Dryosauridae, Milner and Norman, 1984

Rotatori, Filippo Maria, Moreno-Azanza, Miguel & Mateus, Octávio, 2020, New information on ornithopod dinosaurs from the Late Jurassic of Portugal, Acta Palaeontologica Polonica 65 (1), pp. 35-57 : 38-44

Dryosauridae indet.

Figs. 2–4 .

Material.—ML 1851, an almost complete isolated parietal from Praia da Peralta; ML 2321, two partial associated neural arches from Praia de Porto Dinheiro; ML 768, a partial tooth-bearing isolated dentary, ML 2055, associated femur and tibia from Praia do Zimbral; ML 563, isolated femur from Lourinhã coastline; ML 505, isolated tibia from Praia de Vale Pombas. All from Kimmeridgian–Tithonian, Lourinhã, Portugal.

Description.— Cranial skeleton: Parietal ( Fig. 2A ). An isolated almost complete parietal (ML 1851), missing the right lateral half, is 18 mm in length and 22 mm in width. It appears to be sub-triangular in shape, wider anteriorly than posteriorly. The preserved contact with the frontals is W-shaped, presenting an interdigitated suture as seen in Dysalotosaurus ( Janensch, 1955) . The preserved lateral process projects antero-ventrally, having a smooth concave surface. Posteriorly, the lateral processes converge medially to enclose a deep median notch between steep margins. Towards the midline of the notch, the proximal-most part of the nuchal crest is distinguishable although slightly eroded. The ventral surface is concave, flaring in a deep pit anteriorly until the bone margin. Towards the midline pit, two small depressions are preserved.

Dentary ( Fig. 2B ): ML 768 is a fragment of a right dentary bone (length 30 mm, height 7 mm, latero-medial thickness 8 mm), fractured on both caudal and rostral ends, first reported by Mateus (2006) as aff. Dryosaurus sp. It preserves seven tooth positions and one isolated tooth, two erupting teeth and six roots of already worn down teeth. The alveoli display an intra-alveolar border (character 136: 0), but the close proximity of the preserved crowns indicates the absence of intracrown spaces (character 154: 1). In dorsal view, ML 768 is sinuous in shape and the medio-lateral section is slightly concave-convex, towards the lateral surface. The tooth row is convergent anteriorly and divergent posteriorly (character 122: 1). The lateral surface is smooth, bearing seven visible vascular foramina on three different levels character 145: 0). On the medial surface, a deep Meckelian sulcus runs for the entire length of the tooth-row. The margins of the sulcus are neat and straight. Caudally, on the medial surface, and dorsal to the Meckelian sulcus, the splenial sutural contact is preserved and it exhibits a highly dense capillary vascular system. The ventral surface does not possess a ventral flange (character 121: 0). The two preserved crowns are diamond-shaped (character 155: 1) and mesiodistally expanded (character 135: 1) with a smoothly rounded apex. The enamel is asymmetrically distributed (character 140: 1), being present only on the lingual surface of the crown. The crown margin possesses coarsely serrated hooklike non-mammilated denticles (character 139: 1). On the lingual side of the crown, an apicobasally extended primary ridge (character 158: 1) is slightly distally offset (character 156: 1), being surrounded by faint secondary ridges (characters 141: 1, 159: 2). The ridges on the enamel are restricted to the lingual surface of the crown (character 160: 1). The denticles are generally not confluent with the crown ridges character 142: 0), which are less than 10 in number (character 157: 0). The worn down remains of dentary teeth (character 137: 1) do not show the presence of a cingulum between the crown and the root (character 143: 1).

Axial skeleton: Neural arches ( Fig. 3 ): Two small associated disarticulated neural arches (ML 2321) appear to be fractured and subsequently restored. One is almost complete, preserving both transverse processes (height 22 mm, width 51 mm) and missing part of the neural canal. The other element preserves just the right transverse process (height 19 mm, width 35 mm). Both specimens show a low degree of distortion, due to taphonomic processes. The neural spine is low and blade like, as is common in vertebrae of other basal iguanodontians from the cervical or anterior-dorsal series Norman 2004). Anteriorly, two slender and lobed prezygapophyses are present, positioned at about 45° relative to the horizontal. There is a well-marked constriction between the lobe located on the anterior-most end of the prezygapophyses, and the rest of the bony process. Posteriorly to the prezygapophyses, the prezygapophyseal lamina and the anterior centroparapophyseal lamina form a small notch. On the transverse processes, the diapophyses are broken towards the distal edge, showing a sub-triangular transverse section. The parapophyses, located ventrally to the diapophyses shaft, are constrained in the proximal half of the transverse process. Ventral to the parapophyses, on the lateral sides of the neural arches, a shallow depression is present. The two postzygapophyses, lobed like the prezygapophyses, show the same degree of inclination with respect to the horizontal. On the posterior side, wider and deeper notches, in comparison to the notches present on the anterior side, are formed by the postzygodiapophyseal lamina, posterior centrodiapophyseal lamina and centropostzygapophyseal lamina. The same structure is seen in Valdosaurus ( Barrett et al. 2011) . Ventrally, the two lateral walls of the neural canal, present a rugose surface, indicating an incomplete fused condition.

