Trachelosaurus fischeri Broili, 1918

Spiekman, Stephan N. F., Ezcurra, Martín D., Rytel, Adam, Wang, Wei, Mujal, Eudald, Buchwitz, Michael & Schoch, Rainer R., 2024, A redescription of TraCheloSaUrUS fiSCheri from the Buntsandstein (Middle Triassic) of Bernburg, Germany: the first European DinoCephaloSaUrUS-like marine reptile and its systematic implications for long-necked early, Swiss Journal of Palaeontology (10) 143 (1), pp. 1-33 : 8-25

Trachelosaurus fischeri Broili, 1918 3

Holotype. MLU.GeoS.1612, a partially disarticulated skeleton comprising a few isolated skull remains, including a right premaxilla, much of the presacral vertebral column, two sacral vertebrae, several caudal vertebrae, an extensive gastral basket, a right ilium, right? pubis, left? femur, and at least one probable metatarsal.

Diagnosis. A tanysaurian archosauromorph that is defined by the following combination of features (autapomorphies among Triassic archosauromorphs indicated by an asterisk): elongate vertebral column that consists of at least 21 cervical and 27 dorsal vertebrae; neural spines on both the cervical and dorsal vertebrae that are transversely expanded at their distal ends with strongly developed rugosities*; cervical ribs with bifurcating shafts that are short, not or barely extending beyond the length of its corresponding vertebra*; anterior to mid-dorsal vertebrae with very wide, ‘wing-like’ transverse processes; a barrel-shaped torso formed by widely rounded, almost uniformly holocephalous dorsal ribs; an ilium without a preacetabular process; and a stocky femur without a curved shaft.

Horizon. The type and only specimen of Trachelosaurus fischeri was found in a platy sandstone unit within the Chirotheriensandstein, upper part of the Solling Formation (topmost Middle Buntsandstein, probably of earliest Anisian age, see Schoch, 2019; Bachmann et al., 2021).

Locality. Merkel’s quarry, Bernburg an der Saale, Saxony-Anhalt, central Germany.

Ontogenetic assessment. Trachelosaurus fischeri is currently known from a single specimen, MLU. GeoS.1612, comprising one individual (see below). In both the anteriormost cervical vertebra and mid-dorsal vertebrae, the neural arches and centra are unfused or disarticulated. Furthermore, the proximal and distal ends of the femur are poorly ossified. These features are often indicative of skeletal immaturity in diapsids ( Griffin et al., 2021). However, they can also represent typical paedomorphic traits related to aquatic adaptations seen in skeletally mature marine reptiles, including Dinocephalosaurus orientalis ( Griffin et al., 2021; Rieppel, 1989; Spiekman et al., 2024). Considering the presence of several features indicative of an aquatic lifestyle in Trachelosaurus fischeri (high presacral vertebral count, straight femur, and absence of preacetabular process of the ilium), and its close affinities to the marine Dinocephalosaurus orientalis (see below), the latter interpretation is very likely. Therefore, these features are not reliable indicators of relative ontogenetic age for this individual. The presence of strongly rugose neural spines on the presacral vertebrae could represent some indication of maturity, but this cannot be stated confidently.

Remarks. The most recent definition of Tanystropheidae Camp, 1945 was formulated by Dilkes (1998, page 529): “the most recent common ancestor of Macrocnemus , Tanystropheus , and Langobardisaurus and all of its descendants”. Considering the discovery of many new tanystropheid taxa in recent years, as well as the results of our phylogenetic analyses and the novel definitions of Tanysauria clade nov. and Trachelosauridae Abel, 1919 , we propose a revised definition for Tanystropheidae that is stem-based: the most inclusive clade containing Tanystropheus longobardicus ( Bassani, 1886) but not Trachelosaurus fischeri Broili, 1918 , Dinocephalosaurus orientalis Li, 2003 , Protorosaurus speneri von Meyer, 1832 , or Prolacerta broomi Parrington, 1935 . This is a maximum clade definition.

Phylocode registration number. Tanystropheidae is identified in the international clade names repository as registration number 1040.

Description

MLU.GeoS.1612 consists of seven slabs that can be fitted together to form a single block ( Fig. 2B ). The majority of the remains of Trachelosaurus fischeri , including virtually all pre-caudal axial elements, are closely associated in MLU.GeoS.1612.A-B ( Fig. 3 ). Additional remains, including cranial elements and a partial gastral basket, are scattered across MLU.GeoS.1612.C-G ( Fig. 4 ). The slabs additionally preserve indeterminate remains of actinopterygian fishes, as well as several footprints resembling Capitosauroides bernburgensis and a tetrapod ichnotaxon possibly produced by therapsid trackmakers that was described from an unknown locality in the same area ( Buchwitz et al., 2020; Haubold, 1971). The discovery of several tetrapod footprints while examining the slabs (in addition to those previously reported by Broili & Fischer, 1918) requires a more detailed description and analysis that are outside the scope of the present work ( Fig. 2B ).

Skull

Premaxilla. A right premaxilla is exposed in lateral view on MLU.GeoS.1612.D ( Figs. 4 , 5A ). It preserves two teeth in situ, of which the posterior one is larger. The teeth are elongate and incipiently recurved, resembling the dentition of the premaxilla in Tanystropheus spp. (excluding “ Tanystropheus antiquus ”, here and throughout the text), Austronaga minuta, Dinocephalosaurus orientalis , and Gracilicollum latens ( Wang et al., 2023a, 2023b). The teeth lack serrations, as in other tanysaurians ( Ezcurra, 2016; Spiekman et al., 2021b). Their enamel surface is poorly preserved and the apical half of the larger tooth is broken, but the teeth appear to not be striated ( Fig. 5B ), in contrast to the striated crowns of Tanystropheus spp. , Dinocephalosaurus orientalis , and Gracilicollum latens ( Wang et al., 2023b). However, striations are also absent in the likely piscivorous tanysaurian Austronaga minuta ( Wang et al., 2023a). At least three empty alveoli can be discerned, one between the two preserved teeth, and two posterior to the posteriormost preserved tooth. The tooth count of the premaxilla was therefore likely around 5–7. Tooth implantation is either subthecodont or thecodont (sensu Bertin et al., 2018) and the base of the crowns is not fused to the bone.

