Anatomical notes and discussion of the first described aetosaur Stagonolepis robertsoni (Archosauria: Suchia) from the Upper Triassic of Europe, and the use of plesiomorphies in aetosaur biochronology

Skull

Block NHMUK PV R 4787a is a cast of the lower portion of a skull in semi-articulation including much of the lower jaw, quadrate, portions of the palate and maxilla, and the premaxilla (Fig. 1A). These elements represent the left side of the skull, so the cast provides an internal (medial) view. The semi-articulated condition allows for the determination of the skull length, which from the retroarticular process to the tip of the premaxilla is about 240 mm. The dentigerous elements show the presence of at least eight dentary, four maxillary, and four premaxillary teeth. Alick Walker’s PVC cast of this specimen shows even more details (Fig. 1B) including the upper portions of the skull and the braincase, especially the parabasisphenoid. Thus block NHMUK PV R 4787 is the natural mold of a nearly complete skull of S. robertsoni (Walker, 1961). Details of this specimen that are not visible in the cast NHMUK PV R 4787a include: (1) a possible 5th premaxillary tooth crown, or alternatively the tip of the right premaxilla in articulation with the left (Fig. 1B); (2) a portion of the right squamosal and impressions of the skull roof, (3) the left quadrate in articulation with the left articular, and (4) impressions of the braincase, especially the basisphenoid including the left and right basitubera and basipterygoid processes, and the cultriform process is also preserved.

A cast of the right maxilla from NHMUK PV R 4787 (Fig. 2) shows that overall the element is more slender than that of Stagonolepis olenkae (Sulej, 2010) with at least six alveoli (Fig. 2B), but the anterior portion is covered. A very distinct transverse ridge is present on the lateral surface along the anterior and ventral borders of the antorbital fossa, making the fossa extremely pronounced; this ridge also occurs in Stenomyti huangae (Small & Martz, 2013) and Paratypothorax andressorum (Schoch & Desojo, 2016), but is not present in Stagonolepis olenkae (Sulej, 2010), Desmatosuchus (Case, 1922; Small, 2002), Longosuchus (Parrish, 1994), or Neoaetosauroides (Desojo & Báez, 2007). The medial side of the maxillary body is marked by an elongate medial shelf that articulates with the palatal bones (Walker, 1961). The articulations with the lacrimal and jugal are each complex, with regions of overlap between the two bones; this is also visible in Longosuchus meadei (TMM 31185-98), Stagonolepis olenkae (ZPAL AbIII/2454/3), and Aetosauroides scagliai (USFM 11050), and probably present in other aetosaurs as well but obscured in most articulated skulls. Finally, a pneumatic accessory cavity is present on the medial size of the ascending process as described for Desmatosuchus smalli (Small, 2002).

NHMUK PV R 8586 is a cast of ELGNM 38R (Fig. 3), which was figured by Huxley (1877) and features the left side of the internal portion of the snout, including the premaxilla, maxilla, and nasals. The premaxilla measures 63 mm in length and bears four teeth (Fig. 3) with an edentulous anterior portion typical for aetosaurians; four or more teeth also occur in Aetosaurus ferratus (Schoch, 2007) Stagonolepis olenkae (Sulej, 2010), Neoaetosauroides engaeus (Desojo & Báez, 2007), and Paratypothorax andressorum (Schoch & Desojo, 2016), whereas Stenomyti huangae only possesses three (Small & Martz, 2013), and Desmatosuchus lacks any (Small, 2002). The maxilla bears six teeth as preserved, but it is missing the posterior portion. The external naris is 22 mm at its deepest point and 72 mm in length, but missing the posteriormost section. The nasal has a pronounced ridge on the medial edge, which at a position just dorsal to the 3rd premaxillary tooth migrates ventrally to the ventral margin of the nasal where it contacts the premaxilla. The anterior tip of the premaxilla bears a prominent ridge that divides the element into a flat surface that slopes into the external naris, and a second triangular area that slopes anteroventrally (Fig. 3). This ridge is the anterior expansion and also occurs in other aetosaurians such as Desmatosuchus smalli (Small, 2002), Neoaetosauroides engaeus (PVL 4363), and differs from aetosaurians such as Aetosaurus ferratus (Schoch, 2007), Aetosauroides scagliai (PVL 2059), and Typothorax coccinarum (YPM 58121). The premaxilla bears a small dorsal protuberance above the first tooth position that extends dorsally into the external naris (Walker, 1961), which also occurs in Stagonolepis olenkae (Sulej, 2010) and Desmatosuchus smalli (Small, 2002); however, in S. olenkae it is dorsal to the second tooth position (Antczak, 2016) and D. smalli has an edentulous premaxilla (Small, 2002). There is only a slight swelling in this position in Stenomyti huangae (Small & Martz, 2013).