Appendicular skeleton: Femora ( Fig. 4A, B ): The left femur ML 563 is fragmented and heavily distorted. The proximal and distal ends are slightly eroded and fractured. Another, less distorted right femur (ML 2055) measures 250 mm in total length. In ML 563 the general outline of the femoral shaft is strongly bowed anteriorly (character 248: 0). The proximal epiphysis partially preserves the femoral head, which is separated posteriorly by a shallow depression (inter-trochanteric fossa) from the greater trochanter (character 249: 1). The surface of the greater trochanter is flattened (character 252: 1). A broken blade-like surface, which represents the base of the 4th trochanter, is located in the proximal half of the shaft (character 254: 0). Medially, a collapsed surface overlaps the scar of the insertion of Musculus caudofemoralis longus, which is restricted to the medial surface of the shaft (character 256: 1). The distal epiphysis includes both condyles, with the medial larger than the lateral. The two condyles are divided anteriorly by a shallow, V-shaped extensor intercondylar groove (characters 257: 1, 258: 0) and posteriorly by a deep flexor grove. A small lateral process overhangs on the flexor groove opening (character 259: 1). The medial condyle is square shaped and straight, while the lateral presents a slightly inclined anterior edge, and a conspicuous finger-like posterior process (character 260: 1). The medial condyle protrudes cranially towards the lateral condyle (character 261: 1).

As in ML 563, ML 2055 presents the femoral shaft is bowed anteriorly (character 248: 0), being thick and robust in general proportions. The section is sub-triangular in the proximal and mid part of the shaft, becoming more rounded towards the distal epiphysis. The proximal epiphyisis preserves part of the femoral head with a constriction between it and the greater trochanter (character 249: 1). What is preserved of the greater trochanter shows a slightly convex surface (character 252: 0). On the medial surface of the shaft, positioned towards the mid-shaft but slightly proximal (character 254: 0), is a blade, like 4th trochanter (characters 253: 1, 255: 0) that is directed medioventrally. The scar of the Musculus caudofemoralis longus extends more to the base of the trochanter with respect to ML 563, but is still separated from it (character 256: 1). Distally, the epiphysis has both condyles preserved, divided anteriorly by a very prominent U-shaped extensor groove (character 257: 1, 258: 0). This groove is proportionally deeper than in ML 563. Posteriorly, the two condyles are separated by a deeper, fully open, flexor groove (character 259: 0). As in ML 563, the posterior finger-like process of the lateral condyle is strongly inset (character 260: 1), whereas medial condyle does not protrude cranially to the lateral condyle (character 261: 0).

Tibiae ( Fig. 4C, D ): Two partially preserved right tibiae, present various degrees of erosion and fracturing. The specimen ML 505 is a heavily eroded but complete right tibia with some longitudinal fractures along the proximal and distal epiphyses. It measures 235 mm in total length. The tibia ML 2055, associated with the ML 2055 femur, lacks the proximal epiphysis, has various fractures all along the shaft and was restored by epoxy resin, measuring 220 mm.

In the tibia ML 505 the proximal epiphysis exhibits a conspicuous cnemial crest, a well-developed fibular condyle, and a small inner condyle. The cnemial crest possesses a concave lateral edge and a convex medial edge. It is divided from the fibular condyle by a rounded surface (contrary to Valdosaurus, Barrett et al. 2011 ). The inner condyle points latero-posteriorly, being divided from the fibular condyle by a narrow sulcus. The mid-shaft section is sub triangular in cross section, being flattened laterally and sharp medially, having a conspicuous crest departing from the distal-most part of the shaft (similar to the Eousdryosaurus holotype). The overall morphology of the shaft has been partially altered by the reconstruction process, therefore its taxonomical and systematical significance has to be considered cautiously. The distal epiphysis does not preserve any visible features, only the size of the two sub-equal malleoli.

In the tibia ML 2055 the proximal end is laterally compressed and fractured. The section of the mid-shaft is round character 263: 1) with a small crest on the lateral side. The distal epiphysis preserves both malleoli and in anterior view they exhibit a highly rugose surface. A deep sulcus, extends for most of the surface area of the lateral malleolus. The medial malleolus possesses a less extensive notch, while the lateral malleolus is more elongated posterolaterally (character 264: 1). In distal view, the two malleoli are at right angles to one another.

