Archaeopteryx sp.

cf. Archaeopteryx sp.

Note.

The specimen is housed in the palaeontology collection of the State Museum of Natural History Karlsruhe ( SMNK) under the registration numbers SMNK -PAL 10,000 a-c GoogleMaps . Following the historical naming convention of Urvögel, based on repository, we propose to informally refer to this fossil as the “Karlsruhe specimen”.

Locality.

Schaudiberg, near Mühlheim, at Mörnsheim (District Eichstätt), Bavaria, south Germany (48°51'18.01"N, 10°59'13.99"E; Fig. 1 View Figure 1 ), “ Fossilien-Besucher-Steinbruch   GoogleMaps ”, at that time Grundstücksgemeinschaft (common property) Pöschl / Leonhardt.

Horizon.

Mörnsheim Formation, lower Tithonian, undescribed layer, 1.5 m below the marker layer “ Vierte Rosa ” (“ Fourth pink ”; Fig. 2 View Figure 2 ).

Description.

Comments on the orientation of elements. Because the forelimb and the pectoral girdle in particular have undergone considerable changes in orientation during the evolution from early reptiles to birds, a clarification of how we orientate these elements for the description seems necessary. Whereas the scapula is positioned almost vertically with respect to the vertebral column in basal amniotes (see Schwarz et al. (2007)), it is more or less parallel to the vertebral column in birds and derived non-avian paravians (for example, Pei et al. (2017)), including Archaeopteryx ( Wellnhofer 2009) . Thus, the originally anterior side of the scapula becomes the dorsal side, the posterior side the ventral side and so on. We use the bird-like orientation for the description of the scapula, because this seems to be much closer to the real orientation in Archaeopteryx than the original reptile-like orientation of this bone. As for the coracoid, the matter is even more complicated. In modern reptiles, the coracoids are more or less aligned with the scapulae and approach the body mid-line below the latter element. Thus, they have an anterior, posterior, medioventral and laterodorsal edge. In birds, the coracoid is strongly angled towards the scapula (often in a sharp angle of less than 90 °) and the ventral end is twisted against the dorsal end, so that the originally posterior side becomes the lateral side ventrally. This re-orientation and twist of the coracoid seems to have happened gradually during the evolution of birds (see Ostrom (1976); Mayr (2017)). Whereas Ostrom (1976: Fig. 4 B View Figure 4 ) and Carney (2016: fig. II. 8) reconstructed Archaeopteryx with an angle of approximately 90 ° between this bone and the scapula and an almost entirely lateromedially orientated ventral end of the coracoid, Mayr (2017: fig. 1 a) illustrated the coracoid of this taxon still more aligned with the scapula, at an angle of much more than 90 ° and with an anterodorsomedially-posteroventrolaterally orientated distal end. In other basal paravian theropods, in which the shoulder girdle is known in 3 D preservation, the situation varies. In many taxa, such as Velociraptor ( Norell and Makovicky 1999) , Adasaurus ( Perle et al. 1999) , Sinovenator ( Xu et al. 2002) , Mei ( IVPP V 12733 View Materials ; Xu and Norell (2004)) and Sinornithoides ( Russell and Dong 1993) , the coracoid is largely aligned with the scapula, being only markedly flexed medially and slightly angled ventrally. An exception is Buitreraptor , in which the coracoid is more elongate than in other paravians, has a constricted neck, a marked angle towards the scapula and a notable twist ( MPCA 245; Gianechini et al. (2018)); thus, much more resembling the condition in birds, with the ventral end showing a largely mediolateral orientation. However, this is the exception in non-avian paravians and phylogenetic analysis indicates that this condition was acquired convergently to birds in Buitreraptor . Herein, the Archaeopteryx coracoid is considered to have a mediodorsal margin, a laterodorsal margin that articulates with the scapula, a lateroventral margin and a medioventral margin that articulated with the (still unknown) sternum.

