Scott and Claggett—Albian pholadomyid bivalves Texas


615


Figure 4. End-member Comanchean homomyid morphotypes. (1) Plot of height/width to valve volume of nine species shows two fields that overlap


slightly: streamlined field in which valves are higher than wide and cylindrical field in which valves are as wide as or wider than high. (2, 3) Outlines of morphotype anterior and lateral left valve views: (2) H. knowltoni lectotype NMNH 145652; (3) cylindrical morphotype of H. budaensis hypotype UMMP 32854 (based on Perkins, 1961, pl. 24, figs. 6, 7). Scale bars=1 cm.


approximated by the height-to-thickness (width) ratio. The X/Y plot of these parameters (Fig. 8) separates the “streamlined” morphotype of H. knowltoni and H. cymbiformis from the cylindrical form of H. budaensis and H. auroraensis. The volume of the valves is related to the volume of the interior visceral mass, which includes foot, gills, and stomach. Comanchean Homomya species valves increased in size as the visceral mass increased, suggesting that the amount of sediment processed for organic matter increased. The two shape para- digms, streamlined and cylindrical, may reflect a difference in burrowing activity, which may have responded to substrate stability and fluidity. The “streamlined” forms of H. knowltoni, H. kellumi, H. austinensis, and H. cymbiformis first appeared on the


Table 1. Species of Homomya and Panopea reported from Albian Strata, Comanche Shelf, Texas and Mexico. Species


Author Date


“Homomya” jurafacies “Homomya” solida


Bucardiomya [Homomya] alta


Homomya [Pholadomya] knowltoni


Homomya comalensis Homomya tarrantensis Homomya austinensis Homomya kellumi


Homomya vulgaris Homomya bravoensis Homomya auroraensis Homomya budaensis


Formation Location (Map number)


Cragin 1893 Glen Rose Formation Burnett County, Texas (1) Cragin 1893 Glen Rose Formation Parker County, Texas (2)


Roemer 1852 Fredericksburg Group Gillespie County Texas (1)


Hill 1893 Glen Rose Formation Travis County, Texas (3)


Whitney 1937 Glen Rose Formation Comal County, Texas (6) Perkins 1961 Fredericksburg Group Tarrant County, Texas (8) Shattuck 1903 Buda Limestone


Homomya tlahualiloensis Perkins 1961 Aurora Formation Coahuila, Mexico (7) Shattuck 1903 Buda Limestone


Whitney 1911 Buda and Aurora Formations


Homomya cymbiformis Homomya washitae


Panopea [Pleuromya] henselli


Hill


Cragin 1894 Main Street and Grayson fm.


Perkins 1961 Aurora Formation Coahuila, Mexico (7) Travis County, Texas (3)


Travis County, Texas (3)


Böse 1910 Fredericksburg Group El Paso County, Texas (5) Perkins 1961 Aurora Formation Coahuila, Mexico (7)


Tx (3),Coahuila,Mexico (7)


Perkins 1961 Fredericksburg Group Tarrant County, Texas (8) Grayson County Texas (4)


1893 Glen Rose Formation Travis County, Texas (3)


Number of specimens


4 1 1


11 4 7


8 1


10 3 1 2 8


4 2


2


Mean Length


79.8 81.7 45


46.7 64.8 68


50 55


57.9 90


77.3 85.8 88.4


116.5 76.1


63.5


Mean Height


60.2 69 52


32.4 45.9 46.7 43.5 51.5 52


31.2 35


46.9 52.5


41.7 85


46.9


Length/ Height


1.27 1.18 1.45


1.6


1.57 1.48 1.44 1.31 1.37 1.84 1.5


1.83 1.89


1.54 1.38


1.62


Mean Width


53.8 57.2 39


25.8 29


24


57.3 44.1 52.1


33.1 32.5 33.7 49


Height/ Width


1.12 1.21 1.33


1.01 1.07 0.86


36.1 79.5


33.1


1.33 1.34 1.45 1.27 1.1


1.21 1.21


Mean Beak Distance


21 29 12


15 11


Beak/ Length


Beak/ Height


0.265 0.349 0.355 0.42 0.267 0.231


14.9


16.67 17.2 17.3 14.5 6


19


1.18 1.07


1.42


10.8 16.3


na 36


0.328 1.87 0.2


0.311 0.469 0.25


0.254 0.364 0.28


0.314 0.38 0.39


0.167 0.29 0.068 0.09 0.223 0.385 0.12


0.07


0.24 0.08


0.475 0.765


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