Approximately 40% of a skeleton including cranial and postcranial remains representing a new genus and species of basal neotheropod dinosaur is described. It was collected from fallen blocks from a sea cliff that exposes Late Triassic and Early Jurassic marine and quasi marine strata on the south Wales coast near the city of Cardiff. Matrix comparisons indicate that the specimen is from the lithological Jurassic part of the sequence, below the first occurrence of the index ammonite Psiloceras planorbis and above the last occurrence of the Rhaetian conodont Chirodella verecunda. Associated fauna of echinoderms and bivalves indicate that the specimen had drifted out to sea, presumably from the nearby Welsh Massif and associated islands (St David’s Archipelago). Its occurrence close to the base of the Blue Lias Formation (Lower Jurassic, Hettangian) makes it the oldest known Jurassic dinosaur and it represents the first dinosaur skeleton from the Jurassic of Wales. A cladistic analysis indicates basal neotheropodan affinities, but the specimen retains plesiomorphic characters which it shares with Tawa and Daemonosaurus.

The lithological boundary of the base of the Jurassic is the base of the Paper Shales horizon at 4.5 metres. Many British stratigraphers have historically used the first occurrence of the ammonite Psiloceras planorbis to indicate the base of the Jurassic, but elsewhere in Europe two other psiloceratacean ammonites appear before P. planorbis. The thin Langport Member at 2.2 m has been dated as Rhaetian by Swift (1995) using conodonts. The new theropod comes from one of the limestones at the 6 m interval (bed 7 or 9).

(A) The base of the Jurassic showing a series of alternating limestones and mudstones. The new specimen is most likely from the higher of two limestones indicated in yellow that contain a thin shelly horizon, but it was recovered from the fallen blocks in front of the exposure. The upper of the two matches most closely the bed thickness of the slabs with the dinosaur bones. (B) The same beds in stratigraphic context with the highest occurrence of conodonts, the lithological base of the Jurassic and the first occurrence of the ammonite Psiloceras planorbis indicated.

Here we describe a partial disarticulated skeleton, including the skull, of a new genus and species of theropod dinosaur from well-dated marine strata of the southeast coast of Wales (Figs 2 , 3 and 4 ). It represents the most complete theropod from Wales, and one of the most complete from the European Lower Jurassic. The specimen is deposited in Amgueddfa Cymru—National Museum Wales, Cardiff, accession number NMW 2015.5G.1–2015.5G.11 with individual blocks and skeletal elements numbered as in Table 1 ( Fig 5 ).

Despite extensive outcrops of early Mesozoic strata in south Wales, including units of vertebrate-bearing, non-marine and marginal marine facies, the remains of dinosaurs in Wales are exceedingly rare. Excluding the Triassic (Norian) age footprints of Barry, Glamorgan[ 17 ], most Welsh dinosaur records are of isolated bones and teeth from fissure fill deposits where poorly dated sequences infill palaeokarst developed in Carboniferous age limestones[ 18 , 19 ] or from rémanie deposits, such as the so-called Rhaetian bone bed at or near the base of the Westbury Formation[ 20 , 21 ]. A single, large theropod left dentary with teeth was recovered from loose blocks of probable littoral sandstones from Stormy Down, Glamorgan,[ 22 ] and named Zanclodon cambrensis (see [ 21 ]) while Thecodontosaurus caducus was from the Late Triassic of Bonvilston, Glamorgan, south Wales[ 18 ]. This specimen was later given its own genus, Pantydraco[ 23 ].

Theropod dinosaurs are extremely rare in the Lower Jurassic and most reports are of only fragmentary remains[ 1 – 6 ]. This rarity results in a considerable gap in our knowledge of these animals at a time when, indications are, theropods were diversifying rapidly. In Europe Early Jurassic theropods are reported from the Hettangian of Scotland[ 1 ], England[ 2 , 3 , 4 ], France[ 5 ] and Belgium[ 6 ], but all of these occurrences are of fragmentary material, isolated bones, or a few associated elements, with most of it non-diagnostic at generic level ( Fig 1 ). Oddly, most have been obtained from marine or marginal marine strata. A few examples of Lower Jurassic theropods are known from elsewhere; they include the abelisaurid Berberosaurus from the Toarcian of Morocco[ 7 ], Cryolophosaurus from the Sinemurian-Pliensbachian of Antarctica[ 8 ], Syntarsus from the Hettangian-Pliensbachian of South Africa and Zimbabwe [ 9 , 10 ], Podekosaurus from the Pliensbachian to Toarcian of Massachussetts[ 11 ], and from Arizona Dilophosaurus from the Hettangian[ 12 ] and Segisaurus from the Pliensbachian to Toarcian[ 13 ], all of which are Neotheropoda. The presence of the derived therizinosaurid Eshanosaurus in the Lower Jurassic of China, tentatively dated as Hettangian[ 14 ], implies that many other higher theropod taxa should also be represented in the Lower Jurassic, if the dating of Eshanosaurus proves correct. However, a Hettangian age for Eshanosaurus contrasts with all other therizinosaurid occurrences[ 15 ], although the paucity of theropod remains in the Lower Jurassic globally may be the explanation for the disparity. It is imperative that fragmentary remains such as those of Eshanosaurus are identified correctly and reliably dated before any firm conclusions are drawn from their seemingly anachronistic occurrences. Two theropods have been named from the English Hettangian, Sarcosaurus woodi Andrews, 1921[ 4 ] from Barrow upon Soar, Leicestershire, based on an isolated pelvis, vertebra and proximal femur (BMNH 4840/1) and Sarcosaurus andrewsi Huene, 1932 [ 16 ] based on a partial tibia (NHMUK R3542) (see also ref [ 3 ]).

