98
Journal of Paleontology 90(1):92–101
posteroventrally projected (the condition in SMNS 92101). As inSMNS 92101, the lesser tuberosity is strongly developed with stout scars for muscle attachment, and the deltopectoral crest is reduced, forming a thin triangular projection (Fig. 2). The supinator process is broken near its base, but the notch of the ectepicondyle is clearly observed, as is the lateral groove. The ectepicondylar notch is distally located as in SMNS 92101, and anterodistally opened, according to the anterior end of the ectepicondyle (Fig. 2). In ventral view, the attachment buttress for M. coraco-
brachialis longus (Angielczyk et al., 2009), located at the base of the lesser tuberosity, is extremely large, forming a prominent irregular surface (Fig. 2), more developed than inSMNS 92101. The same occurs on the posterodorsal edge of the lesser tuberosity, which has a rougher surface with prominent thin processes. In the distal half, the entepicondyle has a more irregular lateral edge, possibly for attachment of powerful flexor muscles. On the dorsal surface of the entepicondyle, near the posterior edge, there is prominent process with a rough surface not observed in SMNS 92101 (this area is relatively flat in this specimen). This structure seems to be related to muscle attachment and would correspond to the area of origin of M. triceps humeralis medialis or, as an alternative hypothesis, the area for a dorsal component of the antebrachial flexor muscles (Holmes, 1977; Angielczyk et al., 2009). Due to the lack of an entepicondylar foramen in SMNS 92101 and the absence of any evidence of this foramen on the ventral surface of the entepicondyle of SMNS 92100, we are confident that this prominent process has no relation to the absence of this foramen. In ventral view, the capitellum is large but unfortu- nately mostly eroded. However, judging from its outline, it seems to be as prominent as inSMNS 92101. The trochlea is not preserved. Possibly it was poorly developed as inSMNS 92101. In dorsal view, most of the distal half is almost flat, with some small, randomly distributed foramina.
Comparisons.—At first glance, the expanded and twisted prox- imal and distal ends and the robustness of the humerus are features reminiscent of several groups of synapsids, including most Permian nontherapsid families such as varanopids (Varanops and Watongia; Reisz and Laurin, 2004), ophiacodontids (e.g., Ophiacodon; Romer and Price, 1940), and sphenacodontids (Romer, 1922; Romer and Price, 1940), and late Permian and Triassic therapsids, such asmany dicynodonts (e.g., Cistecephalus, Dicynodontoides, Ischigualastia; Cox, 1965; Cluver, 1978; Angielczyk et al., 2009), some basal therocephalians (Cynar- iognathus; Cys, 1967), and some nonmammaliaform cynodonts (e.g., Thrinaxodon, Exaeretodon, Chiniquodon; Bonaparte, 1963; Jenkins, 1971; Abdala, 1999). This condition is also observed, in different degrees of development, in other tetrapods such as diadectomorphs (e.g., Kennedy, 2010) and some parareptiles (e.g., Millerosaurus and Procolophon; Watson, 1957; deBraga, 2003). Nonetheless, in the aforementioned taxa, the deltoid or delto- pectoral crest is usually a bulbous or flaring well-developed process unlike the condition present in SMNS 92101 and SMNS 92100 (Figs. 1, 2). Usually, an expanded proximal end is the product of the
enlargement of the deltopectoral crest for powerful pectoral and deltoid musculature, serving to hold a heavy body with
sprawling forearm orientation (e.g., Romer, 1922, 1956; Cox, 1965) or as a result of an ecological adaptation for swimming or digging (e.g., Cluver, 1978; Hildebrand, 1988; Walker and Liem, 1994; Martinelli et al., 2005). The expanded proximal halves of SMNS 92101 and SMNS 92100 have an uncommon combination of features. We observe in these specimens a reduced deltopectoral crest and a hypertrophied posteroventrally projecting lesser tuberosity. A comparable organization of the processes and muscle attachment sites of the proximal half of the humerus are observed in the late Permian–Early Triassic owenettid procolophonian Barasaurus (unpublished data, Meckert, 1995). Turtles also possess great development of the lesser tuberosity and less conspicuous deltoid and pectoral processes (Romer, 1956; Sterli et al., 2007), but the overall configuration is quite different from what is observed here. In addition, some transversal enlargement of the proximal half of the humerus is observed in some medium- to large-sized archosauromorphs, such as Trilophosaurus (Spielmann et al., 2005, 2008) and archosaurs Postosuchus and Batrachotomus (Gower and Schoch, 2009; Weinbaum, 2013). However, in the latter taxa the lesser tuberosity is much less developed, and overall the distal end is less expanded; the humeri are slender, elongate, and not twisted. SMNS 92100 and SMNS 92101 lack an entepicondylar
foramen (Figs. 1, 2). The presence of this foramen is a plesiomorphy in some tetrapods (e.g., Romer, 1922; Romer and Price, 1940; Kennedy, 2010). This foramen is absent in several amphibians (Romer, 1956), some stem turtles and testudines (Romer, 1956; Sterli et al., 2007; Schoch and Sues, 2015), and almost all Mesozoic archosauromorph groups (e.g., Romer, 1956; Nesbitt, 2011; Ezcurra et al., 2014; the protorosaur Czatkowiella harae Borsuk–Białynicka and Evans, 2009 is apparently the only archosauromorph with an entepi- condylar foramen according to Borsuk–Białynicka and Evans, 2009). Moreover, the lack of an entepicondylar foramen is diagnostic of the procolophonian family Owenettidae (Reisz and Laurin, 1991; Reisz and Scott, 2002). In SMNS 92101 and SMNS 92100, the ectepicondylar
foramen is absent. However, the ectepicondyle has a well- developed ectepicondylar notch and an associated groove for the radial nerve (Romer and Price, 1940), bordered proximo- ventrally by the supinator process. An ectepicondylar notch is recognized in several amniote groups with different degrees of development (Romer, 1922, 1956; Dilkes, 1998; Spielmann et al., 2008; Ezcurra et al., 2014). In articular, a relatively large, distally positioned notch is seen in some basal tetrapods (e.g., diadectomorph Limnoscelis; Kennedy, 2010), some basal nontherapsid synapsids (e.g., Casea, Varanops, Dimetrodon, and Ophiacodon; Romer, 1922; Romer and Price, 1940), some basal archosauromorphs (e.g., Trilophosaurus buettneri and Otischalkia elderae; Hunt and Lucas, 1991; Spielmann et al., 2008), and owenettid procolophonians (Barasaurus and Owe- netta; unpublished data, Meckert, 1995; Reisz and Scott, 2002). In conclusion, the combination of features (i.e., stout
humerus with expanded and twisted proximal and distal ends, lesser tuberosity larger than deltopectoral crest, deep ectepi- condylar notch with prominent supinator process, lack of entepicondylar foramen) noted in SMNS 92101 and SMNS 92100 is only observed in the owenettid Barasaurus from the
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
