Dacentrurus armatus Owen, 1875

Dacentrurus armatus Owen, 1875

Synonymy

Omosaurus armatus Owen, 1875

Stegosaurus armatus Lydekker, 1888

Omosaurus lennieri Nopcsa, 1911a

Dacentrurus lennieri Hennig, 1915a

Dacentrurosaurus armatus Hennig, 1925

Miragaia longicollum Mateus, Maidment & Christiansen, 2009 Dacentrurus longicollum Raven & Maidment, 2017

Holotype: NHMUK OR46013, a partial skeleton, including axial and appendicular elements, and osteoderms ( Owen 1875, Galton 1985).

Type locality and horizon: The exact horizon is unknown. The specimen was discovered in a clay pit at the Old Swindon Hill belonging to the Swindon Brick and Tile Company, Swindon, Wiltshire, UK. Lower part of the Kimmeridge Clay Formation, Upper Jurassic (Kimmeridgian) ( Galton 1985, Martill et al. 2006) .

Referred material: MHNH A ( Nopcsa 1911a, Galton 1991), ML 433(Mateus etal. 2009), ML 433-A(Mateus etal. 2009), MG 4863 (CostaandMateus 2019),thePedrasMuitas( MSGPA)specimen ( Galton 1991, Escaso 2014), the Murteiras ( MSGP B) specimen ( Galton 1991, Escaso 2014), the Atalaia ( MSGP D) specimen ( Galton 1991, Escaso 2014), SHN.LPP 016 ( Escaso et al. 2007a), the RD-10 (Barrihonda-El Humero) specimens ( Cobos et al. 2010, Sánchez-Fenollosa et al. 2022), the CO (Cerrito del Olmo) specimen ( Casanovas-Cladellas et al. 1995), and the CT-28 ( San Cristóbal) specimen ( Figs 2–9) (for more details, see the Supporting Information file).

Locality and horizon of the referred material: All specimens are known from the Upper Jurassic of Europe (for more details, see the Supporting Information file).

Locality and horizon of the CT- 28 specimen: San Cristóbal (CT-28) site in the municipality of El Castellar, province of Teruel, Aragón, Spain. Maestrazgo Basin, Peñagolosa sub-basin, Villar del Arzobispo Formation , Upper Jurassic ( upper Kimmeridgian – Tithonian) ( Fig. 1) .

Emended holotype diagnosis: Anterior caudal vertebrae with (1) short neural spines and expanded and rounded apices; ilium with (2) a wide and short preacetabular process; and (3) a broad base of the preacetabular process and a smooth curvature between the anterior margin of the sacral yoke and the dorsal margin of the preacetabular process; and pubis with (4) a dorsoventrally expanded anterior end of the prepubis.

Supplementary diagnostic characters based on the referred specimens: A premaxilla with (1) an anterior tip that drawn into a point (based on ML 433 ); and (2) an anterolateral margin ventrally projected (based on ML 433 ); a cervical series with (3) at least 17 cervical vertebrae (based on ML 433 ); and (4) at least anterior and mid cervical ribs fused to the vertebrae (based on MHNH A, ML 433 , MG 4863 , the RD- 10 specimen, and the CO specimen); cervical vertebrae with (5) two spinopostzygapophyseal laminae that extend anterolaterally from the top of the postzygapophyses to both sides of the base of the neural spine and culminate on its anterior margin (based on ML 433 , MG 4863 , and the CO specimen); and mid and posterior cervical vertebrae with (6) neural spines positioned in the anterior half of the centrum (based on ML 433 , MG 4863 , and the CO specimen) .

Remarks: Dacentrurus armatus also differs from other Late Jurassic stegosaurs in having dorsal vertebrae with (1) wider than long centra (present in Adratiktlit boulahfa Maidment, Raven, Ouarhache and Barrett, 2020 from the Middle Jurassic of Morocco); and ischium with (2) a straight dorsal surface of the distal shaft (present in Yanbeilong ultimus Jia, Li, Dong, Shi, Kang, Wang, Xu and You, 2024 from the Lower Cretaceous of China).