As for the bones of the forelimb, the re-orientation of the pectoral girdle and the limb also has consequences for the orientation of these elements. Thus, in non-avian dinosaurs with an erect limb posture, usually a proximal and distal end and anterior, medial, posterior and lateral sides are distinguished, whereas the laterally-held stylopod and zeugopod of birds have an anterior, dorsal, posterior and ventral side. Here we adhere to the orientational terms for non-avian dinosaurs, also to avoid confusion with the orientation of the humerus in other reptiles (for example, lepidosaurs), in which the dorsal side of this bone corresponds to the posterior side in non-avian dinosaurs and not to the lateral side (as is the case for the dorsal side of the humerus in birds).

Furcula (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, 6, 7, 12 A, A’). The furcula is seen in anterior aspect. Despite missing the middle part of the U-shaped bone, the tip of the ventral ramus, the ventral margin of the right ramus and fragments of the right terminus, the external mould allows for a precise reconstruction. The furcula is oval in cross-section, with its height about twice its depth. The aperture angle of the U is about 80 °. The slight difference of the aperture angle with other Archaeopteryx specimens (London, Daiting and Thermopolis) is likely due to compaction. The right terminus of the furcula lies in articulation with the acromial region of the scapula as revealed by the digital reconstruction.

Scapula (Figs 5 A View Figure 5 , 6 View Figure 6 , 7 View Figure 7 , 12 A, A View Figure 12 ’). Of the right scapula, only the collum and the anterior half of the glenoid facet is preserved, exposing its medial face. The scapula has rotated anterodorsally against the coracoid at an angle of about 15 ° in a way that the facies articularis coracoidei is now facing the facies articularis humeralis coracoidei. Of the scapular blade, only a partial external mould of the ventral margin is visible. As in other specimens of Archaeopteryx (for example, Mayr et al. (2007); Tischlinger (2009); Rauhut et al. (2018)), a dorsally expanded acromial process is absent and the dorsal margin of the supraglenoidal region is continuous with the dorsal margin of the shaft. Instead, an anteriorly directed acromial process is present, a remnant of which is preserved below the coracoid, as is seen in the tomography scans (Figs 5 A View Figure 5 ’, 12 A, A’). These scans also reveal that the base of this process bulged slightly laterally, as is also the case in the Thermopolis specimen ( Mayr et al. 2007). The medial face dorsal to the dorsal lip of the facies articularis humeralis is concave and shows a slightly thickened medial margin. This part likely represents a constriction at the ventral base of the acromial region that separated the process from the facies articularis coracoidalis. Ventral to this concavity, the anterior margin of the facies articularis coracoidalis becomes evenly convex, forming an elongate ovoid articular face, which is set off from the medial face of the corpus by a low ridge and probably corresponds to the tuberculum coracoideum of modern birds.

With the exception of the above-mentioned concavity, the facies articularis coracoidalis is missing. The dorsal third of the scapular corpus is damaged. The facies articularis humeralis is separated from the facies articularis coracoidalis by a strongly convex bulge and stands at an angle of about 160 ° to the facies articularis coracoidalis. The articular surface of the facies articularis humeralis scapulae is characterised by a shallow oval depression surrounded by a blunt wall on the medial side. As in most paravian theropods and other specimens of Archaeopteryx (for example, Tischlinger (2009); Carney (2016)), the actual articular surface was on the lateral side of the bone and faced lateroventrally.

Coracoid (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, 6, 7, 12). The coracoid is seen in internal aspect. The facies articularis scapulae and the surrounding compacta of the foramen coracoideum are damaged. The compacta of the mediodorsal third of the medial face of the bone is also missing. Most of the mediodorsal half of the facies articularis sternalis is preserved as an external mould. Despite the damage, the shape of the coracoid can be reliably reconstructed.

