulars absent [MR05]; jugal large, forming most of suborbital skull, with long preorbital process, short dorsal postorbital process, and flat temporal lamina [MR05]; jugal anterior tapers to a point [!] & inserts between maxilla & lacrimal [MR05] [2]; jugal dorsal process supports postorbital [MR05]; jugal temporal lamina extend below squamosal & probably contact quadratojugal [MR05]; jugal medial process absent [MR05]; jugal with distinct anterior and posterior ventral margins [MR05]; squamosal large, covering most of temporal region, bearing distinct occipital flange separated by vertical ridge [MR05]; quadrate broad in lateral view, with tall dorsal process [MR05]; basioccipital longer than broad, with anterolateral basal tubera [MR05]; basioccipital forming part of occipital condyle [MR05]; spraoccipital with well and "almost equally developed dorsolateral and ventrolateral processes surrounded by prominent, dorsally directed flanges. The semicircular canals are situated between the two projections." [MR05: 564] [3]; opisthotic stout and compact, with paroccipital process absent [4]; stapes short, with poorly developed shaft & large stapedial foramen [MR05!]; alar (posterior) basisphenoid projections probably contacted basisphenoid tubera [MR05]; parasphenoid completely fused to basisphenoid [MR05]; basipterygoid processes stout & directed anterolaterally [MR05]; parasphenoid cultriform process denticulate [MR05!]; vomer small [MR05]; pterygoids terminating anteriorly on midline between vomers [MR05]; interpterygoid vacuity present posteriorly [MR05]; pterygoid quadrate ramus with separate dorsal flange extending from basicranial articulation to dorsal process of quadrate, supporting elongate epipterygoid [MR05]; lower jaw largely unornamented [MR05]; lower jaw with long dorsal ridge for external jaw adductor [MR05!]; dentary quite straight, with abrupt curvature to symphysis [MR05]; long splenial present, not contributing to symphysis [MR05]; dentary makes up 75% length of jaw [MR05]; retroarticular process absent [MR05!]; articular small and lens-like [MR05]; teeth generally small, homodont, pointed & recurved distally [MR05]; 5 small, pointed, premaxillary teeth [MR05]; caniniforms absent, but 4 anterior maxillary teeth somewhat elongate, others shortening posteriorly [MR05!]; single maxillary tooth row with 18 teeth [MR05]; probably 17 dentary teeth [MR05]; 2 vomerine tooth rows, with medial row continuing pterygoid denticles [MR05]; vomerine lateral denticles autapomorphic [MR05$].

Notes: "!" indicates feature unique among captorhinids, but likely plesiomorphic for Eureptilia. [1] The postfrontal in the referred specimen has a different shape. It is probably simply broken, but the bone labelled "postorbital" in the referred specimen has the correct shape for the postfrontal, and might have been dislodged by a fragment (labelled "postfrontal"), which could, in turn, be a piece of the thoroughly splintered epipterygoid. These skulls were both recovered from split shales. The fossil side was then embeded in resin and apparantly prepared from the opposite (rock) side. This, the authors note, created some difficulties in identifying pieces deeply embedded in the resin, such as the circumorbital bones of the referred specimen. [2] One problem with getting too artistic is that it can obscure details. Our attempts to gussy up the figure from [MR05] obscure this insertion, which is quite clear in the original image. [3] Note that the semicircular canals, i.e. normally the guts of the otic capsule, are described as lying dorsal to the supraoccipital. Ummm. We have concerns about this reconstruction of the occiput and posterior braincase. After struggling with it for some hours, we've decided that we lack sufficient expertise to make any comments at all, beyond this vaguely dissatisfied grunting noise. We therefore grunt, and readers may make of that what they will. [4] The paroccipital process is said to be absent. However, observe the opisthotic illustrated on the left side of the holotype (dorsal view) image here. It shows an unprecedented ventromedially directed process. Ummm. See footnote 3 for explanation of grunting noise.

Protorothyrididae: Small, short legged, lizard-like insectivores. Carroll & others place as early amniote stem group. Hylonomus

Characters: Upper (triradiate squamosal & triradiate post-orbital) [R89] and lower temporal fenstrae; posttemporal fenestra present, bordered by narrow occipital flange of squamosal, tabular (if present), supraoccipital & paroccipital process [R89]; suborbital fenestra consisting of a relatively large hole in the palate located between palatine, ectopterygoid, and maxilla [Other taxa may have foramen in this region, but there is usually no fenestra] [R89]; ossified sternum; $ (primitively) radius long, measuring 70 - 90% of humerus length; ridge-and-groove ankle joint between tibia and astragalus.

Note: Domed skull of Amniotes permits muscles to run vertically. Vertical muscles allow strong static pressure when jaws are closed or almost closed. Increase in muscle bulk would strain periosteal covering of bone. Fenestrae may have evolved as unossified attachment points. Later, evolved as openings which "bulging" of these jaw muscles. Note no obvious change in jaw musculature in primitive forms.

Characters: Lizard-like; some euryapsid (upper temporal fenestra only), but jugal still bifurcated; long necks, with great elongation of neck in some; gracile, presumably cursorial, limbs.

Eureptilia

Captorhinids, Protorothyrids & Araeoscelidans

Abbreviated Cladogram

Contents

Taxa on this Page

The Diapsids

Descriptions