Chapter 11 A PHYLOGENETIC REVISION OF NORTH AMERICAN AND ADRIATIC MOSASAUROIDEA I, _____________________________ ~ " ~r GORDEN L. BELL, JR. INTRODUCTION last comprehensive revision ofmosasauroids was published nearly 30 years ago ,(RusseII, 1967), just I year after the first English edition of the pivotal work ~i'~PhYlogenetic Systematics (Hennig, 1966). Since then phylogenetic systematics, or [!:'.cladistics, has become the premier methodology utilized in biological systematics. 'tlJ'he major objective of this chapter is to place most mosasauroids into a };.phylogenetic systematic framework, identify current taxonomic names that are valid .:,'~within that hierarchical framework, and diagnose those taxa with synapomorphic ;~·characters. ,.' Fossils of mosasauroid lizards have been found on every continent and their ~great diversity is a matter of record, but a modem hypothesis of phylogerdic i:',r'elationships among mosasauroids is necessary in order to test ideas al "Jut ifZJlYolutionary concepts relating to the taxon. The varanid-aigialosaur-mosasaur \".': . lj~Jransition, for example, has been cited as an excellent model of macroevolution !'';(deBraga and Carroll, 1993), without any unambiguous evidence that such a \1 'transition ever took place. (' :1)' New fossils recovered since 1967 must also be added to the analysis. Two ;~;oasal taxa, the Trieste and Dallas specimens, have been found in Slovenia ~;i(Ca\ligaris, 1988:Figure 14) and Texas (Bell, 1993), respectively. New basic taxa ~[have been described and previously unknown taxa were discovered in museum W:J{frawers. There are also better specimens of taxa known previously only from ~ifragments, and invalidated taxa, now known to be distinct and recognizable, that {S. i ;'Ancienl Marine Reptiles 293 Copyright © 1997 by Academic Press All rights of reproduction in any fonn reserved. 294 Gorden L. Bell, Jr. should be resurrected. This study focuses on only those mosasauroid taxa known from North America and three basal taxa (heretofore referred to family Aigialosauridae) from Slovenia and Croatia. Herein I test the more recent hypotheses of mosasauroid relationships (deBraga and Carroll, 1993; Russell, 1967) using modem phylogenetic methods and an enlarged group of basic taxa and characters. Because only common ancestry (sister-group) relationships are testable (Schoch, 1986), no attempt shall be made to identify direct ancestors. Mosasauroidea as used here includes all basic taxa from the families "Mosasauridae" and "Aigialosauridae," as they were used by RusseII (1967), Romer (1956), and many others in a traditional ranked classification. However, no assumption is made at the outset of this study concerning the monophyly of either taxon, and the names will not be formally recognized until their separate monophyletic status is demonstrated. For purposes of comparison, Russell's tree is converted to a c1adogram and shown in Figure 1. POLARITY DECISIONS Preliminary condideration of outgroup relationships was crucial to this analysis and an attempt was made to identify the closest extant sister taxa. Mosasauroids were scored using the same characters that Estes et al. (1988) used for extant squamates. The mosasauroid data (Appendix 2) were added to their matrix and analyzed using PAUP. Strict and Adam's consensuses of the six most equally parsimonious trees placedmosasauroids within Anguimorpha, but provided no better lower-level resolution of their phylogenetic relationships. Final determination of character states at the outgroup node were based on the algorithm described by Maddison et al. (1984). Each of these character states was then used to determine polarity of the ingroup character states. Lack of resolution of the relationships of mosasauroids among the outgroup taxa (Bell, 1993: 10-16) contributed to uncertain polarity in 26 of 142 characters, or 18% of the total number. In the ingroup analysis, these were treated as nonpo1arized and are identified within the data matrix with a question mark as the outgroup state. Also, 10 characters are based on structures or modifications of structures not represented in the outgroups and are also identified in the same manner. For the ingroup taxa within the data matrix, question marks are used for both nonrepresentation of the attribute due to extreme modification, and missing information due to incomplete specimens. The data matrix used in this analysis is contained in Appendix 2. ~ '<=- 0' IJQ ~ ~ ;;.=~ ::c ~ ~. Tribes: o·'" Plioplatecarpinae = Plotosaurini .....0 ffiillIIIIill ~ 0 ='" Mosasaurini '" .,=c 0 Globidensini is: ~ =~ D Prognathodontini ~ Plioplatecarpini Figure l. Phylogeny of Russell (1967), conservatively converted to standard cladogram format for purposes of comparison. 