Metriorhynchidae Fitzinger, 1843 (Zoological Code)

Metriorhynchidae Fitzinger 1843: 35 (as Metriorhynchi), converted clade name (PhyloCode)

RegNum registration number 1017.

Type genus: Metriorhynchus von Meyer, 1832.

Etymology

‘The family of Metriorhynchus ’. The stem Metriorhynch - is based on the genus Metriorhynchus. - idae, is a Latin patronymic suffix, introduced into zoological nomenclature, as -idae, by Kirby (1815: 88). Under Article 29.2 of the Zoological Code, the suffix - idae denotes a family rank within the family-group.

Geological range

Middle Jurassic (early Bathonian) to Early Cretaceous (earliest Aptian) (Cau and Fanti 2011, Chiarenza et al. 2015, Sachs et al. 2020).

PhyloCode phylogenetic definition

The smallest clade within Metriorhynchoidea containing Thalattosuchus superciliosus (Blainville in Eudes-Deslongchamps 1852), Gracilineustes leedsi (Andrews 1913), Metriorhynchus brevirostris (Holl 1829), Rhacheosaurus gracilis von Meyer 1831, and Geosaurus giganteus (von Sömmerring 1816).

Reference phylogeny

Fig. 3.

Zoological Code diagnosis and PhyloCode diagnostic apomorphies

Metriorhynchoid crocodylomorphs with the following unique combination of characters (10): absence of pharyngotympanic tube foramina (some Middle Jurassic metriorhynchid specimens appear to have these foramina, it is currently unclear if the loss of these foramina represent an ontogenetic signal or possibly a phylogenetic signal within Metriorhynchidae) (65.0); in the intertemporal bar, the frontal is wider than the parietal (reversal in Metriorhynchus) (235.1); presence of a true supraorbital notch (285.1); dermic postorbital bar (309.2); dorsal part of the postorbital bar constricted, distinct from the dorsal part of the postorbital (318.1); humerus deltopectoral crest continuous with the proximal surface (753.1); humerus is flattened and has a plate-like appearance (757.1); humerus diaphysis contributes less than 40% of total humerus length (758.1–2); humerus epiphyses are unossified (760.1); radius/ulna are broadly expanded and plate-like (764.1).

Potentially diagnostic characters

The following 51 characters are also diagnostic for Metriorhynchidae but may be more widely distributed within Metriorhynchoidea once more complete specimens of non-metriorhynchid metriorhynchoids are discovered.

Characters unknown in all early diverging metriorhynchoids: lack of scales on the body (874.1).

Characters unknown in early diverging metriorhynchoids more derived than Pelagosaurus: ‘pelagic morphotype’ of the endosseous labyrinth (38.2); cochlear duct shortened (40.1); basisphenoid diverticula cavities indistinct from parent sinus cavity (62.2); no diverticular inflation within the basioccipital (67.0); absence of ‘otoccipital recesses’ (79.0); seven cervical vertebrae (687.2); mid-to-late thoracic vertebrae lack a shallow fossa on the anterior margin of the diapophysis (703.1); shift in neural spine inclination near distal tail (718.1); ventral deflection of distal caudal vertebrae (719.1–3); tail deflection is abrupt, occurring over 5–10 vertebrae (720.2); distal chevrons mediolaterally compressed, with some having a ‘W’-shape created by an anterior process (732.1); forelimbs (humerus + ulna + metacarpal III) are less than 20% of total trunk (presacral vertebrae minus the atlas-axis) length (shared with Aeolodon) (750.4); metacarpal III makes up 20% or more of combined forelimb length (humerus + ulna + metacarpal III) (751.2–3); radiale/ulnare are broadly expanded and plate-like (769.1); radiale and ulnare subequal in length (772.0); metacarpal I is broadly expanded (776.1); hindlimbs (femur + tibia + metatarsal III) are less than 40% of total trunk (presacral vertebrae minus the atlas-axis) length (807.5); forelimbs (humerus + radius) less than 45% of hindlimb (femur + tibia) length (shared with Aeolodon) (808.4); calcaneum tuber poorly developed (829.1); metatarsals II–IV shorter than their respective digits (831.2); absence of dorsal caudal osteoderms (862.1).