The best-preserved skull material is MCZD 2, which consists of seven small blocks that fit together to present much of the skull and the anterior section of the neck (Walker, 1961: figs. 26–29; Gower & Walker, 2002: fig. 1). The material consists of well-preserved bone and is not a natural mold as is most of the S. robertsoni material. This specimen has previously been described in great detail so this will not be duplicated here (Walker, 1961; Gower & Walker, 2002). However, notable is that the basitubera and basipterygoid processes are widely separated anteroposteriorly from each other so that the parabasisphenoid is elongate, supporting what can be observed in NHMUK PV R 4787. Neoaetosauroides engaeus (PVL 5698; Desojo & Báez, 2007), Aetosaurus ferratus (Schoch, 2007), Paratypothorax andressorum (Schoch & Desojo, 2016), and the basal taxon Aetosauroides scagliai (PVSJ 326) also have an elongate parabasisphenoid, so that is the plesiomorphic state within Aetosauria. This differs significantly from the condition in S. olenkae, where the basitubera and basipterygoid processes nearly contact (Sulej, 2010:figs. 1D, 1F) and thus the parabasisphenoid is anteroposteriorly short. In Scutarx deltatylus (Parker, 2016b:fig. 7) and Desmatosuchus spurensis (UMMP 7476) the parabasisphenoid is also anteroposteriorly short; whereas the condition in Calyptosuchus wellesi is unknown (Parker, 2018).

Presacral vertebrae and ribs

In block NHMUK PV R 4784a, which is a cast created from a natural mold and represents the postcranial skeleton of the skull NHMUK PV R 4787 (Walker, 1961), the occipital condyle and left paroccipital process of the skull are present (Fig. 4A). If the cervical count is nine (as in Desmatosuchus spurensis), the four anterior cervicals are missing (including the axis and atlas); however, cervical ribs are present for three of these positions. Especially striking are two very elongate posteriorly projecting cervical ribs underlying the ventral surfaces of the two more posteriorly positioned ribs (Fig. 4A). The elongate ribs (left and right sides) originate where the axis/atlas would be located and are very similar to the greatly elongate cervical ribs found in Alligator (Reese, 1915). As exposed these ribs measure 85 mm in length (the ends are covered), more than three times the lengths of the other exposed cervical ribs. The elongate ribs were not noted by Walker (1961) and have not previously been described for any aetosaur. The axis/atlas and third cervical are present in MCZD 2, but unfortunately are very poorly preserved; however, this block also preserves a very elongate first cervical rib.

The first well-preserved cervical vertebra in NHMUK PV R 4784a is the 8th (Walker, 1961), which is visible in right lateral view and preserves details of the centrum and much of the neural arch and transverse process (Fig. 4B). A disarticulated cervical rib is present across the centrum and probably does not belong to this vertebra. The centrum measures 25 mm in length and the ventral surface is strongly keeled. Keels occur on the ventral surfaces of the cervical vertebrae in Aetosauroides (Desojo & Ezcurra, 2011), Neoaetosauroides (Desojo & Báez, 2007), and Scutarx (Parker, 2016a; Parker, 2016b) while the ventral surfaces of the cervicals lack keels in Longosuchus (Long & Murry, 1995) and Desmatosuchus (Case, 1922; Parker, 2018), and Aetobarbakinoides (Desojo, Ezcurra & Kischlat, 2012). The condition may be variable in Typothorax; ventral keels are absent in the cervicals of the small specimen described by Martz (2002) but present in at least some larger specimens (Heckert et al., 2010), suggesting there may be allometric or ontogenetic variability. The parapophysis in the 8th cervical of NHMUK PV R 4784a is low on the anterior rim of the centrum, but not completely at the base. The transverse process projects laterally and slightly ventrally, is 25 mm long, and bears a flaring sub-rectangular head in lateral view. A distinct posterior centrodiapophyseal lamina (Wilson, 1999) stretches from the base of the transverse process to the posterior portion of the neurocentral suture. This is the “T-beam” structure described by Case (1922) for the posterior cervical and dorsal vertebrae of Desmatosuchus spurensis and described as present in S. robertsoni (Walker, 1961) and Typothorax (Martz, 2002) and also occurs in Paratypothorax (Martz et al., 2013 Fig. 9D). The right prezygapophysis is present but not enough is present to tell if a hyposphene was present. The neural spine is present, but missing the apex.