Remarks.— Cranial skeleton: Parietal: Isolated parietals have not received much attention in literature, what hinders detailed comparisons. ML 1851 differs from Othnielosaurus rex , Hypsilophodon foxii , Camptosaurus dispar , and Dryosaurus altus in being sub triangular in general shape, having less anteriorly expanded lateral processes, a narrower sagittal crest, and proportionally longer posterior lateral processes ( Gilmore 1909; Galton 1974, 1981; Norman et al. 2004). ML 1851 shows striking similarities with mature specimens of Dysalotosaurus lettowvorbecki ( Janensch 1955; Galton 1981; Norman 2004), however, contrasting with the x-shaped morphology of the parietal present in immature specimens of the same species ( Sobral et al. 2012). As in adults of D. lettowvorbecki, ML 1851 possesses a deeply arched lateral process which expands anteriorly, a W-shaped contact with the frontals, proportionately short posterior processes, and a wider sagittal crest ( Hübner and Rauhut 2010; Sobral et al. 2012).

Dentary: The two crowns present in ML 768 shows the characteristic diamond shape typical of iguanodontian dinosaurs (Norman 2004; Galton 2006). The hook like denticles are typical of dryomorphan taxa such as Dryosaurus altus , Dysalotosaurus lettowvorbecki , Iguanodon bernissartensis , Mantellisaurus atherfieldensis (Norman 2004; Galton 2006). The number and position of primary and secondary ridges is extremely variable among species and even within the same tooth row, depending on the tooth position. ML 768 differs from the coeval ankylopollexians in the lack of a marked and mesially offset secondary ridge ( Galton 2006). Instead, as in some specimens of Dryosaurus and Dysalotosaurus ( Galton 1983, 2006; Carpenter and Galton 2018) the single main ridge is positioned towards the midline of the crown, slightly caudally offset.

Axial skeleton: Neural arches: The two isolated and disarticulated neural arches can be ascribed to Iguanodontia on the basis of stout transverse processes and zygapophyses inclined 45° with respect to one another (Norman 2004). Among iguanodontians, the constriction between the lobe of the prezygapophyses and the rest of the bony processes is not well-discussed in literature, but this character is present in Dryosauridae ( Janensch 1955; Galton 1981). On the contrary, ankylopollexians show stout and bulky prezygapophyses, see for instance Camptosaurus dispar , C. aphanoecetes , Mantellisaurus atherfieldensis , and Iguanodon bernissartensis ( Gilmore 1909; Norman 1980, 1986, 2004; Carpenter and Wilson 2008). Furthermore, the presence of anterior and posterior notches constituted by the abovementioned laminae, as seen in Valdosaurus canaliculatus , supports the attribution to Dryosauridae ( Barrett et al. 2011) . The unfused neural arch condition is common among dryosaurid individuals ( Barrett et al. 2011; Barrett 2016; Hübner 2018) since even the largest individuals known so far did not attain complete skeletal maturity ( Horner et al. 2009; Hübner 2012). This peculiar trait may explain why in this case and others reported ( Barrett et al. 2011; Barrett 2016), isolated dorsal and cervical vertebral elements attributed to dryosaurids are often recovered disarticulated. On the contrary, vertebral elements of the caudal series fuse early in ontogeny ( Hübner 2018) and therefore are usually recovered articulated ( Galton 2009; Barrett et al. 2011; Barrett 2016; Hübner 2018).

