2 Icaronycteris Index
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Icaronycteris index (holotype; PU 18150) from the Green River Formation, Wyoming. 2 CONTENTS Abstract ....................................................................... 4 Introduction .................................................................... 5 Relationships and Classi®cation of Eocene Bats: A Historical Overview ........... 12 Relationships Among Extant Lineages of Bats .................................. 31 Goals of the Current Study ................................................... 40 Materials and Methods ......................................................... 40 Taxonomic Sampling and Monophyly .......................................... 40 Outgroups .................................................................. 42 The Data Set ................................................................ 46 Characters Examined in Fossil Bats ............................................ 48 Skull and Dentition ........................................................ 48 Anterior Axial Skeleton .................................................... 64 Pectoral Girdle ............................................................ 70 Forelimb ................................................................. 77 Posterior Axial Skeleton and Pelvis .......................................... 81 Hindlimb ................................................................. 84 Completeness ............................................................... 85 Methods of Phylogenetic Analysis ............................................. 86 Analysis of Character Transformations ......................................... 87 Phylogenetic Results ........................................................... 88 Results of Analyses .......................................................... 88 Analysis 1: All Characters, Fossil Taxa Excluded .............................. 88 Analysis 2: All Characters, All Taxa ......................................... 89 Analysis 3: All Taxa, Characters Limited to Those Scored in Fossil Forms ....... 92 Summary ................................................................... 94 Character Evolution in Early Chiropterans ........................................ 94 Character Transformations Associated with Basal Nodes .......................... 94 Features Diagnosing the Microchiropteran Crown Group ......................... 99 Character Transformations at Basal Nodes: A Functional Perspective ............. 101 Evolution of Echolocation and Foraging Strategies ................................ 107 Timing of the Origin of Flight and Echolocation ............................... 108 Vision and the Evolution Of Echolocation ..................................... 111 Foraging Ecology of Eocene Bats ............................................ 119 Evolution of Foraging Strategies: A Phylogenetic Perspective .................... 129 Classi®cation of Eocene Bats ................................................... 133 Conclusions and Summary ..................................................... 139 Acknowledgments ............................................................ 143 References ................................................................... 143 Appendix 1: Specimens Examined .............................................. 170 Appendix 2; Character Descriptions ............................................. 172 Appendix 3: Data Matrix ...................................................... 180 4 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 235 ABSTRACT The Eocene fossil record of bats (Chiroptera) and obstacle detection but not detection or track- includes four genera known from relatively com- ing of airborne prey. Owing to the mechanical plete skeletons: Icaronycteris, Archaeonycteris, coupling of ventilation and ¯ight, the energy costs Hassianycteris, and Palaeochiropteryx. Phyloge- of echolocation to ¯ying bats were relatively low. netic relationships of these taxa to each other and In contrast, the bene®ts of aerial insectivory were to extant lineages of bats were investigated in a substantial, and a more sophisticated low-duty-cy- parsimony analysis of 195 morphological char- cle echolocation system capable of detecting, acters, 12 rDNA restriction site characters, and tracking, and assessing airborne prey subsequent- one character based on the number of R-1 tandem ly evolved rapidly. The need for an increasingly repeats in the mtDNA d-loop region. Results in- derived auditory system, together with limits on dicate that Icaronycteris, Archaeonycteris, Has- body size imposed by the mechanics of ¯ight, sianycteris, and Palaeochiropteryx represent a se- echolocation, and prey capture, may have resulted ries of consecutive sister-taxa to extant microchi- in reduction and simpli®cation of the visual system ropteran bats. This conclusion stands in contrast as echolocation became increasingly important. to previous suggestions that these fossil forms Our analysis con®rms previous suggestions that represent either a primitive grade ancestral to both Icaronycteris, Archaeonycteris, Hassianycteris, Megachiroptera and Microchiroptera (e.g., Eochi- and Palaeochiropteryx used echolocation. Forag- roptera) or a separate clade within Microchirop- ing strategies of these forms were reconstructed tera (e.g., Palaeochiropterygoidea). A new higher- based on postcranial osteology and wing form, level classi®cation is proposed to better re¯ect hy- cochlear size, and stomach contents. In the con- pothesized relationships among Eocene fossil bats text of our phylogeny, we suggest that foraging and extant taxa. Critical features of this classi®- behavior in the microchiropteran lineage evolved cation include restriction of Microchiroptera to in a series of steps: (1) gleaning food objects dur- the smallest clade that includes all extant bats that ing short ¯ights from a perch using vision for ori- use sophisticated echolocation (Emballonuridae 1 entation and obstacle detection; prey detection by Yinochiroptera 1 Yangochiroptera), and formal passive means, including vision and/or listening recognition of two more inclusive clades that en- for prey-generated sounds (no known examples in compass Microchiroptera plus the four fossil gen- fossil record); (2) gleaning stationary prey from a era. perch using echolocation and vision for orienta- Comparisons of results of separate phylogenet- tion and obstacle detection; prey detection by pas- ic analyses including and subsequently excluding sive means (Icaronycteris, Archaeonycteris); (3) the fossil taxa indicate that inclusion of the fossils perch hunting for both stationary and ¯ying prey changes the results in two ways: (1) altering per- using echolocation and vision for orientation and ceived relationships among extant forms at a few obstacle detection; prey detection and tracking us- poorly supported nodes; and (2) reducing per- ing echolocation for ¯ying prey and passive ceived support for some nodes near the base of means for stationary prey (no known example, al- the tree. Inclusion of the fossils affects some char- though Icaronycteris and/or Archaeonycteris may acter polarities (hence slightly changing tree to- have done this at times); (4) combined perch hunt- pology), and also changes the levels at which ing and continuous aerial hawking using echolo- transformations appear to apply (hence altering cation and vision for orientation and obstacle de- perceived support for some clades). Results of an tection; prey detection and tracking using echo- additional phylogenetic analysis in which soft-tis- location for ¯ying prey and passive means for sta- sue and molecular characters were excluded from tionary prey; calcar-supported uropatagium used consideration indicate that these characters are for prey capture (common ancestor of Hassianyc- critical for determination of relationships among teris and Palaeochiropteryx; retained in Palaeo- extant lineages. chiropteryx); (5) exclusive reliance on continuous Our phylogeny provides a basis for evaluating aerial hawking using echolocation and vision for previous hypotheses on the evolution of ¯ight, orientation and obstacle detection; prey detection echolocation, and foraging strategies. We propose and tracking using echolocation (Hassianycteris; that ¯ight evolved before echolocation, and that common ancestor of Microchiroptera). The tran- the ®rst bats used vision for orientation in their sition to using echolocation to detect and track arboreal/aerial environment. The evolution of prey would have been dif®cult in cluttered en- ¯ight was followed by the origin of low-duty-cy- vionments owing to interference produced by cle laryngeal echolocation in early members of multiple returning echoes. We therefore propose the microchiropteran lineage. This system was that this transition occurred in bats that foraged in most likely simple at ®rst, permitting orientation forest gaps and along the edges of lakes and rivers 1998 SIMMONS AND GEISLER: RELATIONSHIPS OF EOCENE BATS 5 in situations where potential perch sites were ad- The evolution of continuous aerial hawking jacent to relatively clutter-free open spaces. Aerial may have been the ``key innovation'' responsible hawking using echolocation to detect, track, and for the burst of diversi®cation in microchiropteran evalute prey was apparently the primitive foraging bats that occurred during the Eocene. Fossils re- strategy for Microchiroptera. This implies that ferable to six major extant lineages are known gleaning, passive prey detection, and perch hunt- from Middle±Late Eocene