The next three vertebrae are preserved in articulation (Fig. 4B). The parapophysis on the 9th cervical is still situated on the centrum, but slightly higher than its position in the preceding vertebra. The centrum is more elongate as well, measuring 31 mm. Pre- and postzygadiapophyseal laminae (Wilson, 1999) are visible on the 9th and 10th presacrals (Fig. 4B). The 10th presacral is transitional between the cervical and dorsal series as previously described for Desmatosuchus spurensis (Case, 1922; Parker, 2008a; Parker, 2008b). The centrum is more elongate than the previous vertebra with a length of 36 mm. However, there is still a slight ventral keel as in the other cervicals, and although the parapophysis has migrated upwards onto the base of the neural arch, it is not located on the arch itself as in the trunk vertebrae. The neurocentral suture appears to be open.

The 11th presacral is the first true trunk vertebra as the parapophysis would now be situated on the posteroventral surface of the transverse process. Unfortunately this cannot be seen clearly as it is broken away (Walker, 1961: fig. 7i). The centrum has a length of 39 mm and is unkeeled. The 12th presacral is present, but covered by broken ribs. The cervical and trunk vertebrae of S. robertsoni lack oval depressions on the lateral sides just below the neurocentral sutures as in Aetosauroides scagliai (Desojo & Ezcurra, 2011).

Caudal vertebrae

NHMUK PV R 4799b is a PVC cast of an isolated anterior caudal vertebra providing more details of the neural arch and spine (Walker, 1961: figs. 10c-e). The broad transverse processes (left equals 79 mm) do not extend ventral to the base of the centrum; the postzygapophyses are oriented at 45 degrees above horizontal. The centrum is blocky, with equant width and height of about 40 mm, but the entire vertebral height is 112 mm, with the neural spine contributing 40 mm to this measurement. Spinopostzygapophyseal laminae (Wilson, 1999) are present, as is the expanded neural spine table.

Osteoderms

Numerous osteoderms of Stagonolepis robertsoni are preserved as natural molds; however, often they only produce partial casts. Walker (1961) discussed the osteoderms in what is now considered superficial terms, therefore one each of the best preserved dorsal paramedian and lateral osteoderms is redescribed determining estimated position using the technique presented by Parker & Martz (2010). The dorsal paramedian osteoderm (NHMUK PV R 4790a; Fig. 5A) is from the left side, has a width/length ratio of 2.4/1, and an anastomosing surface patterning (Taborda, Heckert & Desojo, 2015), an anastomosing, interlaced network of high ridges surrounding circular and elongate pits closer to the posterior plate margin, and elongate, but irregular grooves on the anterior portion of the osteoderm. The width/length ratio, the slight ventral flexion, and the more medially situated position of the dorsal eminence suggests that this osteoderm is from the anterior caudal region based on comparison with the holotype specimen of Calyptosuchus wellesi (Case, 1932) and a referred specimen of Aetosauroides scagliai (MCP 13a-b-PV). The ornamentation is similar to the pattern in Aetosaurus ferratus, Aetosauroides scagliai, Neoaetosauroides engaeus, Stagonolepis olenkae, and Paratypothorax andressorum in that the ornamentation is non-radial posteromedially, but elongate and radial anteriorly along the full length of the anterior bar. However, in S. robertsoni, the ornamentation on the posterolateral region of the paramedians is distinctly non-radial and faint to absent along the entire posterior margin, whereas in the other taxa the paramedians possess very elongate grooves in the posterolateral portion of the paramedian osteoderm that are nearly parallel to the posterior margin.

