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A baby mosasauroid (Reptilia, Squamata) from the Turonian of Morocco – Tethysaurus ‘junior’ discovered?

Article in Cretaceous Research · November 2013 DOI: 10.1016/j.cretres.2013.09.010

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A baby mosasauroid (Reptilia, Squamata) from the Turonian of Morocco e Tethysaurus ‘junior’ discovered?

Alexandra Houssaye a,*, Nathalie Bardet b a Steinmann Institut für Geologie, Paläontologie und Mineralogie, Universität Bonn, Nussallee 8, 53115 Bonn, Germany b CR2P, UMR 7207, CNRS-MNHN-UPMC, Département Histoire de la Terre, Muséum National d’Histoire Naturelle, CP 38, 8 rue Buffon, 75005 Paris, France article info abstract

Article history: New Middle Turonian mosasauroid remains were discovered in the same large-sized nodule that yielded Received 13 August 2013 a specimen referred to Tethysaurus nopcsai Bardet et al., 2003, from the Goulmima region (southern Accepted in revised form 28 September Morocco). They comprise isolated, fragmentary cranial elements (skull and mandible) and some verte- 2013 brae. Their very small size suggests a juvenile condition, an observation supported also by anatomical Available online (spongious nature of numerous bone parts), micro-anatomical (loose inner spongiosa) and histological (numerous remains of calcified cartilage deep into the centrum; radial vascular canals) data. These bones Keywords: belonged to a basal mosasauroid that cannot be distinguished from Tethysaurus nopcsai to which taxon Mosasauroidea Juvenile we tentatively assign the material. Ó Morocco 2013 Elsevier Ltd. All rights reserved. Turonian Tethysaurus

1. Introduction Member of the Niobrara Formation (Late Coniacianeearliest Cam- panian) in western Kansas (Sheldon, 1990; Everhart, 2002, 2007). Mosasauroids were squamates that invaded the marine realm at The present study documents the first juvenile mosasauroid the beginning of the Late Cretaceous (Cenomanian, c. 98 Ma) and from the Middle Turonian of Morocco, and discusses its systematic became extinct at the end of the Maastrichtian (66 Ma). Various attribution and features that characterize its early ontogenetic ecological grades are encountered within mosasauroids, illus- stage. trating steps in improved swimming abilities and adaptation to a pelagic lifestyle. Currently, three morphotypes are recognized: 1) Institutional abbreviations: MNHN, Muséum National d’Histoire plesiopedal and plesiopelvic mosasauroids with primitive Naturelle, Paris, France; OCP, Office Chérifien des Phosphates, “terrestrial-like” limbs and pelvis; 2) plesiopedal and hydropelvic Khouribga, Morocco. forms lacking a sacrum; and 3) hydropedal and hydropelvic forms with paddles but no sacrum (see Bell and Polcyn, 2005; Caldwell 2. Material and methods and Palci, 2007). Tethysaurus Bardet et al., 2003, from the Middle Turonian of Morocco, has been suggested to have been a plesio- 2.1. Material pedal and hydropelvic form (Bardet et al., 2003). It is considered to be a rather primitive mosasauroid, although its precise phyloge- The present lot (MNHN GOU 4e1-17 and MNHN GOU 5) com- netic position within this taxon remains a matter of debate (Bardet prises isolated, fragmentary cranial bones and vertebrae. Although et al., 2008). all these elements were found together and are of comparable size, To date, remains of juvenile (not embryonic) mosasauroids, the occurrence of what appears to be two left pterygoids (i.e., re- which are considered rather scarce in the fossil record, have been petitive bones), indicates that at least two individuals are repre- discovered solely in the Western Interior Seaway. Examples include sented: MNHN GOU 4 and MNHN GOU 5. These tiny bones originate the Campanian Mooreville Chalk of Alabama (Bell and Sheldon, from the same large nodule that yielded MNHN GOU 3 (from lot 1986; Sheldon, 1987; A.H., pers. obs.) and the Smoky Hill Chalk MNHN 1999-9), a presumably adult specimen of Tethysaurus nopcsai,(Bardet et al., 2003: fig. 2A, E, GeQ). The nodule was collected in the Goulmima region, along the * Corresponding author. Fax: þ49 228 73 3509. southern slope of the High Atlas of Morocco; it is of Middle Turo- E-mail addresses: [email protected], [email protected] (A. Houssaye). nian age (see Bardet et al., 2008 for details). The Goulmima area is

0195-6671/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cretres.2013.09.010 A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215 209 interpreted to have been the centre of a basin resulting from large- shows 11 alveoli, whereas MNHN GOU 5 bears 9 teeth, all broken at scale subsidence during the CenomanianeTuronian transgression their base. Both the anterior palatine and posterior basisphenoid and to have corresponded to an open-marine carbonate platform processes are acute. The lateral ectopterygoid process is perpen- (for details see Bardet et al., 2008). dicular to the main body of the bone. It is very slender in MNHN GOU 4e4 but wider and flatter in MNHN GOU 5. Unfortunately, the 2.2. Methods posterior quadratic process is broken in both specimens.