296 Gorden L. Bell, Jr. PHYLOGENETIC ANALYSIS The data matrix was entered into the PAUP 3.0 program (Sw offord, 1990) and run on an Apple/Macintosh Powerbook 145B computer. The DEL TRAN option of character optimization was selected because it permits assignment of synapomorphies at the most specific taxonomic level. All multistate characters were unordered and all trees were rooted in the outgroup. Analysis was performed using the heuristic mode due to the large number of characters and taxa. Heuristic search options included: simple addition sequence with one tree held at each step, TBR branch swapping performed, the MULPARS option in effect, and steepest descent option not in effect. The analysis produced 99 equally parsimonious trees with a total length of 351 steps. The consistency index for any representative tree is 0.477 with all uninformative characters removed and the retention index is 0.778. Three types of tests were performed to evaluate the quality of the data or the stability of portions of the tree. Such tests have limitations of the number of characters and taxa that can be accommodated, but these problems were circumvented by dividing the tree into various units or component groups. The gl skewness test statistic (Hillis and Huelsenbeck, 1992: 189) measures the amount of signal (phylogenetic information) in a data set and results indicate significant values (95-99% confidence interval) for all of the major clades and most of the subsets of those clades. A set including the more labile taxa, indicated by lack of resolution oftheir phylogenetic relationships, had very low skewness values and were thus not significant. Bootstrapping tests (Felsenstein, 1985) were also run for various parts of the tree. Results identified good to strong support (70-100%) for each of the following taxa: Mosasauridae, Mosasaurinae, Globidensini, Plotosaurini, and five clades containing the basic taxa of Halisaurus, Tyiosaurus, Ectenosaurus, Piatecarpus + P liopiatecarpus, and Prognathodon + P iesiotyiosaurus. Poor support (50-60%) was indicated for "Russellosaurinae," Plioplatecarpinae, and a clade containing the basic taxa of Giobidens. Several bootsrapping tests involving a variety of different taxa within Natantia produced results ranging from less than 50% to 75%. Results for the "aigialosaurs," taxon novum, and YPM 40383 were even lower. Finally, node collapsing tests were performed on the matrix. Adam's consensus trees were calculated using groups of trees with successively larger numbers of minimum steps. A general evaluation of the results indicates poor support for the positions of any of the conservative "aigialosaurs," as well as taxon novum and YPM 40383. The entire constituency of "Russellosaurinae" has poor to moderate support. The clade containing the basic taxa of Halisaurus is fairly well supported as the sister-group to Natantia, as is the c1ade containing Platecarpus + Pliopiatecarpus. The best supported relationships are within and among Phylogenetic Revision of Mosasauroidea 297 Mosasaurinae. CONSENSUS TREES AND PREFERRED HYPOTHESIS OF RELATIONSHIPS There is very little difference in the strict and Adam's consensus trees calculated from 99 trees generated by the phylogenetic analysis. These are illustrated in Figure 2 and Figure 3, respectively. The strict consensus tree was chosen to represent the preferred hypothesis of relationships among Mosasauroidea (Figure 4) because it provides a great deal of resolution, but does not overstep the limitations of the data that are available at this time. It provides but one major surprise and only a moderate amount of incongruency with Russell's tree (see Figure 1) and even less with that of deBraga and Carroll (1993:Figure 4). Unfortunately, the analysis did little to resolve the relationships of basal taxa with derived mosasauroids, largely due to incompleteness of the data. CHARACTER ANALYSIS Skull Characters 1. Premaxilla predental rostrum I: total lack of a bony rostrum (0), or presence of any predental rostrum (1) (Figure 5). In lateral profile, the anterior end of the premaxilla either exhibits some bony anterior projection above the dental margin (Figure 5B-D), or the bone recedes posterodorsally from the dental margin (Figure 5A). The derived condition produces a relatively taller lateral profile with an obvious "bow" or "prow." 2. Premaxilla predental rostrum II: rostrum very short and obtuse (0) (Figure 5D), or distinctly protruding (I) (Figure 5B), or very large and inflated (2) (Figure 5C). In Clidastes a short, acute, protruding rostrum (state 1) produces a "V-shaped" dorsal profile (Russell, 1967:128, 130) and, as far as is known, is peculiar to that genus. An alternative condition, described as V-shaped, include those taxa whose rostral conditions span the whole