Materials and Methods

Basic collecting, preparation and repair The specimen was discovered in March 2014 and ‘rescue’ collected from a minor cliff collapse of well bedded limestones and mudstones at Lavernock Point (Fig 4) by Mr Nick Hanigan and Mr Rob Hanigan. The larger blocks were dried over two weeks under damp newspaper to prevent rapid shrinkage and cracking of a thin mudstone veneer on the limestone surface. The various slabs were cleaned and prepared mechanically to expose bones using sodium bicarbonate abrasive powder, and were X-rayed and CT scanned at the Faculty of Clinical Radiology, University of Manchester. Small broken bones were repaired using superglue. Slabs with bones represented as external moulds were cast in a silicon compound, replicated in fibreglass and resin, and painted to restore missing bones known to have been present and presumed lost to the sea. Extensive searching failed to discover the missing bones on the foreshore. There exists the possibility that some of the specimen remains in the cliff, but repeated careful examination of the cliff face failed to reveal any exposed bones.

Cladistic analysis To establish the phylogenetic position of the new specimen it was analysed in a cladistic analysis comprised of 366 characters and 46 taxa, 18 of which lie outside of Dinosauria. The analysis used the 338 characters and coding of You et al.[24], formerly of Ezcurra and Brusatte[25], with Daemonosaurus included using the coding of Sues et al.[26]. It was possible to atomize 25 of the compound characters (sensu [27]) in the character list of Ezcurra and Brusatte[25], resulting in a further 28 characters. An additional compound character was reduced from three character states to two. To demonstrate the effect of atomizing compound characters an experiment using jacknife procedures was conducted. The jacknife analysis was manually performed using Microsoft Excel to generate random characters for omission in each replicate of the procedure. For each replicate of the jacknife procedure the number of characters utilised and the resulting tree-lengths were recorded. The jacknife procedure was implemented on the original matrix of You et al.[24] and the matrix presented here, minus Daemonosaurus and the new theropod. Two bivariate plots were generated from the recorded data; one analysing difference in tree-length (total matrix tree-length minus jacknife replicate tree-length) vs. the number of characters removed; and the other analysing tree-length vs. the number of characters analysed in each replicate. It was predicted that the former plot would demonstrate the same rate of tree-length decay as characters are removed, whilst the latter would demonstrate an overall difference in tree-length between the two matrices. In the event of the predictions being correct, the two graphs in tandem would demonstrate a difference between the matrices that is a result of a change in character conflict, as opposed to the quantity of characters. The ensemble retention index (RI) which is independent of character number did not change between the original analysis of You et al.[24] and the new analysis, due to the coding being exactly the same. Likewise, a plot of the difference in tree-length vs. the number of characters removed demonstrated the same rate of tree-length decay for both matrices. However, the larger, new matrix was only 15 steps longer despite an additional 25 characters. Additionally, the difference in relative tree-lengths taken from the regression lines of the jacknife plot (tree-length vs. number of characters) was approximately 60 steps less for the larger matrix. Therefore, the jacknife demonstrates the benefits of atomizing compound characters. The final analysis was run in TNT using a ‘new technology’ search. Implied weights were used to adjust character weights during the analysis, in order to combat any coding errors or character conflict that was not resolved by atomizing compound characters. The resulting MPT stored to the computer’s RAM was further analysed with a TBR swapping algorithm in TNT’s ‘traditional’ search function to resolve the maximum MPTs possible. A Bootstrap analysis using 100 replicates in a ‘new technology’ search was also run. Absolute values were stored on the bootstrap tree and it can be found, along with further details on all the procedures discussed here in the supporting information (S1 File).

Nomenclatural Acts The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSID for this publication is: urn:lsid:zoobank.org:pub:2146600F-0A2D-4815-8361-C009021B3513. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS and the University of Portsmouth’s library, Pure.