The bone is hatchet-shaped, with its dorsal margin being two-thirds the length of the facies articularis sternalis. The facies articularis scapularis at the laterodorsal margin of the bone is broken, but appears to have been confluent with medial side of the facies articularis humeralis coracoidei, which has the same length as its scapular counterpart as preserved. The surface of the facies articularis humeralis coracoidei is slightly concave transversely and slightly expanded by a medial lip. The width of this lip increases towards the lateral margin of the coracoid. The outline of the articular face itself is difficult to assess, especially as it is exposed in medial aspect. As in other paravian theropods, most of the articular facet probably faced laterally. It appears to have been a rounded trapezoid in outline, with its ventral margin being one-third wider than its dorsal one. The dorsal three-fourths of the articular face are slightly transversely concave.

As is concluded from the remnants, the mediodorsal margin of the bone must have been evenly convex, curving into the evenly convex sternal (medioventral) margin. The lateral margin of the coracoid is concave. Near its ventral termination, the margin abruptly continues into a blunt and short process (“ sternal process ” of Norell and Makovicky (1999); corresponds to the lateral process of modern birds; see also Mayr et al. (2007)), which also forms the lateral continuation of the facies articularis sternalis. A similarly offset sternal process is also present in Archaeopteryx , Jeholornis and some dromaeosaurids ( Ostrom 1974; Norell and Makovicky 1999; Zhou and Zhang 2002; Mayr et al. 2007). A blunt ridge is present along the posterior side of the lateroventral margin of the coracoid. It is slender and tapers towards the sternal process. The facies articularis sternalis is convex with an increasing curvature in its lateroventral third.

The medioventrally-laterodorsally orientated, oval foramen supracoracoideum pierces the centre of the laterodorsal fourth of the coracoid. Despite the flaked-off ventral margin and surrounding compacta, its shape is clearly visible. Its width is about two-thirds its height.

The tomographic scans reveal a prominent disc-like structure between the coracoid and humeral head, which is still embedded in the matrix. The structure is placed in the dorsal third of the lateral margin and emerges from the anterior surface of the coracoid right next to the lateral margin. The structure is dorsoventrally elongated and bears a convex margin, the tip of which protrudes beyond the humeral head (Fig. 12 A, A View Figure 12 ’). However, its lateral position is probably due to compaction, while in vivo the structure would probably be facing more anterolaterally. Based on its positions, the structure could correspond to the biceps tubercle (coracoid tubercle), resembling that of Archaeopteryx in shape and size ( Mayr et al. 2007; Carney 2016: fig. II. 8).

Humerus (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, 6, 9 B – B’ ’). The right humerus is almost complete, but massively compacted and is seen in posterolateral aspect. It has a maximum length of 57.8 mm (Table 1 View Table 1 ), which is most similar to that of the Munich specimen (57.5 mm; Wellnhofer (2009)). In the middle of the articular facet of the humeral head, there is a shallow concavity that likely represents the contact face with the glenoid fossa. The articular facet of the humeral head is separated from the internal tuberosity by a shallow depression. The proximally rounded semi-ovoid internal tuberosity is proximodistally elongate and extends for about half the length of the humeral neck. Distally, it merges gradually with the medial margin of the shaft.

The proximal part of the humerus that houses the deltopectoral crest and the internal tuberosity is angled at about 30 ° against the shaft, which is almost identical to Archaeopteryx , but different from Alcmonavis ( Rauhut et al. 2019) . Anterolaterally, the humeral head narrows anteroposteriorly and continues into the evenly convex deltopectoral crest that extends over the proximal fourth of the humerus. At its distal end, the deltopectoral crest terminates in a low convexity that further distally sharply turns into the anterolateral margin of the shaft. The distal terminal flange is set off by a small, curved depression, which lies in continuation with the anterolateral margin of the shaft and curves towards the rim of the deltopectoral crest. It appears that this structure slightly warps into the sediment, but is mostly pressed flat. The convexity lies level with the attachment facet of m. pectoralis, which lies on the other side of the humerus in Alcmonavis and in modern birds ( Rauhut et al. 2019; Fig. 5 A View Figure 5 ’). As revealed by the scans, a true facet for the m. pectoralis is not observable on the lateral edge of the anteromedial side of the deltopectoral crest (Fig. 5 A View Figure 5 ’).

