Earliest example of a giant monitor lizard (Varanus, Varanidae, Squamata)

Abstract

Background: Varanidae is a clade of tiny (<20 mm pre-caudal length [PCL]) to giant (>600 mm PCL) lizards first appearing in the Cretaceous. True monitor lizards (Varanus) are known from diagnostic remains beginning in the early Miocene (Varanus rusingensis), although extremely fragmentary remains have been suggested as indicating earlier Varanus. The paleobiogeographic history of Varanus and timing for origin of its gigantism remain uncertain.

Methodology/principal findings: A new Varanus from the Mytilini Formation (Turolian, Miocene) of Samos, Greece is described. The holotype consists of a partial skull roof, right side of a braincase, partial posterior mandible, fragment of clavicle, and parts of six vertebrae. A cladistic analysis including 83 taxa coded for 5733 molecular and 489 morphological characters (71 previously unincluded) demonstrates that the new fossil is a nested member of an otherwise exclusively East Asian Varanus clade. The new species is the earliest-known giant (>600 mm PCL) terrestrial lizard. Importantly, this species co-existed with a diverse continental mammalian fauna.

Conclusions/significance: The new monitor is larger (longer) than 99% of known fossil and living lizards. Varanus includes, by far, the largest limbed squamates today. The only extant non-snake squamates that approach monitors in maximum size are the glass-snake Pseudopus and the worm-lizard Amphisbaena. Mosasauroids were larger, but exclusively marine, and occurred only during the Late Cretaceous. Large, extant, non-Varanus, lizards are limbless and/or largely isolated from mammalian competitors. By contrast, our new Varanus achieved gigantism in a continental environment populated by diverse eutherian mammal competitors.

Figure 1. Holotype locality (circle) for Varanus (Varaneades) amnhophilis nov. taxon, on Samos, Greece.

Star indicates Athens, Greece.

Figure 2. Holotype (AMNH FR 30630) skull elements for Varanus (Varaneades) amnhophilis nov. taxon.

Fragmentary right postorbital (A) and squamosal (B) in lateral view. (C) Right quadrate in lateral view. (D) Fragmentary palatine in ventral view. Right pterygoid in lateral (E) and ventral (F) view. Note the absence of pterygoid teeth. Right side of the braincase (parabasisphenoid, prootic, basioccipital, and otooccipital) in lateral view (G) and medial view (H). (I) Otic region of the braincase in ventral view showing the base of the crista interfenestralis and single opening to the facial foramen. (J) Reconstruction of the braincase in right lateral view with reconstructed areas appearing as semi-opaque shadows. (K) Partial right surangular-prearticular/articular complex in lateral view. (L) Partial right coronoid in lateral view. (M) Reconstruction of the cranium and mandible in right lateral view with reconstructed areas appearing as semi-opaque shadows. All scale bars 10 mm, except in (I) wherein the scale bar is 5 mm. Abbreviations: apoc, paroccipital tuberosity; acpr, anterostapedial process of the prootic crest; bpt, basipterygoid process; bptb, basipterygoid buttress; colf, columellar fossa; cpr, prootic crest (crista prootica); cri, crista interfenestralis; fec, ectopterygoid facet; fo, fenestra ovalis; fsq, squamosal facet (on postorbital); ipr, inferior process; pbs, parabasisphenoid; pcr, posterior crest; ped, hypapophyseal pedicel; poc, otooccipital paroccipital process; prp, prootic paroccipital process; poz, postzygapophysis; pvc, posterior opening of the vidian canal; qpr, quadrate process; sot, spheno-occipital tubercle; syn, synapophysis; tcr, tympanic crest; tpr, transverse process; I–XII, cranial nerves.

Figure 3. Holotype (AMNH FR 30630) vertebrae for Varanus (Varaneades) amnhophilis nov. taxon.