The lateral surface of the premaxilla is marked by a distinct fossa adjacent to the anteroventral margin of the external naris. The posterior end of the main body of the premaxilla is incomplete but likely did not extend much beyond its current preservation. The anterior margin of the premaxilla is rounded in lateral view, similar to the condition in Pectodens zhenyuensis ( Li et al., 2017a) . The prenarial process of the premaxilla is present and moderately well developed, but shorter than in Dinocephalosaurus orientalis , Pectodens zhenyuensis , and Austronaga minuta ( Li, 2003; Li et al., 2017a; Wang et al., 2023a). By contrast, the prenarial process is absent in Tanystropheus spp. and Macrocnemus spp. ( Spiekman et al., 2021b; Table 1 ). The prenarial process of Trachelosaurus fischeri is directed posterodorsally and forms a plate-like structure that only slightly reduces in transverse width posterodorsally, closely resembling the condition in Dinocephalosaurus orientalis and Pectodens zhenyuensis ( Li, 2003; Li et al., 2017a). It has a rounded distal end. The postnarial process is broken off and mostly missing. The broadly rounded concavity between the prenarial process and main body of the premaxilla represents the margin of the external naris. It is large, and far displaced from the anterior margin of the premaxilla, similar to the condition in several tanystropheids and trachelosaurids ( Spiekman et al., 2021b).

?Nasal. A plate-like element with a distinct tapering process preserved in MLU.GeoS.1612.D ( Figs. 4 ; 5C ) was previously tentatively identified as a parietal by Fischer but later reinterpreted as a jugal by Broili ( Broili & Fischer, 1918). However, the morphology of this elongate, plate-like element does not correspond to the known morphology of either of these bones in early archosauromorphs. The identification of this element is hampered, because it is found in isolation and is incompletely preserved, with most of its margins being broken. Furthermore, the overall morphology of Trachelosaurus fischeri appears to be quite derived, and therefore, its cranial morphology might have deviated considerably from that of other early archosauromorphs. It cannot be excluded that the bone represents a palatal element, but it is here considered to most likely represent a nasal. The nasal is large and plate-like in many early archosauromorphs ( Ezcurra, 2016; Spiekman et al., 2021b), and the widely curved margin on one end of the element matches very well with the outline of an external naris. Therefore, this region of the element is interpreted as the anterior end of the nasal. Following this interpretation, the elongate and tapering anterior process of the bone could either represent the anteromedial process, which would have articulated with the prenarial process of the premaxilla, similar to the condition in Macrocnemus spp. (Jaquier et al., 2017), or it could represent the anterolateral process, which would have articulated with the maxilla ventrally, as in Tanystropheus spp. ( Spiekman et al., 2020a).

?Postorbital. Near the putative nasal, another isolated skull bone is preserved ( Figs. 4 , 5D ). It is triradiate, with a single concave margin that ends into two opposing, small processes, and a single, much larger process that is directed perpendicular to the concave margin. This larger process gradually tapers distally, giving it a triangular outline. This element might represent a postorbital. Following this interpretation, the concave margin is part of the orbital rim, and the larger process could represent the posterior process that would have articulated with the squamosal. The exposed surface is mostly smooth but shallowly depressed at the base of the larger process. If this element indeed is a postorbital, this would suggest that it is exposed in internal/medial view and thus represents the left postorbital.

?Postfrontal. An elongate, triradiate element is preserved in isolation adjacent to a gastralium in MLU. GeoS.1612.G ( Figs. 4, 5E ). The largest process is both the longest and the widest. It maintains its transverse width for most of its length, but tapers from its distal third to a point. The other two processes are positioned on the other side of the element, one facing in an opposing direction to the largest process and the other process being almost perpendicular to the other two. Both smaller processes possess a sharp tip distally. The exposed surface of the element is mostly flat, except for a depression at the base of the two smaller processes. The shape of the element is somewhat reminiscent of a postfrontal, which is generally triradiate in early archosauromorphs. It specifically resembles the postfrontal of Dinocephalosaurus orientalis in possessing an elongate lateral process, and small anteromedial and posteromedial processes ( Spiekman et al., 2024). Following this interpretation, the medial margin between the two smaller processes would have articulated with the frontal and parietal, whereas the elongate lateral process would have articulated with the postorbital.

AXial skeleton

Cervical vertebrae. In total, 19 associated postatlantal cervical vertebrae are confidently identified in MLU. GeoS.1612.A-B ( Fig. 3 ) based on their morphology, comprising an elongate centrum with a markedly concave ventral margin in lateral view, and the presence of very closely associated para- and diapophyses on the ventrolateral side of the centrum near its anterior end, corresponding to the general cervical configuration in early archosauromorphs ( Ezcurra, 2016; Spiekman et al., 2021b). One further, disarticulated cervical vertebra is now only preserved as a tiny, poorly preserved fragment ( Fig. 3 , indicated with H*). As is indicated in the plate of MLU.GeoS.1612.A-B in Broili and Fischer (1918, pl. 31), this element clearly represents a cervical vertebra that was well preserved prior to being heavily damaged at a later stage.

All cervical vertebrae are preserved in lateral view. The elements distributed over MLU.GeoS.1612.A-B comprise two articulated series and six isolated elements. The first articulated series is composed of four elements, preserved adjacent to the femur. The second series comprises nine elements and is interrupted posteriorly by the edge of the MLU.GeoS.1612.A slab, but if the curvature of the column is followed, it is clear that this section is still closely associated with the dorsal vertebrae preserved further posteriorly. Therefore, the posterior end of this series is positioned close to the end of the cervical column. Except for the anteriormost element, which is considerably smaller, the centra in the first series are 24–27 mm long, whereas most of those of the second series are approximately 33–35 mm long ( Table 2 ). The centra of the isolated cervical vertebrae that could be measured are 27–31.5 mm long and are thus intermediate in size between the vertebrae in the two articulated series. This gradual size increase from anterior to mid- to posterior cervical vertebrae is also observed in other tanysaurians, most notably Dinocephalosaurus orientalis ( Spiekman et al., 2024) . This suggests that all elements belong to the same individual, and therefore, that the neck of Trachelosaurus fischeri was composed of at least 20 cervical vertebrae (cf. Broili & Fischer, 1918; contra Huene, 1944).

The anteriormost, unambiguous cervical vertebra ( Fig. 6A ) is tentatively identified as the axis (sec. Broili & Fischer, 1918). It is considerably shorter than the succeeding vertebrae ( Table 2 ), and the diapophysis is placed considerably further dorsally along the lateral surface of the centrum than in the postaxial anterior to mid-cervical vertebrae, matching the configuration of the axis in other early archosauromorphs, such as Macrocnemus bassanii , Prolacerta broomi , and Azendohsaurus madagaskarensis ( Gow, 1975; Miedema et al., 2020; Nesbitt et al., 2015). The neural arch is fully detached from the centrum and a distinct longitudinal ridge along the dorsal margin of the centrum demarcates the position of the neurocentral suture. The ventral margin of the centrum is only weakly concave in lateral view compared to the postaxial cervical vertebrae. The neural arch is poorly preserved, but the postzygapophysis is distinct and strongly developed. An elongate, plate-like element positioned anterodorsal to the axial neural arch could represent one of the atlantal neural arches, but it is too poorly preserved to ascertain this confidently. Several small, scattered remains are preserved anterior to the axis. Broili and Fischer (1918) considered these to be remains of the atlas. They are too poorly preserved to corroborate this confidently, and at least one element appears to be a fish scale, but some elements could indeed belong to the atlas based on their overall size and position.

