Fylax thyrakolasus, Prieto-Márquez & Farias, 2021

Prieto-Márquez, Albert & Farias, Miguel Ángel Carrera, 2021, A new late-surviving early diverging Ibero-Armorican duck-billed dinosaur and the role of the Late Cretaceous European Archipelago in hadrosauroid biogeography, Acta Palaeontologica Polonica 66 (2), pp. 425-435 : 427-430

Fylax thyrakolasus sp. nov.

Fig. 2 .

Zoobank LSID: urn:lsid:zoobank.org:act:97E63552-0A0B-4208-8F25-9161CA2731EF

1999 Euadrosauria indeterminate; Casanovas et al. 1999: fig. 2.

2009 Hadrosauroidea indeterminate; Pereda-Suberbiola et al. 2009: fig. 1G.

Etymology: From the Greek thýra, door or gate and kólasi, hell; meaning together with the generic name “keeper of the gates of hell”, in reference to the extreme temporal proximity of this taxon to the Late Cretaceous mass extinction event.

Holotype: IPS-36338 , a nearly complete left dentary with teeth.

Type locality: Fontllonga-R, a locality near the eponymous village, next to the c-13 road that lies between the municipalities of Camarasa and La Baronia de Sant Oisme, Noguera county, in the southern reaches of the Montsec mountain range of Lleida province, in the southern limb of the Àger syncline, northeastern Spain ( Colombo and Cuevas 1993; López-Martínez et al. 1999; Fondevilla et al. 2019). Fontllonga-R is the same locality that Casanovas et al. (1999) designated as simply Fontllonga (see also Sellés and Vila 2015: table 1).

Type horizon: Rocks corresponding to chron C29r within the Figuerola Formation, a few meters from the K/Pg boundary ( Galbrun et al. 1993; Caus et al. 2016; Oms et al. 2016; Fondevilla et al. 2019: figs. 9, 11). The Figuerola Formation spans chrons C31n through C29n and consists of alternating brown, ochre, and reddish marl with sandstone that have been interpreted as fluvial deposits ( Cuevas 1992; Rosell et al. 2001; Riera et al. 2009). This locality is one of the few in the world preserving a continental geological record across the K/Pg boundary ( López-Martínez et al. 1998).

Material.— Holotype only.

Diagnosis.—Non-hadrosaurid hadrosauroid dinosaur characterized by the following unique combination of characters: dorsal region of coronoid process at least as wide anteroposteriorly as 30% of dental battery length (convergent in the lambeosaurine Parasaurolophus tubicen ); coronoid process of dentary lacking ridge on posteromedial surface and inclined anteriorly less than 80° relative to alveolar margin; steeply inclined (i.e., angled less than 45° with coronoid process) and flat occlusal surface of dental battery; dentary tooth crowns 2.8–3.3 times taller than wide, lacking marginal denticles and with two major long ridges on enameled lingual surface.

Description.—The dentary of Fylax thyrakolasus gen. et sp. nov. is nearly completely preserved, except for the missing symphyseal process ( Fig. 2 ). It measures 276 mm in length and 180 mm from the dorsal margin of the coronoid process to ventral margin of dentary, perpendicular to the dental battery. The mandibular ramus is relatively deep, being only 2.7 times longer than tall at its deepest point. The ventral margin is nearly straight in lateral view, displaying only a very gentle sinuous profile ( Fig. 2A 5).