Appendicular skeleton: Femora: The combination of an anteriorly bowed femoral shaft, the 4th trochanter proximally placed, and the medially relegated scar of Musculus caudofemoralis longus are diagnostic for Dryosauridae ( Butler et al. 2008; Barrett et al. 2011; Escaso et al. 2014; Boyd 2015; Dieudonné et al. 2016). Nevertheless, ML 563 and ML 2055 differ from one another in some aspects. ML 563 is generally more gracile than ML 2055 in general proportions. This is herein interpreted as a difference in ontogenetic stages between the two individuals and therefore, as it is discussed in the next paragraphs, ML 563 probably represents a more immature individual than ML 2055. The difference in size is reflected in the different shape and depth of the extensor groove, which is greatly influenced by load. This is consistent from what is currently known in other dryosaurid populations, which show great intra-specific variation of the extensor groove according to the size (see, for instance, femora assigned to Valdosaurus canaliculatus , Dryosaurus altus , Dysalotosaurus lettowvorbecki , Elrhazosaurus nigeriensis in Galton 1981, 2009; Barrett et al. 2011; Hübner 2018). Another difference is the slightly different position of the scar of Musculus caudofemoralis longus. In ML 563 the position of the scar is well separated from the base of the 4th trochanter while in ML 2055 it extends further laterally to the base of the 4th trochanter, although still separate. Although it is generally assumed that a “widely separated scar of Musculus of caudofemoralis longus” is an unambiguous synapomorphy for Dryosauridae , in literature there are various examples of variation of this character even within the same population ( Galton 1981). Furthermore, Valdosaurus and Callovosaurus specimens usually present the scar of the Musculus caudofemoralis longus connected to the base of the 4 th trochanter ( Ruiz-Omeñaca et al. 2006; Barrett et al. 2011) similarly to ML 2055; while Dryosaurus , Dysalotosaurus , Elrhazosaurus , and Eousdryosaurus have it generally more widely separated ( Galton 1981; Escaso et al. 2014) as in ML 563. Besides the inter-specific variation, an incipient intra-specific variation is present within populations of Dryosaurus and Dysalotosaurus ( Galton 1981) , where the insertion scar in some specimens is located more medially than others. As the extensor and flexor grooves, this character may also be influenced by size and varies during ontogeny as suggested by Hübner (2018). Therefore, the insertion scar of the Musculus caudofemoralis longus may be more plastic than previously thought and possibly related to ontogenetic development as well as with phylogeny.

The more mature specimen represented by ML 2055 resembles the overall morphology of an immature ankylopollexian as expected due to marked peramorphic conditions of basal ankylopollexians (i.e., camptosaurid) relative to dryosaurids (see Horner et al. 2009). Nevertheless, this possible attribution to Ankylopollexia is here considered unlikely for ML 2055 since the medial condyle is comparable in size to the lateral condyle, while in Camptosaurus dispar the medial condyle is described as “sensibly more robust” by Gilmore (1909) and the same condition is present in the Portuguese ankylopollexian species Draconyx loureiroi . Furthermore, the flexor groove is widely open, while in most specimens ascribed to the genus Camptosaurus , an incipient lateral expansion overhangs on the flexor groove ( Galton and Powell 1980; Carpenter and Wilson 2008; Carpenter and Lamanna 2015).

Tibiae: The tibia ML 505, despite presenting a high degree of erosion, has many recognizable characters, which indicate dryosaurid affinities. In particular, ML 505 shares with Dryosaurus , Dysalotosaurus , and Eousdryosaurus a conspicuous fibular condyle placed in the midline of the proximal epiphysis and the presence of a wide groove between the posterior, inner, and fibular condyles ( Janensch 1955; Galton 1981; Escaso et al. 2014). Ankylopollexians exhibit a different condition of the proximal epiphysis of the tibia in having the fibular condyle directed posteriorly Norman 2004).

It should be noted that ML 505 has a triangular diaphyseal cross section, while in ML 2055 the cross section is tear-drop. This result is consistent both with incipient intra-specific variation as observed in Dryosaurus and Dysalotosaurus populations ( Galton 1981).

Figures

Fig. 2. Cranial material of Dryosauridae indet. from the Lourinhã municipality, Portugal, Lourinhã Formation, Kimmeridgian–Tithonian. A. ML 1851, parietal in dorsal (A1, A3) and ventral (A2, A4) views. B. ML 768, dentary in lateral (B1), dashed frame indicates area with foramina, dorsal (B2), medial (B3) and ventral (B4) views, detail of dentary tooth (B5).

Fig. 3. Axial skeleton elements of Dryosauridae indet. (A, B) from the Lourinhã municipality, Portugal, Lourinhã Formation, Kimmeridgian–Tithonian compared with of Dryosaurus altus (C), Camptosaurus (“Uteodon”) aphanoecetes (D), and Mantellisaurus atherfieldensis (E). Dorsal vertebrae: ML 2321a (A) and ML 2321b (B), in dorsal (A1, B1), anterior (A2, B2), lateral (A3, A6, B3, B6), posterior (A4, B4), and ventral (A5, B5) views. Dorsal neural arches: YPM 1876 (C), CM 11337 (D), IRSNB 1551 (E), in dorsal view.

Fig. 4. Limb bones of Dryosauridae indet. from the Lourinhã municipality, Portugal, Lourinhã Formation, Kimmeridgian–Tithonian. A, B. Femur, ML 2055 (A), ML 563 (B), in anterior (A1, B1), lateral (A2, B2), medial (A3, B3), posterior (A4, B4), distal (A5, B5), and proximal (A6, B6) views. C, D. Tibia, ML 2055 associated to femur ML 2055 (C), ML 505 (D), in anterior (C1, D1), lateral (C2, D2), posterior (C3, D3), medial (C4, D4), proximal (D5), and distal (C5, D6) views.