Characters unknown in early diverging metriorhynchoids more derived than Teleidosaurus: highly elliptical external nares (shared with Deinosuchus) (96.3); posterodorsal retraction of the anterior margin of the narial fossa, such that it is at least level to the second premaxillary alveoli (103.2–5); external nares at least partially divided by a midline premaxillary septum (105.1–2); premaxilla contributes more than 25% of total rostrum length (120.1); premaxilla is lanceolate in shape, smoothly fitting the rostrum (133.1); of the anterior processes of the surangular, the ventral process is longer than the dorsal process (549.2); premaxilla contacts the anterior margin of the first maxillary alveoli (589.1–2).

Characters unknown in early diverging metriorhynchoids more derived than Magyarosuchus and the unnamed Toarcian taxon: coronoid process considerably dorsal to the tooth-row (487.1); absence of external mandibular fenestrae (512.0); thoracic vertebrae parapophyses are oriented anteriorly (704.1); anterior thoracic vertebrae in lateral view, have the parapophysis dorsal to the diapophysis (705.1); scapula blade reduced, as wide as, or narrower than, the glenoid region (744.2); ilium anterior process is short and robust (787.1); absence of ilium posterior process (794.1); dorsal border of the ilium 21% or less of femoral length (797.1); ilium lacks acetabular depressions with the peduncles being contiguous with the ventral margin (800.1); ischium anterior processes reduced, lacking clear articulation facets (802.1–2); tibia less than 50% of femoral length (shared with machimosaurine machimosaurids and aeolodontin teleosaurids) (809.3–5); middle of the femoral diaphysis flattened (817: 1); femur distal condyles incompletely ossified (822.2); distal surface between the lateral and medial condyles is flat (823.0); absence of gastral osteoderms (836.0); absence of dorsal thoracic osteoderms (838.0).

Characters unknown in early diverging metriorhynchoids more derived than Eoneustes: two major longitudinal neurovascular canals in the rostrum (47.2); dorsal alveolar canal innervates the dental alveoli with a canal leaving the alveolus continuing to the external surface of the rostrum (49.1); nasal lateroposterior processes are well-developed (139.2); nasals reach deep into the preorbital fossa (175.2); jugal reaches deep into the preorbital fossa (reversals in Torvoneustes, Maledictosuchus, and Cricosaurus schroederi) (176.1); posterior maxillary alveoli are ventrally offset relative to other alveoli (due to the ventroposterior curvature of the posterior maxilla) (576.1).

Composition

Neustosaurus, Enaliosuchus, Thalattosuchus, and the subclades Metriorhynchinae and Geosaurinae.

Comments

Authorship: The nomen Metriorhynchidae was first used by Fitzinger (1843: 35), as Metriorhynchi.

Prior phylogenetic definition: Young and Andrade (2009) defined Metriorhynchidae as the least inclusive clade consisting of Metriorhynchusgeoffroyii vonMeyer 1832 and Geosaurusgiganteus (von Sömmerring 1816). Here we have expanded the number of internal specifiers, so that Thalattosuchus, Gracilineustes, Metriorhynchus, Rhacheosaurus, and Geosaurus define Metriorhynchidae. The new definition solves the Thalattosuchus superciliosus issue, as in Wilberg (2015b) Thalattosuchus was recovered as the sister-taxon to Metriorhynchidae sensu Young and Andrade (2009). Now, regardless of the phylogenetic position of this taxon, Thalattosuchus will always be a metriorhynchid. Moreover, the choice of five internal specifiers ensures consistent usage of the internal specifiers within Metriorhynchidae and it’s the major subclades Metriorhynchinae and Geosaurinae.

Discussion: Within Thalattosuchia, Metriorhynchidae are the most diagnostic clade (Figs 1, 22). Unfortunately, the evolutionary timing of many of the postcranial characters that define the group is unknown. This is a consequence of most of the non-metriorhynchid metriorhynchoids having little to no known postcranial skeleton. As such, the diagnostic characters metriorhynchids have in entire regions of the skeleton are still incompletely understood, ranging from their loss of osteoderms (Fig. 22), modification of the manus into flippers (Fig. 23), reduction of pelvis size, modification of the hindlimbs into paddles (Fig. 24), and the development of the hypocercal tail (Fig. 25).