The anterolateral margin of the humeral shaft is slightly concave until the lateral condyle, where the concavity increases. The low bulge distal to the deltopectoral crest is due to compaction. Posteromedially, the internal tuberosity continues on to the humeral shaft with only a slight inflection of its medial margin, unlike the more offset tuberosity in the Thermopolis specimen of Archaeopteryx , for example ( Mayr et al. 2007: fig. 10 b). The posterolateral margin of the humeral shaft runs straight for about half the extent of the deltopectoral crest, then becomes concave until its mid-length. Distal to this point, the posterior margin of the humerus is slightly convex, with a short and shallow concavity proximally adjacent to the medial condyle.

The compacted distal condyles of the humerus are facing anterodistally. The lateral condyle appears regularly ball-shaped and is seen in posterolateral aspect. The medial or radial condyle has about the same size as the lateral one, with a stronger curvature, as is seen in the tomography image.

The fragment of the left humerus (Fig. 9 B-B View Figure 9 ’’) comes from the mid-shaft area. Due to its massive compaction, no anatomical details are preserved.

Radius (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, B, B’, 6, 8, 9 A – A’ ’). The proximal articular head of the right radius is overlain by the distal articular end of the humerus and is impacted (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, 6, 8). The bone exposes its posterior face and is 51.3 mm long. The tomography scans reveal that the proximal end bears a small, medially directed triangular process, which according to its topographical position, may represent the tuberculum bicipitale radii, similar to Alcmonavis , Archaeopteryx , Bambiraptor , Confuciusornis and Cratonavis ( Chiappe et al. 1999; Burnham 2004; Rauhut et al. 2019; Li et al. 2023; Fig. 5 A View Figure 5 ’). However, a compression artefact cannot be ruled out with certainty.

As preserved, the shaft of the radius is half as wide as that of the ulna, until the middle of the bone. From there, the lateral and medial margins diverge until the distal articular head, which is one-fourth thicker than the narrowest diameter of the shaft. The posterior face of the radius is marked by a longitudinal furrow that terminates at the proximal fifth of the bone. The distal terminus of the furrow cannot be identified due to the collapsed compacta. The cross-section shows that the furrow is evidently a result of impaction (see below, Fig. 14 View Figure 14 ). The articular face of the condyle shows multiple punctures, but appears to have been markedly convex mediolaterally.

Only the shaft of the left radius is preserved (Figs 5 B View Figure 5 , 9 A – A View Figure 9 ’’). The proximal end of the bone expands medially, preserving the transition to the articular facet, but not the facet itself. As in the other radius, the medial expansion could indicate the presence of a tuberculum bicipitale radii. However, its morphology is hard to evaluate because the proximal end is missing. The bone is almost straight and shows a longitudinal furrow, which, at both ends, vanishes in fragmented compacta. Like on the contralateral radius, the furrow results from diagenetic impaction (see below, Fig. 14 View Figure 14 ). Whether or not the longitudinal grooves that are also described for Alcmonavis , Jeholornis and various Enantiornithes ( Chiappe and Walker 2002; Sanz et al. 2002; Hu et al. 2015; Rauhut et al. 2019) are similarly caused by diagenetic impaction needs to be investigated.

Ulna (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, B, 6, 8, 9 A – A’ ’). The right ulna (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, B, B’, 6, 8, 9 A – A’ ’) is seen in medial aspect and is massively compressed. Its length is 52.8 mm, which is most similar to that of the Munich specimen (53.5 mm; Wellnhofer (2009)). The shaft of the ulna has a sigmoidal curvature, with the proximal half being posteriorly convex and the distal half being anteriorly convex. The proximal extremity is marked by a blunt, short and evenly rounded olecranon tubercle. The proximal terminus of which is slightly abraded, but likely had a semicircular outline in the preserved view. The proximalmost part of the shaft is parallel-sided. The trochlear notch as well as the radial tubercle are obscured by the dorsal condyle of the humerus. The narrowest point of the shaft is in its middle. The anterior third of the distal extremity – as reconstructed from the distal part of the ulnar shaft – is overlain by the distal fifth of the radius. The surface of the ulna bears four knob-like elevations that, at first glance, resemble quill knobs (but see below: Fig. 13 View Figure 13 ).