In order to investigate histological and micro-anatomical fea- 3.1.4. Opisthoticeexoccipital tures of this material, a single cervical vertebra (MNHN GOU 4e11) Both fused opisthoticeexoccipital complexes (Fig. 1JeM) are was sectioned along the mid-sagittal and neutral transverse planes preserved, the right one (MNHN GOU 4e5) being more complete (for details see Houssaye et al., 2008), after having been photo- than the left one (MNHN GOU 4e6). The distal part is roughly graphed and embedded in polyester resin. The thin sections were triangular. The exoccipital condyle is small and spongious (Fig. 1K). scanned with an Epson V750-M Pro scanner at high resolution The paroccipital process is long and posterolaterally oriented (12,800 dpi) and observed under a LeicaH DM 2500 compound (Fig. 1JeK). Laterally, the paroccipital process exhibits a long trian- polarizing microscope equipped with a LeicaH DFC 420C digital gular articulation facet for the prootic and supratemporal and a long camera. Two indices were calculated by means of the software triangular shallow sulcus. Because the prootic is missing, the ImageJ (Abramoff et al., 2004): 1) the global compactness in dorsolateral borders of the internal auditory meatus cannot be transverse section (Cts), calculated as the total sectional area minus assessed. Just below and separated from it by a rounded bar of bone the area occupied by cavities and the neural canal multiplied by 100 is the obliquely oriented and oval fenestra rotunda. Closely associ- and divided by the total area minus the area occupied by the neural ated medially are two very elongated lenticular foramina separated canal; and 2) the global compactness of the centrum in longitudinal by a thin sheet of bone (Fig. 1J,M), i.e., the larger dorsal jugular fo- section (Cls), calculated as the total area of the centrum minus the ramen (for vagus nerve X and accessory nerve XI, posterior cerebral area occupied by cavities multiplied by 100 and divided by the total branch of the internal jugular vein, and occipital branch of the in- area of the centrum. ternal carotid artery) and the much smaller (about half the length) ventral condylar foramen (for hypoglossal nerve XII), respectively. 3. Results In MNHN GOU 4e6, a small, displaced roof-shaped fragment of bone that could correspond to the supraoccipital (Fig. 1M) is preserved. 3.1. Skull bones 3.2. Mandible 3.1.1. Quadrate The proximal part of a left quadrate (MNHN GOU 4e1; Fig.1AeB) 3.2.1. Dentary as well as the distal part of a right one (MNHN GOU 4e2; Fig.1C) are The posteriormost part of the left dentary, about 20 mm in available, measuring about 18 mm in height. As in Tethysaurus, the length, is preserved (MNHN GOU 4e7; Fig. 2A). It has convex lateral proximal part has the shape of a narrow conch, the suprastapedial and medial surfaces and a flat dorsal one. A complete tooth, most process is slender, obliquely oriented medially and not very probably the distalmost one, is preserved. Although broken, the expanded ventrally so that it is far from contacting the infra- dentary exhibits a long retrodental process posteriorly to this tooth, stapedial process. The stapedial pit is roughly circular and, note- comparable to the one described in Tethysaurus. worthy, only very slightly sunken so that its border is rather unsharp in comparison with the main shaft. The lateroanterior 3.2.2. Splenial tympanic ala does not appear to be developed, although it may have A fragmentary right splenial is available (MNHN GOU 4e8; been eroded. The distal portion exhibits a spongious saddle-shaped Fig. 2B). Although eroded and broken, it does retain an elevated distal condyle and an apparently medially developed medial border. medial wing, which is fractured obliquely just at the level of the splenial foramen and, unfortunately, anterior to the position of the 3.1.2. Basioccipital notched dorsomedial process characteristic of Tethysaurus. The size The basioccipital (MNHN GOU 4e3; Fig. 1DeE) is complete, of the foramen cannot be ascertained but, judging from its anterior measuring 14 mm in both length and width. It bears very large border (the sole one preserved), it