The body-plan diversity within Metriorhynchidae is as remarkable as their divergence from semi-aquatic thalattosuchians (Fig. 1). There is variation in skull shape, particularly between dakosaurins and other subclades (Figs 26, 27), and in postcranial morphology, particularly in the angle and morphology of the tail fluke (Figs 22, 25). While we have learnt much about metriorhynchids over the past two hundred years, there is still much more we do not understand.

Much like the previous use of Steneosaurus as a ‘wastebasket’ taxon, the continued use of Metriorhynchus to place the majority of Middle Jurassic metriorhynchid species (and many Late Jurassic ones) has greatly held back our understanding of the clade. Based on the phylogenetic analyses presented herein (Figs 3–12), the only way to render Metriorhynchus monophyletic would be to place all metriorhynchoids more derived than Opisuchus into the genus Metriorhynchus (as Eoneustes, placed within the genus Metriorhynchus by Mercier 1933, is recovered outside of Euthalattosuchia). The only other way to use the genus Metriorhynchus as ‘traditionally’ defined would be to accept that it is a paraphyletic taxon (and accept the usage of paraphyletic taxa), one from which the other traditionally accepted metriorhynchid genera (Geosaurus and Dakosaurus) evolved. That would place Euthalattosuchia, Metriorhynchidae, Metriorhynchinae, Rhacheosaurini, Geosaurinae, and Geosaurini as subclades within Metriorhynchus. Again, like with the Steneosaurus example above, this would subsume a remarkable level of morphological variation into a paraphyletic genus, creating a genus defined primarily by what it is not.

It is worth noting that the characters Eudes-Deslongchamps (1867 –69: 132–133) used to redefine Metriorhynchus are today known to be present in all metriorhynchids or are variable within the clade (see: Young et al. 2021: 518). It, therefore, should not be a surprise that if we attempt to apply a 19th century ergotaxonomytoday, aframeworkcreatedwhenalimitednumber of metriorhynchid fossils were known, it will result in a paraphyletic taxon. Moreover, this was widely known and accepted in the early 20th century, as in the hand-drawn phylogenies of Fraas (1902) and Mercier (1933) Metriorhynchus was paraphyletic with regard to Geosaurus and Dakosaurus. What complicates matters further, is that Geosaurus was polyphyletic in both ergotaxonomic schemes as well. This resulted in a systematization that placed all Middle Jurassic metriorhynchids in the genus Metriorhynchus, all ‘robust’ Late Jurassic metriorhynchids in the genus Dakosaurus, all ‘gracile’ Late Jurassic metriorhynchids in the genus Geosaurus, and all other Late Jurassic metriorhynchids were placed in Metriorhynchus (Fraas 1902, Andrews 1913, Mercier 1933). In these ergotaxonomies the ‘ Geosaurus morphotype’ evolved independently multiple times [three times in Fraas (1902) and twice under Mercier (1933)], and most Metriorhynchus species were more closely related to one of the Geosaurus lineages or Dakosaurus than to other species assigned to the genus Metriorhynchus. Dakosaurus as defined by Fraas (1902) is also polyphyletic, as it included Plesiosuchus manselii, a Late Jurassic species more closely related to a Middle Jurassic ‘ Metriorhynchus ’ species (Suchodus durobrivensis) than to Dakosaurus maximus (see phylogenetic results herein, and Young et al. 2012a).

To be clear, we are not criticizing the ergotaxonomic systematizations of earlier generations, not only was there a smaller sample set of thalattosuchian fossils, but a different systematic paradigm was in place. But systematics is a science, and over the past 100 years new data has been accumulated from across the world. Hypotheses have to be re-evaluated in the light of new evidence, and we have to ask ourselves, what is the utility of an ergotaxonomy in which the genera included are paraphyletic and/or polyphyletic? How can we discuss the biodiversity or evolutionary trends within Metriorhynchidae but at the same time have multiple lineages called Geosaurus, or Dakosaurus? Especially when there are pre-existing generic names that could be ‘resurrected’ and assigned to these other lineages [as was done by Young and Andrade (2009) and Young et al. (2012a)]. Using Metriorhynchus as a ‘wastebasket’ for all metriorhynchids that do not fall within Geosaurus or Dakosaurus simply obscures the morphological diversity present (e.g. see: Young et al. 2010, 2013, 2021).