(A) Cervical vertebrae 3, 4, and part of 5 in left lateral view. (B) Cervical vertebrae 3 and 4 in posterodorsal view showing the absence of zygosphenes/zygantra and/or pseudozygosphenes/pseudozygantra. (C) Three posterior dorsal vertebrae in ventral view. (D) Reconstruction of AMNH FR 30630 in left lateral view with known parts illustrated on a hypothetical black silhouette for the outline of the animal as a whole. Abbreviations: con, condyle; i, intercentrum; ns, neural spine; pcc, area of precondylar constriction; ped, hypapophyseal pedicel; poz, postzygapophysis; prz, prezygapophysis; syn, synapophysis; zga, zygantrum/pseudozygantrum.

Figure 4. Comparative material of modern Varanus for anatomical comparisons with Varanus amnhophilis.

Braincases of Varanus komodoensis (Australasian clade) (A), Varanus bengalensis (Varanus [Indovaranus] group) (B), and Varanus albigularis (Varanus [Polydaedalus]) group) (C). Ventral views of vertebrae of Varanus nebulosus (Varanus [Indovaranus] group) (D), and Varanus flavescens (E). (F) Varanus albigularis cervical vertebra in left dorsolateral view showing the pseudozygosphene. Varanus komodoensis (A) lacks an accessory prootic crest, Varanus bengalensis (B) possesses a hook-like accessory prootic crest, and Varanus albigularis (C) has a tabular accessory prootic crest. Insets with (D) and (E) show the strong and intermediate precondylar constrictions, respectively. Dotted gray lines show the intersection of hypothetical extensions of the ventrolateral surfaces. With a strong precondylar constriction, the lines intersect anterior to the vertebral condyle, but the intersection occurs beyond the level of the condyle in taxa with weak precondylar constriction. Abbreviations: acpr, anterostapedial process of the prootic crest; con, condyle; cpr, prootic crest (crista prootica); pcc, area of precondylar constriction; prz, prezygapophysis; pzgs, pseudozygosphene; syn, synapophysis.

Figure 5. Temporally calibrated phylogeny of varanids and their outgroups.

Size data are indicated by color included on the known temporal ranges are derived from published accounts , ranges in black indicate taxa without reliable size data. Extant Shinisaurus was used as an outgroup for tree reconstruction, but the shinisaur clade is homogenous in size and extends into the Cretaceous . Some nodes collapsed for space considerations, but the number of included species is in parentheses next to the taxon name (Dataset S1). Maps present the known distributions of the indicated taxa in red. Mosasaur distribution is based on the five basal taxa included in the analysis. Varanus amnhophilis is a nested member of the Indo-Asian A clade and the discordant distribution of that taxon with respect to other Indo-Asian A taxa is illustrated by the map on the lower left.

Figure 6. Size estimates for Varanus (Varaneades) amnhophilis (AMNH FR 30630) and Megalania (Varanus priscus, BMNH 39965 and AMNH FR 6304) based on comparisons of lateral braincase length (BCL) and dorsal vertebral length (DVL).

The open diamonds indicate Varanus (Varaneades) amnhophilis and the open triangles indicate Varanus priscus. The dotted trend lines was calculated using Varanus komodoensis (PCL/BCL, y intercept [yi] = 17.67, x intercept [xi] = −59.67; PCL/DVL, yi = 44.71, xi = 131.4). The solid gray trend line was calculated using extant species from the Indo-Asian A clade of Varanus (PCL/BCL, yi = 22.08, xi = −85.53, R² = 0.918; PCL/DVL, yi = 42.21, xi = −64–96, R² = 0.994). Solid black trendline was calculated using all the data (PCL/BCL, yi = 17.16, xi = 14.11, R² = 0.956; PCL/DVL, yi = 39.93, xi = −24.20, R² = 0.975). Open line drawing represents Varanus prasinus (Vpr), the medium represents Varanus (Polydaedalus) exanthematicus (Vex), and the large represents Varanus komodoensis (Vko)—all to scale. Gray bars indicate predictive size range within 95 percent confidence interval. See text and (Text S1 and Text S3).