would have been fairly large obliquely oriented basal tubera. The condyle is very small, roughly compared to the size of the bone. The lateral wing, although circular and spongious, as compared to the usually horizontal broken, does not seem to have been very tall. reniform shape observed in mosasauroids (e.g., Russell, 1967), including adult specimens of Tethysaurus. The medullar floor is very 3.2.3. Prearticularearticular wide between the basal tubera as compared to that of larger A posterior fragment of a right prearticularearticular, fused as mosasauroid specimens in general and shows several foramina. The usual, has been recognized (MNHN GOU 4e9; Fig. 2CeD). The bone articulating surfaces for the opisthoticeexoccipital complex are is medially curved, i.e., laterally convex and medially concave in large and obliquely oriented. The basioccipitalebasisphenoid su- dorsal view. The retroarticular process, although partially broken, ture is straight, as in adult specimens of Tethysaurus. appears obliquely oriented. The lateral surface extends slightly further than the medial one and both taper anteriorly as usual. The 3.1.3. Pterygoid glenoid cavity seems to be formed by the articular only and is Two incomplete left and near-equal sized pterygoids are pre- composed of a highly spongious bone, also found in larger Tethy- served (MNHN GOU 5; Fig. 1FeG; MNHN GOU 4e4; Fig. 1HeI). In saurus specimens (N.B., pers. obs.). The dorsal surface anterior to the comparison with the pterygoid of MNHN GOU 3, the smaller and glenoid cavity is concave in order to accommodate the surangular. more complete pterygoid described here (MNHN GOU 4e4) prob- ably attained no more than 25 mm in length. The main body of the 3.3. Teeth pterygoid is, as is usual in mosasauroids, dorso-ventrally flattened and roughly sigmoid in shape. It bears very tiny recurved teeth Very few teeth are preserved. The single dentary tooth is about arranged in a fairly straight anteroposterior row. MNHN GOU 4e4 4 mm in height and similar to those of Tethysaurus and some other 210 A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215 A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215 211

Fig. 2. MNHN GOU 4. Goulmima region, southern Morocco; Middle Turonian. A, left dentary MNHN GOU 4e7 in medial view; B, right splenial MNHN GOU 4e8 in lateral view; CeD, right prearticularearticular MNHN GOU 4e9 in medial and dorsal views, respectively. Scale bars equal 10 mm. primitive mosasauroids. It is strongly posteriorly recurved with a constriction. From one vertebra to another, both the presence convex anterior face and a concave posterior one (Fig. 2A). The basal and the number of foramina (subcentral, paracotylar, para- cross-section is roughly circular and the apex sharply pointed. zygosphenal and/or zygantral) varies. There is no carina. The enamel is smooth. The base of the tooth is ankylosed to the jaw bone (i.e., pleurodont insertion). Some pter- 3.4.1. Atlas ygoid teeth are preserved in MNHN GOU 4e4. They are very tiny The right atlas neural arch (MNHN GOU 4e10; Fig. 3AeB) does and posteriorly recurved. not show any dorsoposterior process. Its anterior concavity for articulation with the dorsolateral portion of the occipital condyle is 3.4. Vertebrae rather reniform, inclined on the vertical at about 15e20 in anterior view (Fig. 3A). The synapophyseal process is well-developed. The The vertebral material comprises an atlas neural arch, three anterior border of the spinous process arching around the spinal cervicals, two dorsals, a pygal and a caudal (MNHN GOU 4e10- cord is separated from the condylar articulation by a notch, like in 17). Their original positions along the vertebral column were e.g., Clidastes and Mosasaurus (Russell, 1967; Fig. 3B). estimated after comparisons with extant lizards and fossil pythonomorphs. All centra are procoelous. The vertebrae clearly 3.4.2. Cervical vertebrae lack pachyostosis. Both the cervicals and dorsals (the only Cervicals, that are anterior ones, are the most complete verte- vertebrae with a well-preserved neural arch) are much wider brae (MNHN GOU 4e11-13; Fig. 3CeG). They are rather elongated