Stratigraphic Position and Age Although not found in situ, the new dinosaur has been constrained on lithological grounds to one of two limestone horizons within the Blue Lias Formation of the Lower Lias Group (Fig 4), and specifically within the lower Bull Cliff Member (stratigraphic terminology follows that of Simms[28]) in the Bristol Channel Basin. Unfortunately, this horizon is the one part of the sequence for which a precise date remains contentious as it lies below the first occurrence of the ammonite Psiloceras planorbis (Sowerby, 1824)[29] historically used by British stratigraphers to define the base of the Jurassic[30], and above the last occurrence of a marker conodont for the Triassic, namely Chirodella verecunda Swift, 1995[31]. As a matter of pragmatism, these strata have traditionally been regarded as Late Rhaetian age and are informally called the Pre-Planorbis Beds [28,32,33,34]. The new dinosaur comes from a stratum that is ~3 metres below the first occurrence of P. planorbis and ~4 metres above the last confirmed conodont occurrence. At Lavernock Point there is a distinctive lithological change at the base of the Blue Lias Formation that historically was used by some workers as a lithological definition of the base of the Jurassic[35] and is employed here. In European and North American sections across the Rhaetian/Hettangian (Triassic/Jurassic) boundary the first appearance of a psiloceratacean ammonite is marked by the occurrence of Psilocerus erugatum, followed by P. imitans and P. antecedans, all of which are used to define subzones beneath the first occurrence of P. planorbis. In Austria P. spelae tirolicum, P. cf. pacificum and P. ex gr. P. tilmanni occur in strata below P. planorbis[36] while in Belgium and northeast Russia, Primapsiloceras primulum has been reported from strata below P. planorbis[37,38]. These earliest Jurassic subzones and other psilocerataceans have not been detected in the Lavernock Point sequence of strata. However, ammonites predating Psiloceras planorbis have been found in the basal Blue Lias Formation of Somerset, just 22 km south of Lavernock Point[34]. These include examples of P. cf. erugatum and Neophyllites sp. demonstrating that the basal subzone ammonites of the Jurassic did reach the British Isles. The absence of these very early Jurassic forms at Lavernock is either a reflection of a facies dependency of the earliest Jurassic ammonites that excludes them from nearer shore waters, or (more likely) lack of preservation. In this latter respect it is worth noting that P. planorbis fossils of higher beds in the Lavernock Point Blue Lias Formation are preserved as compressed periostracal films with no aragonite preservation nor calcitic shell replacement, while aragonitic bivalves are entirely missing from the Bull Cliff Member. The fossil assemblage is thus a chemical lag dominated by calcitic fossils (echinoderms, ostreacean and pectenacean bivalves), and phosphatic vertebrate remains. The question remains, should the new dinosaur be attributed to the very latest Triassic, or the very earliest Jurassic? Geochemical considerations may help resolve the precise stratigraphic age of the dinosaur bearing stratum. A number of studies have located isotopic excursions at or near the Triassic-Jurassic boundary (e.g. [39] for sequences in Hungary; [40] for Austria; [41] for Nevada, USA) and the British Isles[42,43]. A study at Lavernock Point[42], found a δ13C org negative anomaly at the top of the Langport Member of the Triassic Penarth Group, while an extensive, integrated analysis of a section on the south side of the Severn Estuary at St Audries’ Bay, Somerset also detected a massive δ13C org negative isotope excursion commencing just above the last conodont occurrences, but before the first ammonite occurrence[43]. This negative excursion has also been detected elsewhere, and in a section in New York State, USA, Ward et al.[44] proposed this excursion to define the base of the Jurassic.

Conodont ages Conodonts have been recorded from the Late Triassic Langport Member of the Lilstock Formation (Penarth Group) at Lavernock Point, some 4 metres beneath the dinosaur discovery site and 7 metres below the first occurrence of Psiloceras planorbis[44]. Examples of Chirodella verecunda Swift, 1995[31] have been collected from Bed 3 of Ivimey Cook[45] (see also [46,31]) and are indicative of the Misikella postehernsteini (Kozur and Mock, 1974)[47] conodont biozone. This conodont biozone is indicative of the Rhaetian Paracochloceras suessi to Cochloceras marshi ammonite biozones of the Tethyan Late Triassic[48,49]. The zonal conodont indicator Misikella postehernsteini has not been reported with certainty from Lavernock Point, but it has been reported from lithologically equivalent strata in the English Midlands[31]. Associated invertebrate fauna. The new theropod skeleton is associated with a variety of marine shelly invertebrate fauna on the same bedding surface including stem ossicles resembling, but not identical to, those of the crinoid Hispidocrinus sp. and portions of test and spines of the regular echinoid cf. Diademopsis sp. In addition associated bivalve fragments seem to be referable to Liostrea (grey, laminated), Plagiostoma (dark grey-black, shiny) Pseudolimea, cf Oxytoma sp. and juvenile examples of cf. Antiquilima (grey, ribbed). This assemblage is regarded as representing a Jurassic fauna.