In addition to their anteroposterior length, the height of the neural spine also seems to gradually increase posteriorly in the cervical vertebrae ( Table 2 ), although the spinous process is covered by sediment in most of them and these, therefore, could not be measured. The anterior articular surface of the centrum is positioned slightly dorsally to the posterior articular surface ( Fig. 6C ), as is the case in many early archosauromorphs ( Ezcurra, 2016). From the concave outline of both articular surfaces of the centrum as exposed in certain elements, it can be discerned that the cervical vertebrae were amphicoelous. The parapophysis is positioned on the anteroventral edge of the centrum in lateral view. The diapophysis is positioned dorsally and slightly posteriorly to the parapophysis and it is placed on a short, wing-like process that gradually fades posterodorsally along the lateral surface of the centrum. The neural arches lack laminae. The zygapophyses are well developed and epipophyses are present dorsal to the postzygapophyses, as occurs in most non-eucrocopodan archosauromorphs ( Ezcurra, 2016; Ezcurra et al., 2023). The posterodorsal extent of the epipophyses is not preserved in any of the elements. There is no fossa lateral to the base of the neural spine. The neural spine is dorsoventrally short, similar to the general condition in tanystropheids and trachelosaurids, except for Tanystropheus spp. and Sclerostropheus fossai , in which the neural spine is almost completely reduced ( Spiekman & Scheyer, 2019; Spiekman et al., 2021b). The anterior and posterior margins of the neural spines are both concave in lateral view, as is the common condition in early archosauromorphs, although in the trachelosaurids Dinocephalosaurus orientalis, Austronaga minuta, and Pectodens zhenyuensis , the neural spine is trapezoidal in outline in lateral view ( Li et al., 2017a; Spiekman et al., 2024; Wang et al., 2023a). The distal ends of the exposed neural spines are gently convex in lateral view, and they are transversely expanded, forming a strongly rugose thickening. A distal transverse thickening of the neural spine also occurs in Augustaburiania vatagini , Czatkowiella harae, Gracilicollum latens, Macrocnemus spp. , and Tanystropheus “conspicuus ” among non-crocopodan archosauromorphs ( Borsuk-Białynicka & Evans, 2009; Scheyer et al., 2020; Sennikov, 2011; Wang et al., 2023b; Wild, 1973), but the degree of rugosity present in Trachelosaurus fischeri is considerably more extensive. There are no intercentra between the vertebrae.

Cervical ribs. Twelve cervical ribs are present on MLU. GeoS.1612.A, including a single partial rib preserved in articulation with its corresponding vertebra; its shaft is oriented parallel to the vertebral column ( Fig. 6C ). At least one additional rib was previously present within the now missing rock fragment located at the intersection between MLU.GeoS.1612.A and B (see Broili & Fischer, 1918, pl. 31, between the cervical vertebrae “9” and “10” therein). All cervical ribs possess a short shaft, not or barely exceeding the length of its corresponding vertebra, which bifurcates distally into two distinct prongs ( Fig. 6B ). This represents a configuration unique among Permo-Triassic archosauromorphs. The ribs possess a distinct anterior free-ending process as in other archosauromorphs, which terminates in a blunt tip. Although prominent, this anterior process is not as long as those seen in Czatkowiella harae , Sclerostropheus fossai , Tanytrachelos ahynis , Ozimek volans , Pectodens zhenyuensis , Dinocephalosaurus orientalis, Austronaga minuta, and Gracilicollum latens ( Spiekman et al., 2021b; Wang et al., 2023a, 2023b). The tubercular and capitular heads of the cervical ribs are positioned closely to each other, as in other early archosauromorphs. The tuberculum forms a prominent process that is rectangular in outline in lateral view, whereas the capitulum only forms a small knob-like protrusion. The rib shaft is lateromedially thin, forming only a lamina between the slightly thickened dorsal and ventral margins. However, the shaft is of considerable height dorsoventrally, giving it a stocky appearance in lateral view. The dorsal margin of the shaft is concave proximally and straight distally, where it is oriented posteriorly and slightly dorsally. The ventral margin of the shaft is straight and would have been oriented parallel to the vertebral column. Consequently, the rib shaft is slightly constricted directly posterior to the tuberculum in lateral view, before widening dorsoventrally further posteriorly. The distal bifurcation of the shaft is formed by posterodorsal and posteroventral prongs, of which the latter extends further posteriorly. Both ends terminate in a sharp tip. They are separated from each other by a wellrounded and deep posterior notch in lateral view.

Dorsal vertebrae. The post-cervical vertebrae are distributed mainly over MLU.GeoS.1612.A. They are disarticulated, except for an articulated series of four elements that are exposed in left lateral view near the edge of the slab ( Fig. 3 ). Due to the disarticulation of the vertebral column, the exact order of the vertebrae cannot be reconstructed. However, the vertebrae are still generally associated in an anterior-to-posterior sequence, which is also corroborated by a gradual morphological shift in the width of the transverse processes, which are very wide laterally in the anterior dorsal vertebrae and which gradually narrow posteriorly ( Table 3 ). Furthermore, the anterior part of the dorsal column is still closely associated with the posterior end of the cervical series, even though the exact cervical–dorsal transition is not preserved. The disarticulated vertebrae are exposed under various angles. The articulated series of four elements and a single displaced element positioned directly posterodorsally to this series represent the posteriormost preserved elements in MLU.GeoS.1612.A ( Fig. 7D ). The preceding elements can be unambiguously identified as dorsal vertebrae due to the morphology of their transverse processes ( Fig. 7A, B ). However, in these posteriormost elements, no clear transverse processes can be discerned ( Fig. 7D ). They have either broken or eroded away due to their angle of preservation, or, more likely, the transverse process was incipient and the rounded, slightly concave surface on their lateral sides represent large rib facets. Because of this morphology, it is difficult to determine whether they represent posterior dorsal, sacral, and/or anterior caudal vertebrae, as was also the consideration of Broili and Fischer (1918). Based on the comparatively large size of its inferred rib articulation site, the posteriormost preserved element in the series is identified as a sacral vertebra, and therefore, the preceding elements represent posterior dorsal vertebrae (contra Huene, 1944, who interpreted these elements as proximal caudal vertebrae, with only the anteriormost element being considered as a potential sacral vertebra). In agreement with our interpretation, the vertebrae anterior to the probable first sacral vertebra possess posterior centrodiapophyseal, prezygodiapophyseal and postzygodiapophyseal laminae, supporting their identification as dorsal elements. Consequently, 27 dorsal vertebrae are preserved in MLU.GeoS.1612.A based on this interpretation, whereas Broili and Fischer (1918) estimated the total number of dorsal and possibly sacral vertebrae at around 20 to 21. Dinocephalosaurus orientalis possesses 28 or 29 dorsal vertebrae ( Spiekman et al., 2024), whereas the trunk of other early archosauromorphs is generally restricted to 12 to 18 dorsal vertebrae ( Spiekman et al., 2021b); the trachelosaurid Pectodens zhenyuensis has only 13 to 14 dorsal vertebrae ( Li et al., 2017a).