The bone fragment of the left ulna is seen in medial aspect and parallels the radius fragment on the same slab (Figs 5 B, B View Figure 5 ’, 9 A-A’ ’). Both articular ends are missing. Distally, the ulna fragment is broken at the base of the distal curvature. Three small, circular knobs are seen, the position of which is almost identical to the three proximal ones preserved on the right ulna. Tomography evidence reveals that these knobs are crystalline, likely calcitic diagenetic artefacts. These framboid-like crystals must have precipitated prior to compaction. Similar knobs are present on the proximal end of the right humerus and the left femoral shaft, as well as on the humerus of Alcmonavis ( Rauhut et al. 2019) , indicating that this artificial knob formation is a taphonomic quality of the Mühlheim locality.

? Carpal (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, 6, 8). Adjacent to the distal fifth of the right radius, there is a roughly sub-quadratic bone fragment. Due to its shortness as preserved, it likely represents one of the proximal carpal bones, possibly the radiale. Unfortunately, the preservation of the carpal is too poor for an exact identification. While the assumed proximal face of the carpal is exposed and abraded, its respective distal face, which is embedded in matrix and, thus, only visible in the tomography (Figs 5 A, A View Figure 5 ’), is preserved in three dimensions. On its assumed anterior face, there is a rounded process that covers half the anterior margin of the bone and is overlain by the radius (Figs 5 A, A View Figure 5 ’). On the respective anterodistal corner of the bone, a pointed triangular process arises, which has barely half the height of the anteroproximal one and covers about one-sixth of the anterior face of the bone. The assumed anterior face between the two processes is deeply concave. This concavity might have accommodated the distal articular end of the radius. The assumed proximal face of the carpal bears a shallow depression.

Metacarpals (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’ 6, 8). The proximal parts of the right metacarpals (mc) II and III are preserved in dorsal aspect. Mc II is about one-third thicker than mc III. The distal third of both bones is missing. Mc II is proximally ovelain by the distal articular head of the radius. Mc III is distally partially overlain by the basal phalanx of digit I. The shaft of mc II bears a longitudinal impaction furrow on its dorsal side. The proximal head of mc III is a transverse ovoid joint, which is angled at about 94 ° against the shaft in the posterior direction. That of mc II is partially broken and covered by sediment. Nothing more can be said because the bones are badly crushed.

Phalanges (Figs 4 View Figure 4 , 5 A, A View Figure 5 ’, 6, 8, 14). Of digit I, about two-thirds of the proximal portion of the first phalanx are preserved in dorsal aspect. The bone bears a fine longitudinal impaction furrow on the dorsal side (see below, Fig. 14 View Figure 14 ). The proximal articular facet of phalanx I- 1 is orientated perpendicularly with respect to the shaft and is set off by a blunt flange, which is one-third wider than the distally adjacent part of the shaft. The posterior half of the distal extremity of the articular face is slightly abraded. The proximal articular head is separated into two condyles by a sulcus that continues as a groove with converging margins on to the undamaged part of the shaft and finally vanishes in a fracture zone. The posterior condyle is visible in posterior aspect and is evenly rounded.

The penultimate phalanx of digit II is completely preserved and seen in lateral aspect, in which the palmar surface is facing away from the radius. It is 19 mm long. The proximal articulation facet is slightly expanded in the palmar direction. The shaft is slightly bent dorsally in its distal third. Like in the other manual bones, longitudinal grooves on the shaft are visible, but are the result of diagenetic impaction. The reconstructed cross-section of the mid-shaft of this phalanx was almost circular (Figs 6 View Figure 6 , 8 View Figure 8 , 14 View Figure 14 , ventral impaction furrow faintly also seen in Fig. 5 A View Figure 5 ’). The distal third of the phalanx is strongly compacted. Its end bears a ginglymus, which is abraded. Laterally, a large ligament pit is present.