in anteriorly than posteriorly, as a result of the marked lateral dorsal view (Fig. 3E). MNHN GOU 4e12, more elongated than projection of the paradiapophyses, but also as the width MNHN GOU 4e13, is the most anterior one. These vertebrae display across the prezygapophyses exceeds that between the post- a ventral hypapophyseal peduncle, which is continuous with the zygapophyses. The neural canal is cylindrical in shape with a flat centrum and located very posteriorly (Fig. 3D, FeG). It ends in an ventral border. It is much narrower and smaller than the cotyle. ovoid facet antero-posteriorly elongated and faces postero- The cotyle is subrectangular in outline in cervicals (where its ventrally (Fig. 3D, FeG). The paradiapophyses project ventrally dorsal and ventral borders are almost straight and parallel) and beyond the ventral border of the cotyle (Fig. 3C, G). The para- becomes much more ovoid in dorsal vertebrae and subrounded in diapophyseal articular facets are located anteriorly, at less than the caudal. The axis of the cotyle/condyle system is oblique. one-third of the centrum length (Fig. 3G). Their dorsal part is poorly Zygapophyses lie clearly above the floor of the neural canal and extended dorso-ventrally, so that the anterodorsal diapophyseal above paradiapophyses. The prezygapophyses are inclined at ridge (sensu Houssaye et al., 2011) is strongly postero-ventrally about 30 to the horizontal and antero-laterally oriented in the inclined (Fig. 3G). The paradiapophyseal dorsal part is inclined at dorsals. In the cervicals, they are inclined at about 45 and more about 30e35 to the horizontal. Prezygapophyseal facets are much anteriorly directed. The zygosphene appears to be larger than the elongated. In lateral view, the neural spine rises directly from the neural canal. Its articular facets are ovoid, strongly inclined on zygosphenal roof; it is rather straight and steeply inclined, at about the horizontal (subvertical), and directed laterally. Where this can 45 (Fig. 3G). be seen (i.e., in cervicals) the posterior border of the neural arch forms a concavity. The neural spine is preserved only in cervical 3.4.3. Dorsal vertebra vertebrae; it markedly widens posteriorly and ends in a knob, In MNHN GOU 4e14 (Fig. 3HeJ), the ventral border of the and does not extend posteriorly beyond the condyle. The centrum paradiapophyses clearly lies above the level of the cotylar is fairly triangular in ventral view and there is no precondylar ventral rim, which would suggest a rather posterior dorsal

Fig. 1. MNHN GOU 4 and MNHN GOU 5. Goulmima region, southern Morocco; Middle Turonian. AeB, left quadrate MNHN GOU 4e1 in medial and lateral views, respectively; C, right quadrate MNHN GOU 4e2 in anterior view; DeE, basioccipital MNHN GOU 4e3 in ventral and dorsal views, respectively; FeG, left pterygoid MNHN GOU 5 in dorsal and ventral views, respectively; HeI, left pterygoid MNHN GOU 4e4 in dorsal and ventral views, respectively; JeL, right opisthoticeexoccipital complex MNHN GOU 4e5 in lateral, medial and posterior views, respectively; M, left opisthoticeexoccipital complex MNHN GOU 4e6 in lateral view. Scale bar equals 10 mm. Abbreviations: a: articulating facet for the prootic and supratemporal; bsp: basisphenoid process; bt: basal tubera; cf: condylar foramen; dc: distal condyle; djf: dorsal jugular foramen; exc: exoccipital condyle; ecp: ectopterygoid process; fr: fenestra rotunda; mf: medullar floor; plp: palatine process; pp: paroccipital process; qp: quadratic process; so?: possible supraoccipital fragment; sp: stapedial pit; stp: suprastapedial process. 212 A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215

Fig. 3. MNHN GOU 4. Goulmima region, southern Morocco; Middle Turonian. AeB, right atlas neural arch MNHN GO 4e10; CeE, cervical vertebra MNHN GOU 4e12; FeG, cervical vertebra MNHN GOU 4e13; HeJ, dorsal vertebra MNHN GOU 4e14; K, dorsal vertebra MNHN GOU 4e15; LeN, pygal vertebra MNHN GOU 4e16; OeQ, caudal vertebra MNHN GOU 4e17; in anterior (A, C, H, L, P), right lateral (B, N, Q), posterior (D, O), dorsal (E, K), ventral (F, I, M) and left lateral (G, J) views. Scale bar equals 10 mm.