The anteriormost dorsal vertebrae that exhibit clear morphological details are exposed in either anterior or posterior view ( Fig. 7A ). The articular surfaces of their centra are slightly concave, suggesting that they are platycoelous to amphicoelous. The articular surfaces are dorsoventrally taller than transversely wide, and their ventral margins are strongly rounded. The neural canal is rectangular in outline; it is roughly square in the most anterior of the elements exposing this feature, whereas in the subsequent elements, it is considerably transversely narrower, resulting in a tall, columnar opening. Only fragments of the neural spine are preserved in these anterior elements, and they were likely relatively low. As mentioned above, the transverse processes in these elements are laterally very wide, being almost three times wider than the height of the articular surface of their corresponding centrum ( Table 3 ). In the anteriormost element preserving fully exposed transverse processes ( Fig. 3 , indicated with e), the dorsal margin of the transverse process slopes gently lateroventrally in anterior or posterior view, whereas in the subsequent elements, the transverse process is fully laterally oriented and its dorsal margin is gently convex. Furthermore, a thin dorsoventrally oriented lamina projects from the transverse process of this element, providing the process a marked wing-like outline in anterior or posterior view. The lateral margin of this lamina is distinctly convex, and it curves ventromedially, terminating ventrally on the ventral portion of the lateral surface of the centrum. The subsequent vertebrae exhibit a similar lamina, but in these elements, the ventral margin of this lamina is distinctly concave, and the lamina terminates more dorsally than in the anteriormost element. Such a prominent lamina extending from the transverse process of the dorsal vertebrae is unusual among non-archosaurian archosauromorphs, but it does occur in Dinocephalosaurus orientalis ( Spiekman et al., 2024) .

Further posteriorly, in the elements that are interpreted here as mid-dorsal vertebrae, the neural arch and centra have disarticulated from each other ( Fig. 7A ); their neurocentral suture was therefore clearly not fused. The neural arches are generally exposed in either dorsal or ventral view, whereas the centra are mostly roughly exposed in lateral view. The centra appear to be transversely constricted, particularly in their anteroposterior midpoint, giving the element an hourglass-shaped outline in dorsal view or ventral view. This feature is likely exaggerated by diagenetic compaction. The ventral margin of the centrum is strongly concave in lateral view, and the articular surfaces are again platycoelous to amphicoelous. The transverse processes in these elements are restricted to the neural arch. As in the anterior dorsal vertebrae, these processes are well developed laterally, which is an uncommon feature among non-archosaurian archosauromorphs. A similar transverse extent of these processes can only be found in Pectodens zhenyuensis , in which they are considerably more slender ( Li et al., 2017a). The transverse processes are projected directly laterally. In dorsal or ventral view, the anterior and posterior margins of the process are gently concave and the distal end, which forms the articular surface for the rib, is anteroposteriorly expanded and laterally flat. Due to the angle of preservation, the ventral lamina of the transverse process is only poorly exposed in this section of the dorsal column. However, it is still clearly present in these elements, albeit more dorsally restricted than in the anterior dorsal vertebrae. The zygapophyses are generally poorly preserved. As in other archosauromorphs, the postzygapophyses are considerably larger than the prezygapophyses, being both transversely wider and anteroposteriorly longer. The neural spine is not preserved in most mid-dorsal vertebrae. However, in some elements, which are preserved under a slight angle from the dorsoventral plane, the neural spine is present but deformed. It is shorter than the height of the articular surface in the corresponding centra. It expands gradually transversely towards its distal end and has a more extensively rugose distal margin than the cervical vertebrae. Even though Tanystropheus spp. , Dinocephalosaurus orientalis , and Ozimek volans also possess a transverse expansion on the distal end of the dorsal neural spines, the expansion is considerably more extensive in Trachelosaurus fischeri ( Spiekman et al., 2021b) . Furthermore, the rugosity present on the distal margin of the neural spines, including vertically oriented striations in some vertebrae, is more reminiscent of that present in certain marine diapsids like Claudiosaurus germaini and Augustasaurus hagdorni ( Carroll, 1981; Sander et al., 1997) rather than early archosauromorphs. A much smaller amount of rugosity only occurs on the spine tables of the dorsal vertebrae of Dinocephalosaurus orientalis among non-archosauriform archosauromorphs ( Spiekman et al., 2021b, in press). The distal surface of the neural spine in Trachelosaurus fischeri is gently convex.

The transverse processes start to distinctly, but gradually, reduce in transverse width from about vertebra q onwards ( Fig. 3 ). Postcervical vertebra x ( Fig. 3 ) is the last element to unambiguously possess a transverse process. It is preserved in close association with the subsequent vertebra, which is the anteriormost element in the fully articulated series of four vertebrae exposed in left lateral view ( Fig. 7D ). The ventral margins of the centra in this series are weakly concave in lateral view, to a lesser extent than the mid-dorsal centra. The centra are slightly longer than tall and lack a lateral fossa. The posterior articular surface of the centrum of the anteriormost element of this series is partially exposed. It is similar to the preceding dorsal vertebrae, being taller than wide with a slightly concave surface. No suture line for the neurocentral suture can be discerned in this series of vertebrae. In the second element of the series and to a lesser extent in the third, the distal end of a poorly developed posterior centrodiapophyseal lamina and clear prezygodiapophyseal and postzygodiapophyseal laminae are present; a prezygodiapophyseal lamina can also be discerned on the final vertebra of the series, the probable first sacral element. An elongate, strut-like element is preserved directly ventral to the anteriormost element of the series. This was interpreted as a haemal arch by Broili and Fischer (1918). However, based on its morphology and relatively wide and flat proximal end, it is most likely a short “lumbar” rib, and thus, the corresponding element is a posterior dorsal vertebra. Another, disarticulated, “lumbar” rib is present somewhat anterior to the same vertebra. The postzygapophyses of these vertebrae are positioned on a considerably higher dorsoventral level than the prezygapophyses. The postzygapophyses are oriented posteriorly and slightly dorsally, whereas the prezygapophyses are of considerable length and oriented anterodorsally. There is no fossa lateral to the base of the neural spine. The distal region of the neural spines expands anteriorly and posteriorly in lateral view. The rugosity of the transversely expanded distal end of the neural spines is composed of clear striations, marked by dorsoventrally oriented grooves in lateral view.