A manual ungual is exposed in lateral aspect on the lateral side of the distal humerus. The proximal articular facet of the ungual is missing, including part of the flexor tubercle, so that standard measurements cannot be taken. The preserved portion has a maximum length of 9.8 mm. However, even this partial length suggests that this ungual is too large to belong to manual digit I or III (9.2 and 6.7 mm long in the similarly sized Munich specimen; Foth and Rauhut (2017)) and its closer proximity to penultimate phalanx II corroborates this identity. A chisel mark pierces the dorsal margin of the ungual level with the flexor tubercle. Additionally, the proximal third of the ungual is compacted. The proximodorsal edge is covered by the humerus. Due to damage in this region, the tomography scans cannot confirm if an ungual lip was present. The ungual is strongly curved, with an aperture angle of the claw arcade being 110 °. The lateral furrow as preserved begins proximally in the ventral third and runs parallel to the ventral margin of the bone. It terminates at the dorsal margin of the ungual close to its tip. There are no remains of a keratinous sheath.

The PCA (Fig. 15 A View Figure 15 ) demonstrates high variability in the morphology of manual unguals in Archaeopteryx and Anchiornis . The greatest factors of variation amongst the specimens included in this analysis (PC 1: 51.13 %, PC 2: 22.39 %) correspond to curvature, dorsoventral height, position of the proximal extent of the dorsal curvature and the morphology of the flexor tubercle. Although these characteristics explain more than 70 % of the total morphological variance captured by the landmarks used here, there is pronounced overlap in morphospace. This demonstrates that there is more intrataxonomic than intertaxonomic variation and suggests that curvature, height, position of the ungual lip and flexor tubercle morphology may make poor characters for taxonomic assignment in these taxa.

Due to the high level of intraspecific variation regarding the morphology of the manual unguals, a CDA is required to separate taxonomic groups based on ungual shape (Fig. 15 B View Figure 15 ). The analysis conducted here diagnoses the Karlsruhe specimen as having stronger affinities for Archaeopteryx than Anchiornis . The same is true for the Mühlheim ( Alcmonavis ) and Haarlem ( Ostromia ) specimens. The manual unguals of Anchiornis are characterised by a shallower ungual curvature, a more distally positioned flexor tubercle and being proximodistally shorter. The manual unguals of Archaeopteryx are characterised by a deeper ungual curvature, a more proximally positioned flexor tubercle and being proximodistally longer.

Hind limbs (Figs 5 C, C View Figure 5 ’, D, D’, 10, 11). The shafts of both femora are preserved, but the articular ends are missing in both elements. Both femoral shafts show a slight medial curvature. The distal half of the shaft of the left femur is pulverised (Figs 5 D, D View Figure 5 ’, 11). The cnemial area is devoid of any identifiable structure. Remains of the left tibial and probably the fibular head, as well as the proximal-most parts of their shafts, are in association with the respective femur.

The better-preserved shaft of the right femur is seen in posterior aspect (Figs 5 C, C View Figure 5 ’, 10). The posterior intercondylar sulcus extends about one-third of the shaft. The condylar area is missing as is the proximal articular end of the bone. As preserved, the cnemial articulation is about twice as wide as the narrowest part of the corresponding shaft.

Of the right tibia, only the proximal end is preserved, and is seen in posterior aspect (Figs 5 C, C View Figure 5 ’ 10, 11). The condylus medialis is subcircular in outline and a little larger than the transversely oval condylus lateralis. The surface immediately distal to the condyli is impacted. The lateral margin of the tibia is formed by the shallowly concave posterior side of the crista fibularis, which begins shortly distal to the condylus lateralis and is terminated distally by a matrix fracture.

Of the right fibula, only the proximal-most fragment is partially preserved, with the articular head missing (Figs 5 C, C View Figure 5 ’, 10, 11). The bone parallels the tibia. Like in the left hind limb, the crural elements are at a right angle with the respective femur.