position (see Houssaye et al., 2011). Paradiapophyses are MNHN GOU 4e15 (Fig. 3K) shows that the narrowest part of located more anteriorly than in anterior cervicals, notably as a the interzygapophyseal constriction is positioned posteriorly. It result of the much more dorso-ventral orientation of their dorsal also reveals a deep notch on the anterior border of the part, which only slightly curves posteriorly. In dorsal view, zygosphene. A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215 213

3.4.4. Pygal vertebra neural canal. Both rings are connected by a few radially oriented The centrum of MNHN GOU 4e16 is slightly shorter than in the trabeculae. In the neural arch, only a single large ring of primary dorsal vertebra and ventrally flattened. Unfortunately, the sacral periosteal bone occurs, while two rings are seen in the neural spine processes are broken off; their bases show that they are located (Fig. 4B). Compactness (Cts) is estimated at 45.2%. Remodelling is anteriorly, originate from the ventral part of the centrum and limited to the base of the neural canal. Primary bone consists of appear to have projected laterally (Fig. 3LeN). unusual parallel-fibred bone. While vascularization consists of radially oriented simple vascular canals in most of the centrum, the 3.4.5. Caudal vertebra latter become essentially longitudinally oriented along the bone This fragmentary vertebra, MNHN GOU 4e17, bears both periphery (Fig. 5D). transverse processes and peduncles for articulation with haemal arches and thus is an anterior caudal (Fig. 3OeP). There appears to 4. Discussion be a beginning of a prezygapophysis. The centrum is much shorter than in the dorsal vertebrae but length exceeds height, which dif- 4.1. Taxonomy fers from the condition observed in most mosasauroids. The cotyle is subrounded. The transverse processes are situated above the All elements are of comparable size and may have belonged to level of the cotylar ventral rim. The haemapophyseal peduncles are the same individual. However, the presence of two left pterygoids located posteriorly on the ventral face of the centrum just anterior suggests that actually two individuals are represented. Yet, the to the condyle. difference in thickness of the ectopterygoid process in the two specimens remains unexplained. It cannot be determined if this 3.5. Micro-anatomical and histological features variation is ontogenetic, intraspecific or interspecific. In view of the fact that none of the bones is crushed, preservational bias can be In longitudinal section, the structure appears lightly built. There ruled out. are compact remains of primary periosteal bone at both the ventral and dorsal borders of the centrum, especially around the neutral 4.1.1. Vertebrae transverse plane (NTP; Fig. 4A). The rest of the section corresponds Vertebrae are procoelous, as in most other squamates. Within to a spongiosa whose trabeculae tend to be oriented in sagittal that group, the development of a hypapophysis of central origin direction. The intertrabecular spaces are rather large so that the with an articulated distal element, the obliquity of the axis of the spongiosa is loose. The compactness of the centrum (Cls) is esti- cotyleecondyle system, the posterior position of the narrowest part mated at 54.9%. Periosteal bone corresponds to parallel-fibred bone of the interzygapophyseal constriction as seen in dorsal view, and but with numerous large cells of rather random shape and not the presence of articulated caudal chevron intercentra, are char- neatly organized parallel to the main direction of bone deposition acteristic of the Varanoidea sensu Lee (1997), i.e., the Varanidae, (Fig. 5A). This is reminiscent of the unusual parallel-fibred bone Helodermatidae and Pythonomorpha sensu Lee (1997) following described in hydropelvic mosasauroid long bones by Houssaye et al. Lee (2009) (Hoffstetter and Gasc, 1967; Lee, 1997). The phylogenetic (2013). Vascularization consists of radially oriented primary position of the Pythonomorpha within squamates remains unclear vascular canals (Fig. 5B). Of particular interest are the numerous (see Caldwell, 2012; Gauthier et al., 