Dorsal ribs. More than 20, mostly disarticulated, dorsal ribs are preserved in MLU.GeoS.1612.A-B ( Fig. 3 ). Three further dorsal ribs are present in the MLU. GeoS.1612.C, E–F ( Fig. 4 ), of which two are largely complete and a third preserves the distal end of a shaft. The anteriormost preserved dorsal rib in MLU.GeoS.1612.A is found in articulation with a small fragment of an anterior dorsal vertebra ( Fig. 7C ). The tuberculum and capitulum of this rib are separated from each other by a very small concave gap between them. All subsequent ribs, including the element far displaced and preserved in MLU.GeoS.1612.F, are holocephalous and the single head of the ribs becomes gradually narrower in more posterior elements. The anteriormost dorsal ribs are slightly shorter than those in the mid section of the trunk ( Table 4 ). The dorsal rib shafts have a strong curvature in anterior or posterior view, and therefore, the torso of Trachelosaurus fischeri was barrel-shaped, as was also observed by Broili and Fischer (1918). The shaft maintains its width along its entire width and terminates distally in a blunt tip. A distinct groove is present along the surface of the shaft in most ribs.

Sacral vertebrae. The probable first sacral vertebra is preserved in articulation with the last three dorsal vertebrae. The morphology of the element resembles that of the posteriormost dorsal vertebrae (see above) with the exception of the presence of a larger rib facet that is situated on both neural arch and centrum, as occurs in the sacral vertebra of other early archosauromorphs ( Fig. 7D ). Posterodorsal to the inferred first sacral vertebra, another element is preserved. Like the preceding vertebrae, it is exposed in left lateral view, but it is poorly preserved and much of its left lateral surface is broken. This likely represents the second sacral vertebra, but it does not provide further useful information. No sacral ribs are preserved.

Caudal vertebrae and ribs. Elements of the caudal vertebrae are preserved across MLU.GeoS.1612.A-C, G. The most anterior elements are preserved in association in MLU.GeoS.1612.C adjacent to the partially articulated gastral basket. They comprise at least two very poorly preserved centra, a single, more complete vertebra, and three relatively well-preserved caudal ribs ( Fig. 8 ). The most complete vertebra (β) is exposed in lateral view and has a very gently concave ventral margin. It is quite short anteroposteriorly ( Table 5 ). The neural arch is poorly preserved and likely mostly comprises a transverse process that has been shifted relative to the centrum, so that it is exposed in ventral view. The outline of the transverse process is subrectangular, being longer lateromedially than wide anteroposteriorly. One of the caudal ribs, which is preserved distal to the transverse process, likely disarticulated from this vertebra. Based on the width of the transverse process and the vicinity of the caudal rib, the vertebra is identified as a proximal caudal element. The preserved caudal ribs on MLU.GeoS.1612.C vary in length but all are relatively stocky elements that maintain their width along most of their length.

The remains of at least 20 caudal vertebrae are exposed in the MLU.GeoS.1612.A-B ( Fig. 3 ). Most of these elements are represented by isolated centra that are scattered across MLU.GeoS.1612.A, adjacent to the dorsal vertebrae and the ilium. They slightly decrease in size in the opposite direction of the dorsal vertebrae on the slab. Only the anteriormost element on MLU.GeoS.1612.A preserves remnants of transverse processes, which are very poorly developed ( Fig. 7D ). The remaining elements do not appear to have possessed transverse processes, and they are therefore interpreted as distal caudal vertebrae, an identification that agrees with their diminutive size. A single, poorly preserved neural arch, exposed in dorsal view, is present on MLU.GeoS.1612.B. Its neural spine is mostly broken. Parts of an additional vertebra are located on both sides of the crack delimiting MLU. GeoS.1612.A and B. Transverse processes, relatively much longer in the latter element, are present in both of these vertebrae, suggesting that they represent middle caudal vertebrae.

Gastralia. A large articulated, partial gastral basket is preserved across MLU.GeoS.1612.C-E ( Figs. 4 , 8 ). The gastralia are closely packed together. In addition, a large amount of disarticulated gastralia are scattered across all slabs ( Fig. 2B ), indicating the presence of a very extensive gastral basket in agreement with the large dorsal vertebral count. They are all elongate, slender elements, similar to those seen in early archosauromorphs generally ( Ezcurra, 2016). Two distinct types are present. The first type constitutes relatively smaller elements that have a gently continuous curvature. The second, larger type, represents elements that are slightly bowed, resulting in a gentle V-shaped outline. A few of the latter type possess two long prongs on one side. These elements thus have the outline of a two-pronged fork. In the partial gastral basket, the smaller, lateral elements articulate on either side of a larger, V-shaped element. The lateral gastral elements medially approach, but do not reach, the bend of the larger elements.

Appendicular skeleton

Ilium. A right ilium is preserved near the edge of MLU. GeoS.1612.A. It is exposed in lateral view and virtually complete, only missing the posteriormost end of the postacetabular process ( Fig. 9A ). The supra-acetabular crest is either poorly developed or strongly diagenetically compressed and only a slight convexity is present at the dorsalmost level of the acetabulum, resembling the condition in non-eucrocopodan archosauromorphs ( Ezcurra et al., 2023). The main body of the ilium is sloped on each side in lateral view, anteroventrally on the anterior margin and posteroventrally on the posterior margin, with the latter forming the steepest angle with the horizontal plane. There is no posterior heel on the distal end of the ischiadic peduncle. The ventral margin of the ilium is subtriangular, and the articular surfaces for the pubis and ischium are subequal in anteroposterior length. There is no ridge on the lateral surface of the iliac blade and, although it is damaged, the dorsal margin seems to have been continuously convex. The postacetabular process is mostly posteriorly, and also slightly dorsally, oriented and gradually reduces its dorsoventral height posteriorly. The ventral half of the lateral surface of the postacetabular process lacks the depression present in several tanystropheids ( Ezcurra, 2016). A preacetabular process is absent, as is the case in Dinocephalosaurus orientalis ( Spiekman et al., 2024) , and consequently, the anterior margin of the iliac blade is gently rounded. A tuber on the anterior margin of the iliac blade, a feature previously identified in Tanystropheus longobardicus , Raibliania calligarisi , Fuyuansaurus acutirostris , the Hayden Quarry tanystropheid, and some specimens of Macrocnemus bassanii (sensu Pritchard et al., 2015; see also character 266 of Spiekman et al., 2021b for further explanation of this feature), is absent in Trachelosaurus fischeri .

Pubis. A large, incomplete, and poorly preserved element was tentatively identified as a pubis by Huene (1902) and a coracoid by Broili and Fischer (1918). It is here identified as a right(?) pubis. The element has a rounded outline, with a single, relatively flat, margin, and a distinct pubic foramen ( Fig. 9C ). This configuration corresponds closely with the rounded pubis present in Dinocephalosaurus orientalis ( Spiekman et al., 2024) . The pubis is found adjacent to the associated gastral basket at the edge of MLU.GeoS.1612.E in relatively close proximity to the sacral series, ilium, and hind limb elements ( Fig. 2B ).