Description

Axial skeleton

Dorsal vertebrae: The dorsal series was found in almost full articulation and comprises 10 free dorsal vertebrae ( Figs 2–4) and two dorsosacral vertebrae ( Fig. 5). The most anterior dorsal vertebra is considered the second vertebra in the dorsal series. Dorsal vertebrae D2–D9 are in the site and retain the original position in which they were found ( Figs 2, 3). In contrast, D10 and D11 were extracted and isolated ( Fig. 4). DS1 and DS2 are fused to the sacrum, forming the synsacrum ( Fig. 5).

The dorsal centra are amphicoelous, wider than tall and long (Supporting Information, Table S1), taller than long (Supporting Information, Table S1), heart-shaped (or subcircular), with smooth concentric ridges in the articular facets, and some present a smooth ventral keel. The lateral surfaces are gently concave anteroposteriorly and gently convex dorsoventrally. As we advance in the series, the centra are proportionally longer (Supporting Information, Table S1). The pedicels in the anterior dorsal vertebrae are short ( Fig. 3A, B), whereas those in mid and posterior dorsal vertebrae are proportionally more elongated ( Figs 3C–E, 4). A dorsoventrally directed medial keel is present on both the anterior and posterior surfaces of the pedicels ( Figs 3, 4). The keel is more pronounced in the posterior surfaces and strongly expanded posteriorly in the most posterior dorsal vertebrae of the series ( Fig. 4B, G). The neural canals are in the bottom half of the pedicels. Although their morphology cannot be observed correctly in several vertebrae, they appear as teardrop-shaped in anterior view and subcircular in posterior view ( Figs 3B, 4). The parapophyses are not well preserved in most of the vertebrae, but are subcircular or oval in outline, apparently larger in the mid dorsal vertebrae, and more dorsally situated (until at the same level of prezygapophyses) as we advance in the series ( Figs 3C, 4). The prezygapophyses can be observed laterally in CT-28-142 ( Fig. 3C), CT-28-143 ( Fig. 3C), CT-28-144, and CT-28-146 and totally in MAP-4575 and MAP-4576 ( Fig. 4). In general, the prezygapophyses do not extend or slightly extend beyond the anterior articular facet of the centrum. The flat and oval articular facets of the prezygapophyses face dorsomedially and have a dorsal process located at the posterior region of the lateral margins in some vertebrae. This dorsal process is completely preserved in the left prezygapophysis of CT-28-143 ( Fig. 3C) and CT-28-144. Although being fragmented, its presence can be noted at least in CT-28-142 and MAP-4576 ( Figs 3C, 4G). The postzygapophyses are approximately located at the same level as the prezygapophyses and project beyond the posterior articular facet of the centrum. Similar to those in the prezygapophyses, the articular facets of the postzygapophyses are flat and oval; however, they face lateroventrally and are posteriorly separated by a wide groove ( Figs 3, 4C, H). The diapophyses extend at a variable angle along the dorsal series, with a greater angle to the horizontal in the mid dorsal vertebrae and lower in the anterior and posterior dorsal vertebrae. The cross-section of the diapophyses is triangular (T-shaped) ( Fig. 3C), excluding the last two free posterior dorsal vertebrae (MAP-4575 and MAP-4576), in which the diapophyses are dorsoventrally compressed ( Fig. 4), in a similar manner to the diapophyses of the dorsosacral vertebrae ( Fig. 5). Finally, the neural spines are lateromedially compressed but with transversely expanded apices ( Figs 3, 4).

Two dorsosacral vertebrae are fused to the sacrum, forming the synsacrum ( Fig. 5). Compared with those of the free dorsal vertebrae, the centra of the dorsosacral vertebrae are considerably smaller (Supporting Information, Table S1). The centrum of DS1 is wider than tall and long (Supporting Information, Table S1). The anterior articular facet is flat and heart-shaped ( Fig. 5A). The ventral surfaces of both centra are heavily eroded; therefore, the presence of a ventral keel cannot be ascertained ( Fig. 5F). In DS1, the pedicel is slightly developed, and the neural canal is wide and subcircular, possibly affected by taphonomic processes ( Fig. 5A). The prezygapophyses are like those present in free dorsal vertebrae, although the articular facets face with a lesser angle, being more laterally projected. The dorsoventrally compressed diapophyses are L-shaped in anterior view ( Fig. 5A) and are partially fused to each other and the first sacral rib. Finally, the neural spines are short, lateromedially compressed with slightly transversely expanded apices, and fused to each other and to the rest of the synsacrum ( Fig. 5).