The most salient attribute of the dentary of F. thyrakolasus gen. et sp. nov. is its disproportionately massive coronoid process. In particular, the dorsal region of the coronoid process is greatly enlarged, being 31% of the length of the dental battery. Among hadrosauroids, only the lambeosaurine Parasaurolophus tubicen displays a similar ratio (e.g., 30% in NMMNH P-25100; Fig. 3 ). All other examined hadrosauroid species have lower ratios, the majority of them ranging 18–25% ( Fig. 3 ). However, when compared to the minimum width of the ascending ramus of the coronoid process, the dorsal region of this process in F. thyrakolasus gen. et sp. nov. is less expanded than in hadrosaurids. Thus, in Hadrosauridae and Claosaurus agilis (e.g., YPM 1190), the dorsal region of the coronoid process is at least 1.5 times wider than the ascending ramus, whereas in F. thyrakolasus gen. et sp. nov. and other early diverging hadrosauroids the dorsal region is less than 1.5 times wider than the ramus (SOM 1, Supplementary Online Material available at http://app.pan.pl/SOM/app66-PrietoMarquez_ CarreraFarias_SOM.pdf). The medial surface of the dorsal region of the coronoid process in F. thyrakolasus gen. et sp. nov. is paddle-shaped, slightly deeper than wide. Most of what is seen of the coronoid process above the lingual side of the dental battery consists of this medial surface of the dorsal region. A short acute apex is present along the posterodorsal margin of the coronoid process. The anterior and posterior margins of the ascending ramus of the coronoid process are parallel. As in the vast majority of hadrosaurids SOM 2) and Tethyshadros insularis ( Dalla Vecchia 2009) , the coronoid process of F. thyrakolasus gen. et sp. nov. is angled anteriorly less than 80° relative to the dorsal margin of the alveolar sulci of the dental battery (74° in the case of IPS-36338) (contra Pereda-Suberbiola et al. 2009a). Unlike in many early diverging hadrosauroids such as Sirindhorna khoratensis ( Shibata et al. 2015) or Protohadros byrdi ( Head 1998) , but as in hadrosaurids ( Prieto-Márquez 2010b) and a few hadrosaurid outgroups like Penelopognathus weishampeli ( Godefroit et al. 2005) and Telmatosaurus transsylvanicus ( Weishampel et al. 1993) , there is no prominent longitudinal ridge on the medial surface of the ascending ramus of the coronoid process of F. thyrakolasus gen. et sp. nov.

The lateral surface of the dentary near the base of the coronoid process is substantially expanded laterally in IPS-36338, so that the angle between the lateral surface of the dentary and that of the region posteroventral to the coronoid process is 150°. This angle is also less than 165° in hadrosaurids ( Prieto-Márquez 2010b) and hadrosauroids Penelopognathus weishampeli ( Godefroit et al. 2005) and Telmatosaurus transsylvanicus ( Weishampel et al. 1993) . Other early diverging hadrosauroids like Probactrosaurus gobiensis ( Norman 2002) or Eolambia caroljonesa ( McDonald et al. 2012) feature greater angles indicative of less expanded lateral surfaces near the base of the coronoid process. The coronoid process is well offset laterally from the lateral surface of the mandibular ramus, so that there is a well-developed concave platform separating the dental battery from the base of the coronoid process, as in hadrosaurids ( Prieto-Márquez 2010b) and some hadrosaurid outgroups like Bactrosaurus johnsoni ( Prieto-Márquez 2011) or Jeyawati rugoculus ( McDonald et al. 2010). A deep and large adductor fossa exists between the medial surface of the ascending ramus of the coronoid process and the posterior extent of the dental battery. The base of this fossa connects medioventrally with a deep sulcus the extends adjacent to the ventral margin of the dentary. This sulcus wedges gradually anteriorly until disappearing into the ventral margin of the dentary past mid-length of the dental battery ( Fig. 2A 5).

The incompletely preserved anterior end of the dentary is ventrally deflected, forming 13.5° with the long axis of the dental battery in medial view. The point of inflexion between the relatively straight ventral margin of the mandibular ramus and the start of the deflection occurs anteriorly past the mid-length of the dentary, precisely at 72% of the length of the dental battery. Because the symphyseal process is not preserved, it is uncertain whether this region of the dentary was further curved ventrally. Anterior to the first alveolar sulcus, the dorsal margin of the dentary abruptly slopes ventrally, indicating a relatively short (less than 20% of the length of the dental battery) proximal edentulous margin preceding the predentary, as in most early diverging hadrosauroids like Bactrosaurus johnsoni ( Prieto-Márquez 2011) , Gilmoreosaurus mongoliensis ( Prieto-Márquez and Norell 2010) or Tethyshadros insularis ( Dalla Vecchia 2009) .

The dental battery of F. thyrakolasus gen. et sp. nov. contains 29 alveolar positions, of which 17 preserve teeth. There are up to three replacement teeth per alveolar sulcus at the deepest point of the dental battery. The occlusal surface displays a maximum of two functional teeth, unlike the three teeth present in hadrosaurids ( Prieto-Márquez 2010b). The longitudinal axis of the dorsal margin of the dental battery is obliquely oriented, converging anteriorly with the lateral surface of the mandibular ramus ( Fig. 2A 5), as in most non-hadrosaurid hadrosauroids ( Prieto-Márquez 2010b; Prieto-Márquez et al. 2019), except Plesiohadros djadokhtaensis (Tsogbataar et al. 2014) . This is unlike the condition in Hadrosauridae , where the dorsal margin of the dental battery is parallel to the lateral surface of the mandibular ramus ( Prieto-Márquez 2010b). Also unlike in hadrosaurids, but as in some early diverging hadrosauroids like Sirindhorna khoratensis ( Shibata et al. 2015) and Penelopognathus weishampeli ( Godefroit et al. 2005) , the dental battery ends anterior to the posterior margin of the coronoid process ( Fig. 2A 2 ) (contra Casanovas et al. 1999).