This anastomosing ornamentation in S. robertsoni radiates from an elongate, but narrow, raised dorsal eminence that contacts the posterior osteoderm margin as in most non-desmatosuchine taxa that possess an eminence; in desmatosuchines, the eminence often contacts the posterior margin (Parker, 2018; Parker & Martz, 2010). This eminence is rounded rather than distinctly pyramidal as in Typothorax (Martz, 2002) and Longosuchus (Parker & Martz, 2010), and offset medially from the center of the osteoderm as in nearly all aetosaurian posterior trunk and anterior caudal paramedians. The anterior portion of the osteoderm bears a raised, transverse, smooth strip of bone called the anterior bar (Long & Ballew, 1985) that bears anterolateral and anteromedial projections as in most non-desmatosuchine aetosaurs. The anterior bar maintains an even width across the lateral portion of the osteoderm, but thins significantly medially before expanding again at the anteromedial projection. This distinct medial thinning is termed ‘scalloping’ (following Parker, 2016b). This feature occurs in several other aetosaurians including Aetosauroides scagliai (PVL 2073), Calyptosuchus wellesi (UCMP 126844; Parker, 2018), Scutarx deltatylus (Parker, 2016b), and Paratypothorax andressorum (SMNS 5721). The medial edge is straight and the lateral edge slightly sinuous in dorsal view. In posterior view the osteoderm is moderately flexed.

The lateral trunk osteoderm (NHMUK PV R 4789a; Fig. 5B) is from the right side based on the presence of a distinct beveling of the anteromedial corner of the anterior bar, which represents an articulation surface for the anterolateral process of the adjacent paramedian osteoderm. The bar is thin but stretches across the entire anterior margin of the osteoderm. The osteoderm is trapezoidal in dorsolateral view and a ridge-like dorsal eminence that contacts the posterior margin divides the osteoderm into distinct dorsal and lateral flanges. The dorsal flange is roughly trapezoidal in dorsal view, whereas the lateral flange is slightly larger and sub-rectangular in dorsolateral view. The surface ornamentation is anastomosing and very faint in the posterior portion of the osteoderm, as seen in the paramedian. The osteoderm is slightly flexed ventrally and the angle between the two flanges is obtuse. In dorsolateral view the lateral margin is gently rounded. The medial margin is angled posteromedially, corresponding with the shape of the adjacent paramedian osteoderm.

The ventral osteoderms (NHMUK PV R 27404 [negative cast of holotype ELGNM 27R]; NHMUK PV R 4789a; Figs. 6A, 6B) are rectangular, bear an anterior bar, and possess an ornamentation of randomly arranged oblong pits, the ‘drops’ which Agassiz (1844) used to formulate the genus name. These differ from what is seen in Calyptosuchus wellesi (UCMP 27225; Parker, 2018), Aetosauroides scagliai (MCP13-a-b-PV; Desojo & Ezcurra, 2011), and Typothorax coccinarum (Martz, 2002; Heckert et al., 2010). These taxa also have rectangular ventral osteoderms, but the ornamentation arrangement is more radial and consists of very elongate furrows and ridges, even in Typothorax, where paramedian osteoderm ornamentation is distinctly pitted. Coahomasuchus kahleorum (Heckert & Lucas, 1999) and Aetosaurus ferratus (Schoch, 2007) preserve more equant, overlapping ventral osteoderms; but the surface ornament on these specimens is poorly preserved and difficult to comprehend. Where they are preserved, however, they appear to consist of finer pits, less densely packed, but nonetheless have a strong radial distribution as in Coahomasuchus chathamensis (Heckert, Fraser & Schneider, 2017) and the previous taxa. The ventral osteoderms of S. robertsoni also differ from Stenomyti huangae where the osteoderms are subrounded rather than square and broadly separated rather than overlapping (Small & Martz, 2013).