Femur. A large, robust element is preserved adjacent to the anterior cervical vertebrae in MLU.GeoS.1612.A ( Fig. 9B ). It is strongly crushed due to diagenetic compression and its surface is highly fractured. The element is broken in two pieces, with the central portion of the shaft missing, but it is preserved as a natural mould. This element is identified as a left femur (sec. Huene, 1902, 1944; Broili & Fischer, 1918). It lacks distinct condyles on both the proximal and distal ends, as well as the sigmoid or curved shape present in the femora of most terrestrial neodiapsids. Instead, the shaft is straight. The shape and proportions of this element are similar to the femur of Dinocephalosaurus orientalis , in which the reduction of the above-mentioned features is related to the formation of flipper-like limbs for aquatic propulsion ( Chen et al., 2014; Spiekman et al., 2024). The widths of both ends of the element roughly correspond with the width of the acetabulum as preserved on the ilium ( Table 6 ), providing additional evidence that it represents a femur. One edge of the element is distinctly raised relative to the rest of the bone, and this is tentatively identified as an internal trochanter. Based on this interpretation, the bone represents the left femur with the widest end, positioned adjacent to the anterior cervical vertebrae, identified as the proximal end. From the proximal end, the element gradually reduces in width until about two-thirds along the length of the shaft. From there, the shaft increases in width again to the expanded distal end. The proximal end is slightly wider than the distal end. The exposed proximal margin is roughly straight, whereas the distal margin is gently and continuously convex. No distinct medullary cavity appears to be present on either of the exposed sides of the broken shaft, although this could be the result of the strong compression of the bone.

Metatarsal. An elongate, pillar-like element is preserved directly adjacent to the ilium in MLU. GeoS.1612.A ( Fig. 9A ). Based on its relative size and elongation, as well as its location adjacent to the ilium, it most likely represents a metatarsal (sec. Huene, 1944). Broili and Fischer (1918) identified this element as either a metatarsal or metacarpal. The ends are roughly straight to slightly convex and they are slightly expanded relative to the shaft, with one end, presumably the proximal end, being slightly more expanded.

A similar but considerably smaller and incomplete element is preserved near the edge of the slab, posterior to the ilium ( Fig. 3 ). Based on its considerably smaller size, yet similar pillar-like morphology, this element could represent a metatarsal (e.g., metatarsal I), a metacarpal, or a phalanx.

Unidentified remains

Several other remains preserved on MLU.GeoS.1612 can be assigned to Trachelosaurus fischeri based on their relative size and type of preservation. However, these elements could not be identified because of the lack of preserved diagnostic features ( Figs. 3 , 4 ).

Phylogenetic results

Spiekman et al. (2021b) matrix. The SCT is generated from 52 MPTs of 1240 steps recovered in the equal weights analysis. It is poorly resolved, with a massive polytomy composed of all members of Tanysauria, in which only the internal relationships within Trachelosauridae are resolved (Additional file 1). The application of the iterPCR protocol resulted in the pruning a posteriori of Sclerostropheus fossai , Raibliania calligarisi , Augustaburiania vatagini , Elessaurus gondwanoccidens , Fuyuansaurus acutirostris , and Gracilicollum latens. The resulting RSCT recovered a large tanystropheid clade ( Fig. 10A ). Together with Trachelosauridae , Tanystropheidae forms the more inclusive Tanysauria, a clade that was also recovered as monophyletic in previous iterations of this matrix ( Spiekman et al., 2021b, 2024; Lu & Liu, 2023; Wang et al., 2023a, 2023b). Tanysauria has the following synapomorphies in all the MPTs: premaxilla without a palatal shelf on its medial surface (character 11: 1 → 0), a thick anterior margin of the premaxilla results in the external nares being posteriorly displaced (character 31: 0 → 1), postaxial intercentra absent (character 182: 0-> 1), axis neural spine expanded anterodorsally in lateral view (character 186: 0 → 1), parapophysis of posterior dorsal vertebrae positioned entirely on neural arch (character 212: 0 → 1), ratio of anteroposterior length of vertebral centrum versus proximodistal length of corresponding haemal spine in anterior caudal elements between 0.64–0.81 (character 224: 0 → 1), scapular blade largely posteriorly directed and semi-circular in outline with a continuously curved anterior/dorsal margin (character 228: 0 → 1), humerus without thick subcilindrical tuberosity on ventral margin of the deltopectoral crest (character 244: 0 → 1), ratio of width of the distal end of the metacarpal I versus its total length between 0.38 and 0.43 (character 259: 0 → 1), and only one element of distal tarsals 1 and 2 present in mature individuals (character 292: 0 → 1). Tanystropheidae possesses the following synapomorphies common to all MPTs: haemal spine maintains breadth along its length (character 223: 2 → 1), ilium with low brevis shelf (character 269: 0 → 1), and ratio of length of metatarsal IV versus proximodistal length of metatarsal V between 3.65 and 5.15 (character 305: 1 → 3).

Although the topology of the tanystropheid clade is generally similar to the recent results in Spiekman et al. (2024), there are some notable differences. Compared to this previous iteration, more tanystropheid taxa were found to be unstable by the iterPCR protocol, resulting in a less-inclusive RSCT. Luxisaurus terrestris is found as the sister taxon to all remaining tanystropheids in the RSCT, which corresponds to the results of Lu and Liu (2023, note that Fuyuansaurus acutirostris was identified as an unstable OTU in the present analysis). The clade composed of the three Macrocnemus taxa forms the sister group to all other tanystropheids except Luxisaurus terrestris, similar to the result in Spiekman et al. (2024), when not considering the unstable OTUs Fuyuansaurus acutirostris and Augustaburiania vatagini . The Tanystropheus OTUs form a monophyletic group, whereas in Spiekman et al. (2024), Raibliania calligarisi was found as the sister taxon to Tanystropheus longobardicus . This result was found in 49 out of 52 MPTs of the current analysis, with Raibliania calligarisi recovered as the sister taxon to Ozimek volans in the remaining three MPTs. As in the analysis of Spiekman et al. (2024), AMNH FARB 7206 and Tanytrachelos ahynis form a trichotomy together with the Tanystropheus clade in the RSCT. However, whereas Langobardisaurus pandolfii was also part of this polytomy in the RSCT of Spiekman et al. (2024), this taxon is here recovered as the sister taxon to the node of this polytomy. Amotosaurus rotfeldensis and Ozimek volans are sister taxa to each other in the RSCT of Spiekman et al. (2024), but here they are recovered in a polytomy together with the clade that includes Langobardisaurus pandolfii, AMNH FARB 7206, Tanytrachelos ahynis , and the Tanystropheus OTUs in the RSCT.

