Fig. 3.5 The hand morphology of basal theropods, maniraptorans, and birds in a phylogenetic context illustrating the controversy of digit identity in birds.

Dinosauria, only the maniraptorans possess eggs with modern avian characteristics (e.g. egg asymmetrical with an air sac, shell composed of multiple layers with prismatic condition) (Figure 3.4) (Varricchio et al., 1997; Grellet-Tinner & Chiappe, 2004; Grellet-Tinner et al., 2006;Varricchio & Jackson, 2004a,b). The development of increasing parental care within the phylogenetic tree of ther-opods as it approaches Aves can be documented through the discovery of nesting adults such as the oviraptorid Citipati osmolska (Figure 3.3) (Clark et al., 1999). What can be inferred from the fossil record on nonavian maniraptoran behavior suggests both brooding and parental care, as in living birds (Grellet-Tinner & Chiappe, 2004), as well as precociality (Erickson et al., 2001) and paternal care (Varricchio et al., 2008), which are inferred to be the ancestral conditions for modern birds (McKitrick, 1992).

Histological studies of bone tissue have revealed that avian growth rates had their origins within Dinosauria (Erickson et al., 2001, 2007).

There is a general trend that can be observed within Mesozoic avians, from primitive growth in Archaeopteryx with multiple visible lines of arrested growth (LAGs), as present in many other dinosaurs, to the development of the modern strategy of rapidly achieving terminal size in shorter periods of time higher in the clade (Chinsamy et al., 1995; Chinsamy & Elzanowski, 2001; Erickson et al., 2009). Like Archaeopteryx, the more derived Cretaceous enantiornithines grew slowly, with adult specimens preserving multiple lines of growth (Chinsamy et al., 1995). The basal ornithuromorphs Patagopteryx and Hollanda, taxa more closely related to modern birds than either Archaeopteryx or the enantiornithines, show a single LAG, suggesting more rapid growth (Chinsamy et al., 1994, 1995;Bell et al., 2010) compared to birds outside the ornithuromorph clade. The modern bone microstructure of the ornithurine Hesperornis, a derived ornithuro-morph closely related to modern birds, indicates that growth patterns comparable to those of living birds may have been achieved prior to the origin of the crown clade (Chinsamy et al., 1995).

The developmental plasticity that characterizes the osteogenesis of modern birds (Starck & Chinsamy, 2002) may also have developed even earlier within avian evolutionary history; de Ricqles et al. (2003) suggested that the Early Cretaceous Confuciusornis had uninterrupted growth with rates comparable to modern birds. This claim, however, is not supported by a recent mor-phometric study, which suggests a slower and possibly discontinuous growth pattern for Confu-ciusornis (Chiappe et al., 2008). The bone histology of this abundant but geographically restricted Mesozoic fossil may need to be re-evaluated in light of the significant difference in size between specimens (Chiappe et al., 2010).

Most of the arguments made by opponents of the theropod origin of birds are weak (Welman, 1995; Feduccia, 1996; Kurochkin, 2006) and there is very little cladistic support for an alternative hypothesis (Witmer, 1991; James & Pourtless, 2009). Until recently, the most compelling criticism of the theropod ancestry of birds was based on the differential interpretation of the ossified digits of the hand in modern birds and nonavian thero-pod dinosaurs (Figure 3.5). During the development of the manus in extant bird embryos, five points of condensation appear and only the middle three develop, presumably II-III-IV of the I-II-III-IV-V buds. This has led embryologists to infer a II-III-IV manual formula for living birds (alular, major, and minor, respectively; Burke & Feduccia, 2007). However, the three digits present in nonavian maniraptorans are interpreted as corresponding to digits I-II-III of the ancestral penta-dactyl hand (I-V) (Shubin, 1994), based on interpretations of the early theropod fossil record (Sereno & Novas, 1992; Sereno, 1993; Padian & Chiappe, 1998). Basal theropods such as Herrera-saurus and Eoraptor retain five digits but display reduction in the outer two digits IV and V, leading to the interpretation that more advanced thero-pods retain digits I-II-III (Figure 3.5; Sereno 1993). This discrepancy in the perceived homologies of the hands of nonavian theropods and birds had led some researchers to exclude Aves from Theropoda

(Feduccia & Nowicki, 2002; Kurochkin, 2006; Burke & Feduccia, 2007), while others have attempted to resolve this apparent inconsistency with hypotheses regarding gene regulatory mechanisms, in which the identity of the digit is transferred so that a given digit arises from a different condensation (Wagner & Gauthier, 1999). While this frame shift is possible, and known to occur in a number of vertebrates (Shapiro, 2002), new evidence also suggests the possibility that the inferred pattern of reduction within theropods may have been misinterpreted, as the Triassic fossil record is admittedly extremely fragmentary (Galis et al., 2005). This is supported by Morse's Law (which states that in most tetrapod lineages digits V and I become reduced prior to other digits), for which theropods are currently considered one of a few exceptions (Shubin, 1994), and the recent discovery of a cer-atosaur theropod with digital reduction in digits I and IV and no digit V (Xu et al., 2009a). Despite the systematic placement of this taxon outside Teta-nurae (the theropod clade that includes manirap-torans and birds), the new ceratosaur theropod Limusaurus inextricablis shares derived features with tetanurans suggesting a close relationship, and the reinterpretation of the tetanuran manus as II-III-IV (Figure 3.5; Xu et al., 2009a). The disparity in the manual morphology between birds and nonavian dinosaurs has long been pitched by opponents as a major flaw in the theropod hypothesis (Feduccia, 1996, 2001; Kur-ochkin, 2006). While this debate cannot be considered closed based on the reduction of digit I in one taxon, it is important to consider the highly fragmentary nature of the Triassic and Early Jurassic theropod fossil record. As demonstrated by Limurasaurus, new discoveries have the potential to radically change our interpretation of early theropod evolution. For this reason the digits of the avian hand are here referred to as the alular (I or II), major (II or III) and minor (III or IV).

Even though it is now generally accepted that birds are theropod dinosaurs nested within the clade Maniraptora (Turner et al., 2007; Chapter 1), the sister taxon to Aves within this clade is debated and differs between cladistic analyses

(Holtz, 1994; Gauthier, 1986; Sereno, 1997; Forster et al., 1998a; Norell et al., 2001; Huang et al., 2002; Makovicky et al., 2005; Novas & Pol, 2005; Golich & Chiappe, 2006; Turner et al., 2007). Most cladistic analyses place one of two groups as the closest relative to birds: Dromaeosauridae or Troodontidae, and these two clades are often considered to form a more inclusive clade, Deinonychosauria (Forster et al., 1998a; Sereno, 1999; Benton, 2004). Each of these clades possesses a different combination of avian characters distributed amongst the included taxa, suggesting that these groups are more closely related to birds than other theropods. Recent discoveries have identified the bizarre manirap-torans Epidexipteryx (Zhang et al., 2008a) and Anchiornis (Xu et al., 2009b) as apparently even closer to birds than the deinonychosaurs, forming a clade with Aves termed Avialae (Xu et al., 2009b). However, these claims need to be examined in greater detail. The search for the definitive closest relative of birds continues but the overwhelming evidence in favor of a manir-aptoran origin and its wide acceptance within the scientific community allows us to follow this hypothesis for the remainder of this chapter.

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