Dorsal ribs: Several partial dorsal ribs and fragments were found. Most of them are still in the field next to the dorsal vertebrae ( Fig. 2). They consist of proximal, medial, and distal parts that come from the anterior to posterior region of the ribcage, and from the left and right sides. In general, they are curved medially, with a reduced tuberculum, well-developed capitulum, and long shaft. The capitula are anteroposteriorly compressed and have a slightly expanded proximal end, with a rugose articular facet. As we advance in the series, the capitula tend to elongate, become more slender, and lie at a greater angle to the horizontal ( Fig. 2). The tubercula are anteroposteriorly compressed, and their articular facets are slightly expanded and rugose. The crosssections of the proximal and distal halves of the shaft are triangular (T-shaped) and oval, respectively.

Sacral vertebrae and ribs: The synsacrum consists of two dorsosacral vertebrae and five sacral vertebrae (sacrum) ( Fig. 5). The sacral centra are co-ossified but can be distinguished by intercentral sutures that represent the junctions between articular facets ( Fig. 5F). As we advance in the series, the centra are wider and shorter (Supporting Information, Table S1). A ventral keel is absent ( Fig. 5F). Dorsally, as also occurs with the centra, the neural spines are fused together ( Fig. 5C–E). The spines are short and lateromedially compressed, but with rounded and transversely expanded apices ( Fig. 5A–E). S5 ( Fig. 5B) exhibits a notably wider than tall (Supporting Information, Table S1), slightly concave, and oval posterior articular facet. The pedicels are short, with a subcircular neural canal that occupies the entire surface. In both laterals, the sacral rib is completely fused to the centrum up to the level of the postzygapophyses, which do not extend beyond the posterior articular centrum facet and have their flat and oval articular facets facing lateroventrally.

There are five sacral ribs that are anteroposteriorly expanded medially and laterally ( Fig. 5F). They are fused medially to the centra and laterally to each other ( Fig. 5F). All the sacral ribs are slightly posterolaterally projected. However, the angle of inclination of the ribs increases from the first to the last sacral rib, being almost vertical in the fourth and fifth ribs ( Fig. 5F). The sacral ribs are separated from each other by four oval fenestrae, the smallest of which is located between the first and second sacral rib, and the largest is between the second and third sacral rib, very similar in size to the fenestrae located between the third and fourth sacral vertebrae ( Fig. 5F). In dorsal view, the neural arches of the dorsosacral vertebrae and the dorsal surfaces of the sacral ribs are co-ossified, forming a solid and flat sacral yoke ( Fig. 5E).

Caudal vertebrae: The anterior caudal series comprises the first (MAP-4577) ( Fig. 6A–E), second (MAP-4578) ( Fig. 6F–J), third (MAP-4579) ( Fig. 6K–O), fourth (MAP-4580) ( Fig. 7A–E) and fifth (MAP-4581) ( Fig. 7F–J) caudal vertebrae. The centra are wider than tall and long (Supporting Information, Table S1), taller than long (Supporting Information, Table S1), oval or heart-shaped, and have smooth concentric ridges in the articular facets. As we advance in the series, the centra are proportionally longer (Supporting Information, Table S1), with a less oval and more heart-shaped morphology. C1 ( Fig. 6A–E) is platycoelous (or slightly opisthocoelous), whereas the following vertebrae are slightly amphicoelous ( Figs 6F–O, 7A–J). Two symmetrical and large foramina can be observed in the anterior articular facet of C1 ( Fig. 6A). The lateral surfaces of the centra are slightly concave, with transverse processes in their upper halves. The ventral surface of the five centra lack a keel, groove, and chevron facets. C1 has very dorsoventrally expanded transverse processes, with closed proximodorsal canals ( Fig. 6A, C). Theyareposterolaterallydirected, anteroposteriorlycompressed, and triangular. Both transverse processes of C2 are incomplete, but very similar to those of C1, although less dorsoventrally expanded and anteroposteriorly compressed. The proximodorsal canals are open in C2 (although we do not discard the possibility that they are broken) ( Fig. 6F, H). In contrast, the subtriangular transverse processes of C3–C5 are posteroventrally directed, neither dorsoventrally expanded nor anteroposteriorly compressed, distally tapered, and retain a dorsal process in the proximal region ( Figs 6K, M, 7A, C, F, H). The pedicels are slightly dorsally developed but strongly lateromedially expanded. The neural canals have a subcircular outline in anterior view and subtriangular or oval outline in posterior view. The prezygapophyses project dorsoanteriorly and extend beyond the anterior centrum facet. The articular facets of the prezygapophyses face dorsomedially and are flat and oval. However, the flat and oval articular facets of the postzygapophyses face ventrolaterally and do not extend beyond the posterior facet of the centrum. Finally, the neural spines are short, increasing slightly in length from C1 to C5 (Supporting Information, Table S1), with rounded and transversely expanded apices.

