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Journal of Paleontology 92(4):611–633
to basal upper Albian Fredericksburg Group, and the upper Albian to lower CenomanianWashita Group (Scott et al., 2003). Different subdivisions into formations reflect regional litho- logical changes from North Texas, south to the San Marcos Arch, and to West Texas (Fig. 2). Homomyid species occur in each group in Texas and northern Mexico (Fig. 2) and evolved across this time span. Ranges are relatively long, some up to eight million years and others only one to two million years. The Trinity Group is characterized by the first appearances of Homomya knowltoni (Hill, 1895) and Homomya comalensis Whitney 1937; two other species are known only from the Trinity, but their generic assignment is uncertain. Four new Homomya species diversified in the middle-lower upper Albian Fredericksburg Group. One Trinity species, H. comalensis, ranges into the Fre- dericksburg basal marl member of the Person Formation on the San Marcos Arch, and H. knowltoni persisted into the basal for- mation of the Washita Group. Homomya washitae Cragin, 1894 spans the Albian/Cenomanian boundary, and three species are found only in the lower Cenomanian Buda Limestone (Fig. 2). Homomya specimens are diverse and abundant in the Aur-
ora Limestone in the Sierra de Tlahualilo in western Coahuila, Mexico (Perkins, 1961). The upper member of the Aurora is composed of fossiliferous, interbedded thick-bedded and nodu- lar marly limestone up to 110m(365 ft) thick. Perkins correlated the upper member with the middle-upper Albian Fredericksburg and Washita groups in North Texas. Perkins recognized an older Texigryphaea mucronata Zone and a younger Homomya kellumi Zone. The former species is characteristic of the Fredericksburg, which correlates the lower part of the upper Aurora member with middle-upper Albian. The H. kellumi Zone correlates the upper part of the upper Aurora member with the upper Albian Washita Group. In Texas, this species is in the Fort Worth Formation in the middle part of the Washita, which is characterized by the upper Albian ammonite species Mortoniceras lasswitzi (Young, 1957) (Fig. 2). Also in the H. kellumi Zone is the caprinid rudist Kimbleia albrittoni (Perkins, 1961), which ranges throughout the upper part of the upper Albian Washita in Texas (Fig. 2) (Scott et al., 2016). Thus, the H. kellumi Zone in the Aurora correlates with the middle part of the Washita up to the Albian/Cenomanian contact.
Statistical comparison of species
To identify consistently Comanchean species of Homomya, Liopistha (Sergipemya), and Panopea, dimensions of key mor- phological properties were measured (Fig. 3; Table 1). Because most specimens are preserved as internal or composite molds and most are broken, shell heights are more commonly complete than lengths. Two sets of species are compared, a smaller- sized set of H. knowltoni; H. tarrantensis Perkins, 1961; H. tlahualiloensis Perkins, 1961; H. kellumi Perkins, 1961; and H. comalensis (Fig. 4) and a larger-sized set of H. cymbiformis Perkins, 1961; H. austinensis Shattuck, 1903; H. vulgaris Shattuck, 1903; H. budaensis Whitnet, 1911; and H. auroraensis Perkins, 1961 (Fig. 5). The difference between heights of populations of
H. knowltoni and H. tarrantensis are statistically insignificant (two-tailed, paired Student’s t test is 0.3001), and their strati- graphic ranges overlap in the Fredericksburg Group (Fig. 4).
Figure 3. Outlines of a typical Comanchean homomyid showing measured dimensions on Table 1. (1) Right valve lateral view; (2) dorsal view; (3) anterior view. Scale bars=1 cm.
Likewise, the difference between heights of populations of H. kellumi and H. tlahualiloensis are statistically insignificant (two-tailed, paired p=.1195, CI=4.637) and both are in the Fort Worth Formation in the middle part of the Washita Group (Fig. 5). Heights of populations of H. cymbiformis, H. austi- nensis, H. vulgaris, and H. budaensis-auroraensis are also sig- nificantly different (Fig. 6). Statistical analyses were performed using equations in Excel spreadsheets and GraphPad software (
www.graphpad.com/quickcalcs/ttest2/). Taxonomic implica- tions will be discussed in the Systematic Paleontology section. Five species appear to be quite similar, so they are com-
pared on X/Y plots (Fig. 7): length/height (L/H) and height/ width (H/W). The difference between the L/H ratios of H. knowltoni and H. tarrantensis is statistically insignificant (two-tailed p=.2004, CI=0.0686), but the H/W ratio is significantly different (two-tailed p=.0230, CI=−0.1040). This statistic indicates that H. knowltoni is significantly wider than H. tarrantensis; however, this difference is not a practical criterion on which to base a species concept. The H/W data points of H. knowltoni overlap those of H. tarrantensis in the Fredericksburg Group (Fig. 7). The difference between H/W ratios of the two younger species in the Washita Group, H. kellumi and H. tlahualiloensis, is also statistically insignif- icant (two-tailed p=.0097, CI=−0.18229); the lengths of too few specimens were available to form significant populations. These two species are very similar to each other and are dis- tinctly larger than the two Trinity and Fredericksburg group species (Fig. 7). The lower Cenomanian species, H. austinensis, is relatively longer and higher than the other species. This group of Comanchean Homomya species became larger during the early Albian to early Cenomanian time span, approximately 16 million years.
Evolution of homomyid functional morphotypes
Two end-member morphotypes are represented by the “streamlined” Homomya knowltoni, which is an elongate, slightly inflated form with a relatively high umbo, and the cylindrical Homomya budaensis, which is a very elongate, tubular, inflated form with a very low umbo. The geometry of these shell forms is approximated by a cylindrical outline, the volume of which is [π × radius2 × length]; the radius is taken as half the width. The inflation of the valves is
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