Trachelosaurus fischeri is found as the sister taxon to the clade composed of Dinocephalosaurus orientalis and Austronaga minuta, with Pectodens zhenyuensis forming the sister taxon to the clade that encompasses these three taxa. Together, these four taxa form Trachelosauridae . Trachelosauridae is defined by the following synapomorphies common to all the MPTs: jugal without posterior process (character 42: 0 → 1), ventral process of postorbital ends much higher than the ventral border of the orbit (character 61: 1 → 0), anterior process of the squamosal continues along the posterior margin of the ventral process of the postorbital and contacts the jugal (character 64: 0 → 1), presence of 11–13 cervical vertebrae (character 195: 1 → 2), mid-dorsal vertebrae with a very wide and “wing-like” transverse process and a distinct connection from the process to the corresponding centrum through a lamina (character 207: 0 → 1), holocephalous anterior dorsal ribs (character 213: 1 → 0), ratio of total length of the humerus versus the total length of the femur between 0.84 and 0.91 (character 248: 1 → 2), astragalus without posterior groove (character 290: 0 → 1), metatarsal V without a hook-shaped proximal end (character 300: 1 → 0), and ratio of length of metatarsal IV versus proximodistal length of metatarsal V between 1.25 and 1.90 (character 305: 1 → 0). The clade comprising Trachelosaurus fischeri, Austronaga minuta, and Dinocephalosaurus orientalis has the following common synapomorphies: more than 13 cervical vertebrae (character 195: 2 → 3), 23 or more dorsal vertebrae (character 204: 0 → 1), ratio of length versus height of the centrum at the level of its posterior articular surface in posterior dorsals between 0.83–1.25 (character 205: 1 → 0), presence of flipper-like limbs (character 239: 0 → 1), ilium without preacetabular process (character 264: 1 → 0), ilium without supra-acetabular crest (character 270: 1 → 0), and femur with linear shaft (character 286: 0 → 1). Finally, Trachelosaurus fischeri is defined in all the MPTs by the following autapomorphies: anterior to mid-cervical vertebrae with a spine table (character 180: 0 → 2), cervical ribs short and shaft parallel to the neck (character 199: 2 → 1), and anterior to mid-dorsal vertebrae with gradual transverse expansion of the neural spine (character 203: 0 → 1).

CoArTreeP matrix. The SCT of all the MPTs found in the analyses using the six different concavity constant values (k = 19 − 24) is generally very well resolved, with a few polytomies (see homoplasy indices and fit values in Table 7 ). This topology is mostly congruent with those of the SCTs found in the analysis 1 of Ezcurra et al. (2023) (in which all analyses were also conducted under implied weighting with k = 19 − 24), and the SCTs of Müller et al. (2023) and Paes-Neto et al. (2023) (these two analyses were conducted only under equal weighting). The only contradictory results are the position of the Long Reef proterosuchian as more closely related to the Erythrosuchidae + Eucrocopoda clade than to Proterosuchidae (contrasting with Ezcurra et al., 2023), the placement of Tawa hallae as an herrerasaurian and not as closer to Neotheropoda (contrasting with Ezcurra et al., 2023, but as in Müller et al., 2023), and some aspects of the internal relationships of Tanysauria that are described in detail below.

Trachelosaurus fischeri is recovered in all the MPTs of our analyses within a taxonomically broader Trachelosauridae that is also composed of Dinocephalosaurus orientalis, Gracilicollum latens, Fuyuansaurus acutirostris, Austronaga minuta, and Pectodens zhenyuensis ( Fig. 10B ). This group is positioned within a taxonomically larger clade of non-crocopodan archosauromorphs, i.e., Tanysauria, which is also composed of Jesairosaurus lehmani and Tanystropheidae . Tanysauria is supported by the following six synapomorphies (character states present in Trachelosaurus fischeri indicated with an asterisk, here and below): premaxilla with five or more tooth positions (character 42: 2 → 1*), postorbital-squamosal contact continues ventrally for much or most of the ventral length of the squamosal (character 127: 0 → 1/2), parietal extending over interorbital region (character 160: 0 → 1), interclavicle anterior margin with a median notch (character 407: 0 → 1), humerus with moderate medial development of the entepicondyle, being poorly projected from the level of the shaft (character 425: 1 → 0), and postfrontal medial margin lateral to parietal (character 832: 0 → 1).

The clade that is composed of Trachelosauridae and Tanystropheidae has the following 12 synapomorphies: maxilla without anterior maxillary foramen (character 52: 1 → 0), maxilla with a separate dorsal apex, forming a distinct process differentiated from the rest of the maxilla by strongly concave or sharply flexed margins (character 58: 0 → 1), postfrontal participating in the border of the supratemporal fenestra (character 123: 0 → 1), axis with anterodorsally expanded neural spine (character 329: 0 → 1), fourth or fifth cervical centrum length versus height of the anterior articular surface greater than 2.92 (character 331: 3/4 → 6/7*), dorsal vertebrae with fan-shaped neural spine in lateral view (character 363: 0 → 1*), scapula and coracoid unfused with each other in non-early juvenile individuals (character 384: 0 → 1), scapula with anterior margin of the blade straight or convex along entire length in lateral view (character 390: 1 → 0), scapula with constriction distal to the glenoid anteroposteriorly longer than half the proximodistal length of the bone (character 391: 1 → 0), humerus with transverse width of the proximal end versus total length of the bone <0.25 in non-early juvenile individuals (character 416: 1 → 0), ischium with posterior process that extends from the posterodorsal margin (character 488: 0 → 1), and ilium with subtriangular postacetabular process, tapering posteriorly in lateral view (character 688: 0 → 1*).

Five character states are optimised as synapomorphies of Trachelosauridae : premaxilla with anteroposteriorly deep base of the prenarial process (= nasal process) (character 35: 0 → 1*), jugal without posterior process (character 100: 2 → 0), neck with ten or more vertebrae (character 324: 2 → 3*), metatarsal V without a hook-shaped proximal end (character 577: 1 → 0), and axis with postzygapophysis confluent with the neural spine, not protruding posteriorly from the base of the neural spine (character 897: 0 → 1*). Pectodens zhenyuensis and Austronaga minuta form a clade at the base of Trachelosauridae that is supported by the presence of premaxilla with anteroposterior length of the main body versus its maximum dorsoventral height> 5.00 (character 28: 2 → 4), and jugal with strongly posterodorsally oriented ascending process in an angle equal to or lower than 45º in lateral view when articulated with the maxilla (character 627: 0 → 1). Fuyuansaurus acutirostris is recovered as the sister taxon to all other trachelosaurids to the exclusion of the Pectodens + Austronaga clade, and this group is supported by the presence of anterior-middle and sometimes posterior postaxial cervical vertebrae with distally restricted transverse expansion of the neural spines (not mammillary process) (character 321: 0 → 1*).