2012). However, whatever the remains of calcified cartilage far from the cotylar and condylar rims exact phylogenetic relationships of these squamate groups, it ap- (Fig. 5C); they extend deep towards the neutral point (i.e., there pears that the presence of a zygosphene/zygantrum system within were growth started sensu Buffrénil et al., 2008). Remodelling is these taxa defines the Pythonomorpha (Lee, 1997). General verte- rather limited; it occurs essentially near the core of the section. bral morphology is clearly distinct from that observed in snakes, as In transverse section, the organization is typical of what is seen shown notably by the presence of a notched zygosphene, the in several extant squamates (see Houssaye et al., 2010). In the greater width across the prezygapophyses than across the post- centrum, a ring of primary periosteal bone surrounds the bone zygapophyses and the lack of prezygapophyseal processes periphery. Another ring of secondary lamellar bone envelops the (Houssaye, 2010). Within the Pythonomorpha, the clear absence of

Fig. 4. MNHN GOU 4e11. Goulmima region, southern Morocco; Middle Turonian. A, longitudinal section of the centrum; B, semi-transverse vertebral section. Scale bars equal 2 mm. NTP: neutral transverse plane. 214 A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215

Fig. 5. MNHN GOU 4e11. Goulmima region, southern Morocco; Middle Turonian. AeC, longitudinal section of the centrum; D, semi-transverse vertebral section. A, B, D, cortex showing the occurrence of large cell lacunae and radially oriented simple vascular canals. C, important remains of calcified cartilage in the osseous trabeculae. pachyostosis and osteosclerosis distinguishes the present taxon present up to the first caudal vertebrae, the occurrence of para- from most stem-ophidiomorphs and from plesiopelvic and ple- cotylar foramina and the caudal centra in which length exceeds siopedal mosasauroids (sensu Bell and Polcyn, 2005; Caldwell and height. Originally, the hypapophyseal peduncles were described as Palci, 2007; Palci and Caldwell, 2007; see Houssaye, 2013). It is triangular in T. nopcsai but a reanalysis of this character has now difficult to make comparisons with plesiopedal and plesiopelvic shown that they are in fact ovoid like in the new material. To sum mosasauroids and with stem-ophidiomorphs, as most specimens of up, the vertebral characters clearly indicate a basal mosasauroid. those groups are preserved articulated on slabs with only one side exposed. Nevertheless, the shape of the centrum in ventral view is 4.1.2. Skull elements distinct from the Y-shape observed in the non-pachyostotic stem- The new material differs from several of the above-mentioned ophidiomorphs Dolichosaurus and Coniasaurus, which is marked by taxa by its quadrate. Indeed, in all plesiopedal and plesiopelvic a strong width reduction just behind the paradiapophyses mosasauroids for which this bone is clearly observable (e.g., (Caldwell, 1999, 2000). Moreover, the zygosphene of Coniasaurus is Aigialosaurus, Carsosaurus, and Komensaurus), as well as in not so strongly notched (Caldwell, 1999) and the articulation of the Yaguarasaurus (Páramo, 1994), Russellosaurus (Polcyn and Bell, pre- and postzygapophyses is nearly horizontal in Coniasaurus and 2005), Pannoniasaurus (Makadi et al., 2012) and Romeosaurus Dolichosaurus (Caldwell, 1999, 2000; A.H., pers. obs.). Haasiasaurus, (Palci et al., 2013), the quadrate has a conch-like shape with a the single plesiopedal and plesiopelvic mosasauroid with vertebrae suprastapedial process that is highly developed ventrally, reaching preserved three-dimensionally, displays paradiapophyses that more than half the overall length of the quadrate. In our specimen, project laterally much more strongly and a centrum that is pro- as in adult Tethysaurus, the quadrate is rather narrow and bears a portionally longer (A.H., pers. obs.). The centrum of Mesoleptos also poorly ventrally developed suprastapedial process, which does not is much more elongated; the paradiapophyses and pre- and post- reach mid-length of the bone. zygapophyses project further laterally and the articulation of the The dentary described