Ilium

Described in detail by Walker (1961), the ilium of Stagonolepis robertsoni is autapomorphic. The shapes and sizes of the processes of the iliac blade are particularly diagnostic for aetosaurian taxa. S. robertsoni possesses an elongate postacetabular blade that ends in an acute angled end in medial view (Fig. 7). Most other aetosaurians have a squared-off end of the postacetabular blade such as Aetosauroides scagliai (PVSJ 326; PVL 2073), Calyptosuchus wellesi (UCMP 32422), Longosuchus meadei (TMM 31185-40) and Neoaetosauroides engaeus (PVL 3525). Typothorax coccinarum (UMCP 35255) has a postacetabular blade that has a squared off end and is very short, barely extending past the posterior edge of the ischiadic peduncle.

The preacetabular blade of S. robertsoni differs from almost all aetosaurians in that it is very anteroposteriorly short in that it does not extend to the anterior margin of the pubic peduncle, very narrow, and ventrally hooked (Fig. 7). Most aetosaurians have preacetabular blades that extend anteriorly to the edge of the pubic peduncle, and are thick and triangular in lateral view, such as Aetosauroides scagliai (PVL 2073), Desmatosuchus spurensis (UMMP 7476), and Calyptosuchus wellesi (UCMP 32422). Neoaetosauroides engaeus (PVL 3525) and Typothorax coccinarum (UCMP 35255) both have thin preacetabular blades and the element in T. coccinarum is very strongly hooked ventrally; however, in both taxa the preacetabular blades are much more elongate than in S. robertsoni, even extending past the anterior margin of the pubic peduncle.

Two ilia similar to that of S. robertsoni are TMM-31100-1 and UMMP 7322. TMM-31100-1, assigned to Lucasuchus hunti (Long & Murry, 1995), has the elongate and acute angled tip of the postacetabular blade, as well as a narrower and slightly ventrally hooked preacetabular blade; however, the preacetabular blade is more anteroposteriorly elongate than that of S. robertsoni extending to the margin of the pubic peduncle. UMMP 7322, referred to Desmatosuchus spurensis by Long & Murry (1995), also has an acutely angled postacetabular blade end and a relatively narrow and slightly ventrally hooked preacetabular blade, but again the preacetabular blade is more elongate, in this case extending slightly anterior to the anterior margin of the public peduncle.

Overall the ilium most resembles that of Aetosaurus ferratus (Schoch, 2007) with a short, narrow and ventrally hooked preacetabular blade, but the posterior acetabular blade end is not as acutely angled in medial view.

Pubis

An odd characteristic of the pubis of S. robertsoni is the presence of two large pubic foramina, the uppermost of which represents the obturator foramen (Walker, 1961). This was unique until the discovery of the same feature in the pubis of Scutarx deltatylus (Parker, 2016b). The pubis is poorly known in many aetosaurs; however, Desmatosuchus spurensis (MNA V9300) has only a single opening.

Use of symplesiomorphies in aetosaurian taxonomy

The first formal diagnosis of S. robertsoni (Heckert & Lucas, 2000:1556) was based on plesiomorphies that do not even comprise a unique combination of characters within Aetosauria and was intended to only diagnose the genus. This conservative assessment of the taxon has allowed other specimens (e.g., Aetosauroides scagliai, Calyptosuchus wellesi) from other areas (North and South America) to be easily assigned to the genus, mainly for the purpose of building a global terrestrial vertebrate biostratigraphy for the Upper Triassic (e.g., Lucas & Heckert, 1996; Heckert & Lucas, 1999; Heckert & Lucas, 2000; Heckert & Lucas, 2002; Lucas, Spielmann & Hunt, 2007; Lucas, 2017). However, detailed comparison demonstrates that these taxa all bear unique combinations of characters that allow them to be differentiated (e.g., Desojo & Ezcurra, 2011; Parker, 2018). Assignment of specimens to genera can be subjective and based upon the taxonomic philosophy of the researcher. I have argued elsewhere (e.g., Parker, 2018) that utilizing monotypic genera in aetosaurian work can aid in removing some of the ambiguity regarding use of these taxa in broader scale studies where genera are often used as a proxy for species. Diagnoses should be apomorphy-based, at the species level, or provide unique combinations of characters. Thus it is important that holotype specimens be reexamined utilizing discrete apomorphies in a phylogenetic context (Nesbitt, Irmis & Parker, 2007; Irmis et al., 2007; Nesbitt & Stocker, 2008; Parker, 2013).