Trachelosaurus fischeri is positioned among the more deeply nested trachelosaurids, being the sister taxon to a clade composed of Dinocephalosaurus orientalis and Gracilicollum latens. The clade that includes Trachelosaurus fischeri , Dinocephalosaurus orientalis , and Gracilicollum latens is supported by the following three synapomorphies: 14 or more cervical vertebrae (character 324: 3 → 4*), ilium without or with incipient preacetabular process (character 460: 1 → 0*), and ilium without supra-acetabular crest (character 667: 1 → 0*). Dinocephalosaurus orientalis and Gracilicollum latens are found as sister taxa to each other because of the presence of anterior cervical ribs with anterior free-ending process (= accessory process) extending anterior to the prezygapophyses of the corresponding vertebra when in articulation (character 350: 1 → 2).

The base of Tanystropheidae is represented by a trichotomy composed of Luxisaurus terrestris, Augustaburiania vatagini , and a clade formed by more deeply nested species. Trachelosaurus fischeri is excluded from this clade, because it lacks postaxial anterior and middle cervical vertebrae with a distinct longitudinal lamina extending along the lateral surface of the centrum at midheight (character 340: 0 → 1).

The resampling frequencies are low (<70%) around the base of Tanysauria, but with a few exceptions. The Trachelosauridae + Tanystropheidae clade has absolute and GC frequencies of 90% and 87%, respectively. Among tanystropheids, high resampling frequencies occur within the genus Macrocnemus (> 90%), whereas other frequencies are all lower than 70% to the exclusion of an absolute frequency of 76% for the Tanystropheus spp. + Raibliania calligarisi clade.

Figures

Fig. 4 Overview of MLU.GeoS.1612.C-G,preserving the cranial and proximal caudal remains, as well as a tentatively identified pubis and partial gastral basket.The rib marked with †‡ corresponds to the rib that is indicated the same in Table 4. Abbreviations: car, caudal rib; cav, caudal vertebra; dr, dorsal rib; fisc, fish scales; fr v, fragmentary vertebra; gas,gastralia; na, nasal;pmx, premaxilla; po, postorbital; pof, postfrontal; pub, pubis

Fig. 1 Geographic and geological setting. Locality and stratigraphic context of Merkel’s Quarry, where Trachelosaurus fischeri was found. Modified from Schoch (2019)

Fig. 2 Overview of MLU.GeoS.1612 and its sedimentary features.A, Historical label on the slab.B, Overview of MLU.GeoS.1612, the holotype of Trachelosaurus fischeri; the bracketed lower case letters correspond to the different slabs of which the specimen is composed.The asterisks (*) indicate tetrapod footprints preserved on the slabs.C–D Slab in cross-section, with trough cross-lamination (C) and the two intervals with an arrow pointing at their boundary (D). E Sinuous and ramified structures on the top surface (opposite to the surface preserving bones and footprints), including a close-up on the right. F Round structures.G Squared structure.H Tetrapod footprint (left manus) previously mentioned in Broili and Fischer (1918). Structures in F–H are in convex hyporelief,and their location is marked with matching upper case letters on the overview of the specimen in B

Fig. 3 Overview of MLU.GeoS.1612.A–B preserving the majority of the presacral vertebrae and appendicular elements. Cervical vertebrae are indicated with capitalised letters, dorsal and sacral vertebrae are indicated with lower case letters, blue dots indicate bifurcated cervical ribs.The element indicated with an asterisk represents a badly broken cervical vertebra, which could be identified based on plate 31 of Broili and Fischer (1918). The two ribs indicated with † and ‡, respectively, correspond to those listed in Table 4. Abbreviations: aue,autopodial element; cav, caudal vertebra; fe, femur; fr v, fragmentary vertebra; il, ilium; mt, metatarsal; sv, sacral vertebra

Fig. 5 Close-ups of the cranial elements of Trachelosaurus fischeri. A Right premaxilla in lateral view. B The posterior of the two preserved premaxillary teeth. C Possible nasal. D Possible postorbital. E Possible postfrontal. Abbreviations: anp, anterior process; conc, concavity; latp, lateral process; ponp, postnarial process; pop, posterior process; prnp, prenarial process

Fig. 6 Close-ups of selected cervical vertebrae and ribs of Trachelosaurus fischeri. A Anteriormost preserved cervical vertebrae (elements A–C), including the tentatively identified axis and possible atlantal elements. B Cervical ribs with bifurcating distal ends. C Mid-cervical vertebrae (elements L–N). Abbreviations: at, atlantal element; atna, atlantal neural arch; atr, atlantal rib, axc, axis centrum; axna, axis neural arch; cap, capitulum; cv,cervical vertebra; cvr, cervical rib; diap,diapophysis;epp, epipophysis; feap,free-ending anterior process; fs,fish scale; lam, lamina; ns, neural spine; pap, parapophysis;pdp, posterodorsal process; poz, postzygapophysis; prz, prezygapophysis; pvp, posteroventral process; rug, rugosity; tub, tuberculum

Fig. 7 Close-ups of selected dorsal and sacral vertebrae and ribs of Trachelosaurus fischeri. A Anterior and mid-dorsal vertebrae (elements a-o). B Posterior dorsal vertebrae (elements s-u). C Anteriormost dorsal rib. D Posteriormost dorsal vertebrae and sacral vertebrae.Abbreviations: cap, capitulum; ce, centrum; diap, diapophysis; lam, lamina; na, neural arch; nc, neural canal; ns, neural spine; pap, parapophysis;pcdl, posterior centrodiapophyseal lamina; podl, postzygodiapophyseal lamina; poz, postzygapophysis; prdl, prezygodiapophyseal lamina; prz, prezygapophysis;rf, rib facet; rug, rugosity; sv, sacral vertebra; tub, tuberculum; tvp, transverse process

Fig. 8 Close-up of the proximal caudal region and partial gastral basket of Trachelosaurus fischeri. The caudal centra marked with α, β, and γ correspond to the elements that are indicated the same in Table 5. Abbreviations: cac, caudal centrum; car,caudal rib; cav, caudal vertebra; gas, gastralia; prz, prezygapophysis; sv,sacral vertebra; tp, transverse process

Fig. 9 Close-ups of the appendicular elements of Trachelosaurus fischeri. A Right ilium in lateral view and tentatively identified metetarsal. B Left femur in ventral view. C Right (?) pubis in medial(?) view. Abbreviations: ac, acetabulum; af, articular facet; cv, cervical vertebra; for, foramen; ilb, iliac blade; isp, ischial peduncle; itt, internal trochanter; mt, metatarsal; pac, postacetabular process; pfo, pubic foramen; pup, pubic peduncle

Fig. 10 Results of the phylogenetic analyses.A RSCT of all MPTs of the analysis of the Spiekman et al. (2021b) matrix (consistency index: 0.35; retention index: 0.53). Bootstrap GC frequencies>50% are listed above each branch and Bremer supports>1 are listed below each branch. B SCT of all MPTs of all analyses of the CoArTreeP matrix under implied weighting (k values = 19–24). No-zero weight symmetric resampling absolute (left) and GC (right) frequencies are listed above each branch