above, with its peculiar long retrodental pre- and postzygapophyses is poorly inclined on the horizontal process, is also reminiscent of Tethysaurus (Bardet et al., 2003). The (A.H., pers. obs.). single dentary tooth preserved cannot be distinguished from those Within the Mosasauroidea, the present material is distinct from of other basal mosasauroids. most hydropedal mosasauroids notably in the lack of reduction or vertical orientation of the zygapophyses in the dorsal region, the 4.2. Juvenile condition occurrence of a well-developed zygosphene (at least up to rela- tively posteriorly situated dorsals) and the obliquity of the cotyle/ Small size, as well as the numerous spongy parts in many skull condyle system. Amongst the most primitive forms of the lineages elements, and the poor ossification of the cotyle and condyle sug- of hydropelvic mosasauroids, the new material differs from Dalla- gest a juvenile (but not embryonic) condition. The histological data saurus in which paradiapophyses project much more strongly provide clearer indications of such an ontogenetic stage. Indeed, laterally and the centrum of which is more elongated and cylin- the numerous remains of calcified cartilage in the trabeculae of drical (Bell and Polcyn, 2005) and from Pannoniasaurus in which endochondral origin deep in the centrum are unlike the adult the facet on the hypapophyseal peduncle is circular in shape. condition where this is restricted to the condylar and cotylar sur- However, vertebral characters described for Tethysaurus nopcsai faces, and the very low degree of bone remodelling indicates a ju- (Bardet et al., 2003) all occur in the present material: the obliquely venile condition (Houssaye et al., 2010; Houssaye and Tafforeau, oriented cotyle/condyle system, the elliptical shape of the cotyle 2012). Moreover, by comparison with what was described in ju- and condyle, the strong zygospheneezygantrum articulations and venile specimens of Clidastes (Houssaye and Tafforeau, 2012), the the deep notch on the zygosphene, the zygapophyses probably occurrence of radially oriented canals that are replaced only by A. Houssaye, N. Bardet / Cretaceous Research 46 (2013) 208e215 215 longitudinally oriented ones in the outer cortex also is indicative of Caldwell, M.W., 1999. Description and phylogenetic relationships of a new species the juvenile stage. In addition, in spite of the fact that the vertebra of Coniasaurus Owen, 1850 (Squamata). Journal of Vertebrate Paleontology 19, 438e455. that was analyzed histologically for the present study is a cervical, Caldwell, M.W., 2000. On the aquatic squamate Dolichosaurus longicollis Owen, 1850 whereas that of the supposedly adult specimen of Tethysaurus (Cenomanian, Upper Cretaceous), and the evolution of elongate necks in e nopcsai was a dorsal (Houssaye, 2008), the spongiosa clearly is squamates. Journal of Vertebrate Paleontology 20, 720 735. Caldwell, M.W., 2012. A challenge to categories: “What, if anything, is a mosasaur”. much looser in this specimen, although there is no difference in Bulletin de la Société Géologique de France 183, 7e34. compactness (Cls ¼ 54.9% vs 55.0% in the adult analyzed; Caldwell, M.W., Palci, A., 2007. A new basal mosasauroid from the Cenomanian Cts ¼ 45.2% vs 44.6%; see Houssaye, 2008), which is also in accor- (Upper Cretaceous) of Slovenia with a review of mosasauroid phylogeny and evolution. Journal of Vertebrate Paleontology 27, 863e880. dance with changes observed in growth series of Clidastes Everhart, M.J., 2002. Remains of immature mosasaurs (Squamata: Mosasauridae) (Houssaye and Tafforeau, 2012). Thus, histological and micro- from the Niobrara Formation (Late Cretaceous) argue against nearshore nurs- anatomical data confirm the anatomical observations. eries. Journal of Vertebrate Paleontology 12 (Supplement to no. 3), 52A. ‘ ’ Everhart, M.J., 2007. Remains of young mosasaurs from the Smoky Hill Chalk (Upper Based on comparisons with a standard mosasauroid skull such Coniacian-Lower Campanian) of western Kansas. Second Mosasaur Meeting, as that of Platecarpus (see Russell, 1967), this material hints at a Abstract Booklet and Field Guide, Sternberg Museum of Natural