1050
Koskinonodon perfectus (Case, 1922) Figures 2–5
1922 Buettneria perfecta Case, pls. 1, 2A, 2B, figs. 1–3. 1956 Eupelor fraasi jonesi Colbert and Imbrie, p. 449, pls. 25.3, 25.4, 26.1, 26.2, figs. 7, 8.
1965 Metoposaurus fraasi jonesi Chowdhury, p. 42. 1993 Buettneria perfecta Hunt, p. 78, figs. 7–9. 2007 Koskinonodon perfectum Mueller, 2007, p. 225.
Holotype.—Complete skull (UMMP 7475) from the Dockum
Group, Crosby County, Texas,USA(Case, 1922, pls. 1, 2A, 2B, figs. 1–3).
Referred material.—Partial skull with associated ribs and intercentra (PEFO 35392) from the Chinle Formation, Petrified Forest National Park, Arizona, USA (Figs. 2–5).
Remarks.—Previously, the inclusion of the lacrimal into the orbital margin was the sole diagnostic character of Koskinono- don, which separated it from Metoposaurus, but the subsequent observation of a comparable condition to the former in the latter by more recent workers (Sulej, 2002, 2007; Brusatte et al., 2015; but see Lucas et al., 2007) has rendered it invalid. Therefore, the diagnosis of Koskinonodon is in need of revision, particularly with regard to its differentiation from other large-bodied meto- posaurids. However, we do not attempt to amend the diagnosis in this paper in the absence of a broader revision of the Meto- posauridae, which similarly requires extensive revision to evalu- ate existing autapomophies of the clade and to assess the potential for morphological differences to be attributable to intraspecific variation, ontogeny, or factors other than speciation. Addition- ally, although this specimen provides additional information on K. perfectus, the paucity of juveniles of other taxa inhibits an appropriate comparison of autapomorphies at this ontogenetic stage. Our taxonomic assignment of this material is based on the marked differences fromthe diagnosis of the smallmetoposaurid, Apachesaurus gregorii, and the current biogeographic distribution of metoposaurids in which K. perfectus is the only large-bodied metoposaurid west of Texas.
Description.—PEFO 35392 consists of a partial skull associated with several ribs and intercentra. The skull is modestly well preserved, with landmark features such as the pineal foramen, the left orbit, the left stapes, the left otic notch, and both occipital condyles visible in dorsal view, and features such as the parasphenoid and pterygoid visible in ventral view. The skull measures ~15.4cm from the posteromedial margin of the postparietals to the posterior margin of the orbit. The dorsal surface of the skull (Fig. 2.1, 2.2) preserves a
good amount of detail and landmark features, although it has been fractured in several locations, and the original sutures are not visible. Where the ornamentation is preserved, much of the
Journal of Paleontology
skull is characterized by the typical honeycomb pattern found in temnospondyls, mostly consisting of semi-circular pitting of variable size with a small area of elongate grooves in the posterolateral lobe of the temporal region. The pits contain small, distinct foramina that provide a method to distinguish fragmentary temnospondyl material from ornamented osteo- derms of amniotes such as aetosaurs, which lack these foramina (Witzmann et al., 2010). The posterior region of the skull is the best preserved. A large anteromedial portion of the skull, probably the snout, is present but obscured by encrusting matrix and bone splinters, which could not be removed without damaging the skull because this area is very thin. The left orbit is mostly preserved with the entire lateral and posterior margins intact (Fig. 2.1, 2.2). Portions of the anterior and anteromedial margins of the orbit are missing, and the posteromedial portion is dislocated into the orbit. Several postcranial elements are pressed into the dorsal
surface of the skull, including two nearly complete ribs and one partial rib posterior to the left orbit and a caudal intercentrum anterior to the pineal foramen; these are discussed further below. The pineal foramen is well defined, circular, and connected to a slightly raised ridge that extends both anteriorly and posteriorly along the midline (Fig. 2.1, 2.2). The best-preserved and most- diagnostic portion of the skull is the posterior region, containing the otic region and the occiput. The otic notch (Fig. 2) is the most common feature used to identify specimens of North American metoposaurids, and in this specimen, it is relatively deep in comparison to described specimens of A. gregorii (Spielmann and Lucas, 2012). The surface is smooth and uninterrupted except for a small fracture that caused slight dislocation of the squamosal directly upward, suggesting that the incision is not the product of taphonomic damage. Further supporting the conclusion of a well-preserved otic region is the presence of an articulated stapes, the cylindrical distal end of which can be seen projecting posterolaterally in dorsal and occipital views (Figs. 2.1, 2.2, 3). The exposed region of the stapes is identical in form to that seen in other metoposaurids (Case, 1931; Dutuit, 1976; Sulej, 2007; Spielmann and Lucas, 2012; Brusatte et al., 2015). Medially, the otic notch forms a continuous surface with the tabular and the postparietal. Although no posterolaterally projecting tabular horn is present, this margin of the tabular appears to be weathered; thus we cannot conclusively determine whether the tabular horn was absent in this individual. The occiput can be seen projecting posteriorly beyond the posterior extent of the dorsal skull table. The right otic notch, in addition to most of the right side of the cranium, is not preserved in this specimen. The palatal surface of the skull (Fig. 2.3, 2.4) is similar to
the dorsal surface in that the posterior portion is the best preserved and that the anterior portions are heavily damaged. The basal plate of the parasphenoid is fully preserved, although its sutures to the exoccipitals and the pterygoids are not identifiable. The basal plate is roughly diamond-shaped, with
Figure 2. Skull of juvenile Koskinonodon perfectus (Case, 1922) (PEFO 35392) from the Chinle Formation of the Petrified Forest National Park, AZ, USA: (1) photograph of dorsal view, (2) interpretive illustration of dorsal view, (3) photograph of ventral view, (4) interpretive illustration of ventral view. Abbreviations: cp=cultriform process; ect=ectopterygoid; ic=intercentrum; fr=frontal; ipw=interpterygoid window; ju=jugal; lac=lacrimal; mbf=mass of bone fragments; oc=occipital condyle; on=otic notch; orb=orbit; par=parietal; pin=pineal foramen; po=postorbital; pof=postfrontal; ppar=postparietal; psp=parasphenoid; pt=pterygoid; qj=quadratojugular; r=rib; sq=squamosal; sta=stapes; stf=subtemporal fenestra; sup=supratemporal; tab=tabular. Scale bars=5 cm.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238