MAP-4582 is the only preserved caudal vertebra from the posterior region ( Fig. 7K–O). The centrum is amphicoelous, wider than tall and long (Supporting Information, Table S1), taller than long (Supporting Information, Table S1), and strongly heart-shaped (apple-shaped). Marked concentric ridges are observed in both anterior and posterior facets of the centrum ( Fig. 7K, M). The lateral surfaces are concave, with small transverse processes in the upper half of the centrum ( Fig. 7L). In ventral view, a wide groove and an incomplete chevron fused in the posterior margin are present ( Fig. 7O). The pedicels are short and narrow. The neural canal is taller than wide, and oval in outline ( Fig. 7K, M). The prezygapophyses are finger-like and project anteriorly beyond the anterior centrum facet, with articular facets facing dorsomedially. The postzygapophyses and the neural spine are not preserved.

Pelvic girdle

Ilium: Two ilia are preserved and not fused to the synsacrum ( Fig. 8). Both ilia are mostly broken, but some relevant characters are distinguishable. The preacetabular processes of both ilia are partially preserved and are short and wide ( Fig. 8A, C, D). The anterior tip is broad and rounded. Posteriorly, the preacetabular processes become slightly wider, and the dorsal and lateral surfaces are divided by a more distinct break in slope ( Fig. 8B, C). The lateral margins are concave in dorsal and lateral views and nearly straight in ventral view. The pubic peduncle is prominent and projects anteroventrally ( Fig. 8B, C). The acetabular and postacetabular regions are preserved only in the right ilium ( Fig. 8E). The ischial peduncle is low and its articular facet is rugose, slightly concave, and oval. The supracetabular process is partially preserved, large, and presumably rounded. Finally, the postacetabular process is short and rounded, with a concave ventral surface.

Ischium: CPT-4291 is a right ischium ( Fig. 9A, B). It is subtriangular in lateral and medial views. The acetabular margin is concave and separates the iliac peduncle from the pubic peduncle. The iliac peduncle is subrectangular and wider anteroposteriorly than the pubic peduncle dorsoventrally. It projects dorsoanteriorly, and its articular surface is teardrop-shaped, being narrow in the anterior region and becoming wider posteriorly. The subcircular pubic peduncle is dorsoventrally expanded and projects anteriorly when viewed laterally and medially. The shaft of the ischium is flat, lateromedially compressed, and tapering posteroventrally. Distally, the dorsal margin of the shaft is straight.

Pubis: MAP-4574 is a complete right pubis ( Fig. 9C–F). The prepubis process is short and robust. The proximal end is dorsoventrally expanded and rounded, with a slightly convex and smooth medial surface and a convex and rugose lateral surface. The acetabular process is large and semicircular in lateral view ( Fig. 9D), with a straight ventral margin and convex anterior, posterior, and dorsal margins. The lateral surface is deeply concave ( Fig. 9D), whereas the medial surface is convex ( Fig. 9C). The obturator notch is opened posteriorly ( Fig. 9E, F). It is lateromedially wide and can be observed in posterior, dorsal, and ventral views. In contrast, the postpubis process is long and slender. The distal region is fractured and slightly displaced. The distal end is dorsoventrally expanded and rounded. The lateral surface is rugose and slightly convex, whereas the medial surface is smooth and slightly concave.

Hindlimb

Femur and tibia: The remains of the hindlimb comprise a partial femur (CT-28-127) and a partial tibia (CT-28-130) ( Fig. 2). The posterior surface of the femur is partially broken and displaced, and the proximal end is not preserved. The tibia preserves only the distal region, the posterior surface of which is completely broken, and several fragments of the diaphysis. The bad state of preservation of the hindlimb does not allow us to describe and compare the fossils.