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. 2022 Nov 25:10:e14461.
doi: 10.7717/peerj.14461. eCollection 2022.

A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana

Elías A Warshaw  1   2 , Denver W Fowler  1
Affiliations

A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana

Elías A Warshaw et al. PeerJ. .

Abstract

Here we describe a new derived tyrannosaurine, Daspletosaurus wilsoni sp. nov., from Judithian strata (~76.5 Ma) intermediate in age between either of the previously described species of this genus. D. wilsoni displays a unique combination of ancestral and derived characteristics, including a cornual process of the lacrimal reduced in height relative to D. torosus and more basal tyrannosaurines, and a prefrontal with a long axis oriented more rostrally than in D. horneri and more derived tyrannosaurines. The description of this taxon provides insight into evolutionary mode in Tyrannosaurinae, lending strength to previous hypotheses of anagenesis within Daspletosaurus and increasing the resolution with which the evolution of this lineage can be reconstructed. Cladistic phylogenetic methods, stratigraphy, and qualitative analysis of the morphology of relevant taxa supports an anagenetic model for the origin of morphological novelty in this genus, highlighting the predominance of anagenetic evolution among contemporary dinosaur lineages.

Keywords: Anagenesis; Daspletosaurus; Paleontology; Tyrannosaur.

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Conflict of interest statement

The authors declare they have no competing interests.

Figures

Figure 1
Figure 1. Map of the area of discovery of BDM 107, holotype of D. wilsoni sp. nov.
Nearby towns (Hinsdale, Glasgow, Saco) and highways (US-2) are labeled. Dashed lines indicate county boundaries; “Jack’s B2” site indicated by star.
Figure 2
Figure 2. Premaxillae of BDM 107.
Shown in lateral (A), medial (B), and rostral (C) views. Abbreviations are as follows: nf, neurovascular foramina; smp, symphysis. Scale is 10 cm.
Figure 3
Figure 3. Left maxilla of BDM 107.
Shown in lateral (A) and medial (B) views. Abbreviations are as follows: af, antorbital fossa; aof, antorbital fenestra; ma, maxillary antrum; mxf, maxillary fenestra; pmr, promaxillary recess; pmx, promaxillary fenestra; ps, palatal shelf; nf, neurovascular foramina; ns, neurovascular sulci. Scale is 10 cm.
Figure 4
Figure 4. Right jugal of BDM 107.
Shown in medial (A) and lateral (B) views. Abbreviations are as follows: cp, cornual process; po, pneumatic opening; pop, postorbital process. Scale is 10 cm.
Figure 5
Figure 5. Left lacrimal of BDM 107.
Shown in lateral (A), medial (B), and dorsal (C) views. Abbreviations are as follows: cpa, cornual process apex; fr, frontal ramus; po, pneumatic opening; rda, rostrodorsal ala; rr, rostral ramus; vp, ventral process; vr, ventral ramus. Scale is 10 cm.
Figure 6
Figure 6. Left postorbital of BDM 107.
Shown in lateral (A), dorsal (B), medial (C), caudal (D), and rostral (E) views. Abbreviations are as follows: cdt, caudodorsal tuberosity; dtf, dorsotemporal fossa; fc, frontal contact; lsc, laterosphenoid contact; sop, subocular process; sos, supraorbital shelf. Scale is 10 cm.
Figure 7
Figure 7. Left squamosal of BDM 107.
Shown in lateral (A), medial (B), and rostral (C) views. Abbreviations are as follows: cp, caudal process; ltf, laterotemporal fenestra; pcs, postorbital contact surface; po, pneumatic opening, qjp, quadratojugal process; rmm, rostromedial margin of pneumatic recess. Scale is 10 cm.
Figure 8
Figure 8. Right quadratojugal of BDM 107.
Shown in lateral (A) and medial (B) views. Abbreviations are as follows: dqc, dorsal quadrate contact; jr, jugal ramus; sc, squamosal contact; vqc, ventral quadrate contact. Scale is 10 cm.
Figure 9
Figure 9. Right quadrate of BDM 107.
Shown in medial (A) and lateral (B) views. Abbreviations are as follows: op, orbital process; mc, mandibular condyles; po, pneumatic opening; pqf, paraquadrate foramen. Scale is 10 cm.
Figure 10
Figure 10. Right dentary of BDM 107.
Shown in lateral (A) and medial (B) views. Abbreviations are as follows: dc, dentary chin; dg, dentary groove; mcf, Meckelian foramen; mg, Meckelian groove; nf, neurovascular foramina; ns, neurovascular sulci; pt, pathology; rk, rugose knob. Scale is 10 cm.
Figure 11
Figure 11. Right splenial of BDM 107.
Shown in medial (A) and lateral (B) views. Abbreviations are as follows: dcs, dentary contact surface; mhf, mylohyoid foramen. Scale is 10 cm.
Figure 12
Figure 12. Results of the cladistic analysis.
Grey nodes denote Daspletosaurus, star denotes D. wilsoni, and numbers by each node are bootstrap support. Skull reconstruction represents the holotype of D. wilsoni, BDM 107 (known material in white).
Figure 13
Figure 13. Time-calibrated sequence of Daspletosaurus chronospecies.
Ages (left) are in Ma and are based on Carr et al. (2017) and Fowler (2017) for D. torosus and D. horneri. Representative skulls are, from top to bottom: D. horneri, MOR 590; D. wilsoni, BDM 107 (known material in white); D. torosus, CMN 8506. Stars represents the temporal position of adjacent specimens. Accompanying characters represent synapomorphies of progressively more exclusive clades represented by each taxon (e.g., D. wilsoni + more derived tyrannosaurines, D. horneri + more derived tyrannosaurines). No clear demarcations are drawn between taxa along the depicted lineage, given the relative paucity of specimens and the subjectivity intrinsic to species delineations of anagenetic lineages; ages of taxa are therefore imprecise. Scale is 10 cm.
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