Tooth crowns exhibit relatively tall diamond-shaped enameled lingual faces. The height/width ratio of the crowns ranges 2.8–3.2. No marginal denticles are observed; instead, the margins of these crowns are smooth. As in many early diverging hadrosauroids like Sirindhorna khoratensis ( Shibata et al. 2015) , Probactrosaurus gobiensis ( Norman 2002) , Bactrosaurus johnsoni ( Prieto-Márquez 2011) , Plesiohadros djadokhtaensis (Tsogbataar et al. 2014) , or Tethyshadros insularis ( Dalla Vecchia 2009) , the enameled lingual surfaces of IPS-36338 feature two major ridges, one of them slightly more prominent than the other. This pair of long ridges extend apicobasally and are accompanied by a few fainter, shorter accessory ridges. The primary most prominent ridge is posteriorly offset from the midline of the crown. The occlusal surface is steeply inclined (forming less than 45° with the ascending process of the coronoid process) and flat, as in Bactrosaurus johnsoni ( Prieto-Márquez 2011) . This condition is reminiscent of the steep and planar occlusal surfaces of Prosaurolophus maximus and Saurolophus spp. ( Erickson et al. 2012).

Remarks.— Pereda-Suberbiola et al. (2009a) noted that in the dental battery of Fylax thyrakolasus gen. et sp. nov. the distal tooth crowns are not more erupted and worn than the ones near the center of the battery. It is certainly common among hadrosauroids to display progressively more worn teeth (i.e., shallower as seen rising in labial view above the alveolar margin of the dentary) towards the posterior region of the dental battery, particularly nearing the level of the coronoid process (e.g., Bactrosaurus johnsoni, AMNH FARB 6553; Prosaurolophus maximus, MOR 447-8-20-87; Lambeosaurus lambei, TMP 81-37-1; Hypacrosaurus stebingeri, MOR 549; or Corythosaurus casuarius, ROM 858). However, the teeth of the anterior two fifths of the dental battery in IPS-36338 are missing, so it is not possible to ascertain whether the extent of eruption and wear would remain more or less constant throughout the dental battery, or gradually more worn posteriorly as in other hadrosauroids.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 2. Dentary of the hadrosauroid dinosaur Fylax thyrakolasus gen. et sp. nov. (IPS-36338, holotype) from the uppermost Maastrichtian Fontllonga-R locality; in posterior (A1), medial (A2), dorsal (A4), anterior (A5), lateral (A6), and ventral (A7) views.A detailed lingual view of the tooth crowns appears in A3.

Fig. 3. Distribution of the ratio between the maximum width of the dorsal region of the coronoid process (C) and the length of the dental battery D) in a sample of hadrosauroid dinosaurs. Taxon abbreviations: Ac, Acristavus gagslarsoni; Am, Amurosaurus riabinini; Ar, Aralosaurus tuberiferus; Ay, Arenysaurus ardevoli; Ba, Bactrosaurus johnsoni; Bl, Blasisaurus canudoi; Br, Brachylophosaurus canadensis; cfCo, cf. Corythosaurus sp.; Ch, Charonosaurus jiayinensis; Co, Corythosaurus sp.; Eda, Edmontosaurus annectens; Edr, Edmontosaurus regalis; Eot, Eotrachodon orientalis; Ft, Fylax thyrakolasus; Gra,?Gryposaurus alsatei; Grl, Gryposaurus latidens; Hya, Hypacrosaurus altispinus; Hys, H.stebingeri; Krn, Kritosaurus navajovius; Lml, Lambeosaurus lambei; Ma, Maiasaura peeblesorum; Pat, Parasaurolophus tubicen; Pbr, Probrachylophosaurus bergei; Pl, Plesiohadros djadokhtaensis; Pn, Penelopognathus weishampeli; Pr, Prosaurolophus maximus; Pt, Protohadros byrdi; Saa, Saurolophus angustirostris; Sao, Saurolophus osborni; Te, Telmatosaurus transsylvanicus; Ts, Tsintaosaurus spinorhinus; Vel, Velafrons coahuilensis. Silhouettes were downloaded from http:// phylopic.org and drawn by Pete Buchholz (https://creativecommons.org/licenses/by-sa/3.0/), Scott Hartman (https://creativecommons.org/licenses/bync-sa/3.0/) and Craig Dylke (https://creativecommons.org/publicdomain/zero/1.0/).