History, Hays, skull length of no more than 100 mm and a total body length of Kansas, 16. about 3 m. Gauthier, J.A., Kearney, M., Maisano, J.A., Rieppel, O., Behkke, A.D.B., 2012. Assem- bling the squamate tree of life: perspectives from the phenotype and the fossil record. Bulletin of the Peabody Museum of Natural History 53, 3e308. Hoffstetter, R., Gasc, J.P., 1967. Observations sur le squelette cervical et spécialement 5. Conclusion sur les hypapophyses des sauriens varanoides actuels et fossiles. Bulletin du Muséum National d’Histoire Naturelle 39, 1028e1043. Houssaye, A., 2008. A preliminary report on the evolution of the vertebral micro- Although it cannot be absolutely proved, our analysis strongly anatomy within mosasauroids (Reptilia, Squamata). In: Everhart, M.J. (Ed.), suggests that the new material represents a basal mosasauroid that Proceedings of the Second Mosasaur Meeting. Fort Hays State University, Hays, is indistinguishable from Tethysaurus nopcsai. Its juvenile condition Kansas, pp. 81e89. is demonstrated by anatomical, micro-anatomical and histological Houssaye, A., 2010. A new aquatic pythonomorph (Reptilia, Squamata) from the Turonian (Late Cretaceous) of France. Comptes Rendus Palevol 9, 39e45. features. This new material, found in the same nodule that had Houssaye, A., 2013. Paleoecological and morphofunctional interpretation of bone yielded an adult Tethysaurus nopcsai, is therefore interpreted as mass increase: an example in Late Cretaceous shallow marine squamates. e corresponding to one (or two) juvenile specimen(s) of Tethysaurus Biological Reviews 88, 117 139. Houssaye, A., Tafforeau, P., 2012. What vertebral microanatomy reveals about the nopcsai. ecology of juvenile mosasaurs (Reptilia, Squamata). Journal of Vertebrate Paleontology 32, 1042e1048. Houssaye, A., Buffrénil, V. de, Rage, J.-C., Bardet, N., 2008. An analysis of vertebral Acknowledgements ‘pachyostosis’ in Carentonosaurus mineaui (Mosasauroidea, Squamata) from the Cenomanian (early Late Cretaceous) of France, with comments on its phylo- genetic and functional significance. Journal of Vertebrate Paleontology 28, 685e We warmly thank L. Cazes (Muséum National d’Histoire 691. Naturelle, Paris, France) for the preparation of the thin sections, P. Houssaye, A., Bardet, N., Rage, J.-C., Pereda Suberbiola, X., Amaghzaz, M., Amalik, M., Loubry (MNHN, Paris, France) and G. Oleschinski (Bonn Universität, 2011. A review of Pachyvaranus crassispondylus Arambourg, 1952, a pachyostotic marine squamate from the latest Cretaceous phosphates of the margin of the Bonn, Deutschland) for the photographs. Thanks to the authorities southern Tethys. Geological Magazine 148, 237e249. of Morocco (Ministère de l’Energie et des Mines, Rabat; Université Houssaye, A., Lindgren, J., Pellegrini, R., Lee, A.H., Germain, D., et al., 2013. Micro- anatomical and histological features in the long bones of mosasaurine mosa- Cadi Ayyad, Marrakech) for providing logistic and administrative e fi saurs (Reptilia, Squamata) implications for aquatic adaptation and growth facilities for eldwork. We are also thankful to J. W. M. Jagt rates. PLoS ONE 8, e76741. http://dx.doi.org/10.1371/journal.pone.0076741. (Natuurhistorisch Museum Maastricht, Maastricht, The Houssaye, A., Mazurier, A., Herrel, A., Volpato, V., Tafforeau, P., Boistel, R., Netherlands) and M. W. Caldwell (University of Alberta, Edmonton, Buffrénil, V. de, 2010. Vertebral microanatomy in squamates: structure, growth and ecological correlates. Journal of Anatomy 217, 715e727. Canada) for fruitful comments that improved the manuscript. A.H. Lee, M.S.Y., 1997. The phylogeny of varanoid lizards and the affinities of snakes. acknowledges financial support from the A. v. Humboldt Philosophical Transactions of the Royal Society B352, 53e91. Foundation. Lee, M.S.Y., 2009. Hidden support from unpromising data sets strongly unites snakes with anguimorph ‘lizards’. Journal of Evolutionary Biology 22, 1308e 1316. } Makadi, L., Caldwell, M.W., Osi, A., 2012. The first freshwater mosasauroid (Upper References Cretaceous, Hungary) and a new clade of basal mosasauroids. 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