This is especially true when taxa are being utilized for intercontinental biostratigraphic correlations (Irmis et al., 2010). For an example of how taxa may be assigned based on plesiomorphies for biostratigraphic correlation, a recent discussion on global biostratigraphy for the Late Triassic proposes a new hypothesis that Neoaetosauroides engaeus is a junior subjective synonym of Aetosaurus ferratus (Lucas, 2017). Support is given based on the width/length ratios and ornamentation patterns of the paramedian osteoderms (both symplesiomorphies for Aetosauria based on their presence in the non-stagonolepidid Aetosauroides scagliai) as well as an interpretation of similar sutural patterns of the skull based on published figures (Lucas, 2017:369). However, this interpretation ignores several key characters such as the presence of a laterally expanded premaxillary tip in Neoaetosauroides engaeus (Desojo & Báez, 2007), which is apomorphic for many aetosaurians (Parker, 2016a; Hoffman, Heckert & Zanno, 2018) although absent in Aetosaurus ferratus (Schoch, 2007) as well as Aetosauroides scagliai, which polarizes the character distribution. Among other character differences with A. ferratus, N. engaeus also lacks a well-developed antorbital fossa. Thus, synonomy between the two taxa is based on overall general similarity and symplesiomorphy rather than discrete synapomorphies, and currently unsupported. The practice of using plesiomorphic characters to assign specimens and to synonymize taxa has long been out of favor in vertebrate paleontology and should no longer be acceptable (e.g., Sereno, 1990; Padian, Lindberg & Polly, 1994; see further discussion in Nesbitt & Stocker, 2008). Data that utilize this approach should not used for broader scale studies until the synapomorphy distributions among included taxa are fully evaluated and supported.

Status of the holotype of S. robertsoni

The holotype of S. robertsoni is the impression of a fragment of the ventral carapace of a single specimen (ELGNM 27R) that shows several partial rows and columns of imbricated predominantly rectangular osteoderms (Fig. 11; Huxley, 1877:pl.1, fig. 1). These osteoderms have an anterior bar and a surface pattern of numerous drop-shaped pits radiating from the osteoderm center; hence the name Stagonolepis, which means “drop scale”. Since the initial descriptions by Huxley (1877) and Walker (1961) subsequent authors have assigned other species to Stagonolepis robertsoni based on similarities of the dorsal paramedian osteoderms (e.g., Murry & Long, 1989; Long & Murry, 1995; Lucas & Heckert, 2001; Heckert & Lucas, 2002; Lucas et al., 2007b). However, none of these authors have addressed the diagnostic status of the type specimen.

Ventral osteoderms are known from several other aetosaurian taxa including Coahomasuchus kahleorum, Calyptosuchus wellesi, Scutarx deltatylus, Neoaetosauroides engaeus, Aetosaurus ferratus, and even the non-aetosaurian Revueltosaurus callenderi. However, as described above presently the ornamentation in the ventral osteoderms of the holotype specimen (ELGNM 27R) of S. robertsoni differs from that of other aetosaurians in that it consists of randomly arranged oblong pits separated by a latticework of thin ridges. These pits are tightly packed and cover the majority of the osteoderm surface. This differs from what is seen in other aetosaurian taxa where the ornamentation consists of narrow grooves and oblong pits radiating from a central point on the osteoderm, and presently S. robertsoni is currently valid based on this character as well as the unique combination of characters listed and discussed above. A similar patterning is found in what is thought to be ventral osteoderms in the purported aetosaurian Chilenosuchus forttae Casamiquela, 1980 (Desojo, 2003); however, the aetosaurian affinities of this taxon are uncertain (Desojo et al., 2013). Regardless even if C. forttae is shown to unambiguously represent an aetosaur, it can still be distinguished from S. robertsoni by the ornamentation of the dorsal osteoderms, which lack a radial pattern in C. forttae (Desojo, 2003). However, this might necessitate the designation of a neotype specimen to conserve the name Stagonolepis robertsoni as the holotype specimen will no longer be diagnostic and the taxon valid only based on a unique combination of characters (Parker, 2008b; Parker, 2014).