| Alvarezsauroids | |
|---|---|
 | |
| Skeletal mount ofPatagonykus | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Maniraptora |
| Clade: | †Alvarezsauria Bonaparte, 1991 |
| Superfamily: | †Alvarezsauroidea Bonaparte, 1991 |
| Type species | |
| †Alvarezsaurus calvoi Bonaparte, 1991 | |
| Subgroups | |
Alvarezsauroidea (from the Argentine historian, writer and physicianGregorio Álvarez) is a group of smallmaniraptorandinosaurs. The group was first formally proposed by Choiniere and colleagues in 2010, to contain the familyAlvarezsauridae and non-alvarezsaurid alvarezsauroids, such asHaplocheirus,[2] which is the basalmost of the Alvarezsauroidea (from the LateJurassic,Asia). The discovery ofHaplocheirus extended the stratigraphic evidence for the group Alvarezsauroidea about 63 million years further in the past. The division of Alvarezsauroidea into theAlvarezsauridae and the non-alvarezsaurid alvarezsauroids is based on differences in theirmorphology, especially in their hand morphology.
Fossils of alvarezsauroids were described since the 1990s, with many alvarezsauroids being found in Asia and other members also known from North America, South America and possibly Europe.[2] The body length of the derived members of Alvarezsauroidea range between 0.5 and 2 m (1 ft 8 in and 6 ft 7 in), but some members may have been larger.[3]Haplocheirus, for example, was among the largest member of the Alvarezsauroidea. Because of the size ofHaplocheirus and its basal phylogenetic position, a pattern of miniaturization for the Alvarezsauroidea is suggested. Miniaturizations are very rare in dinosaurs, but convergently evolved inParaves.[2]
The differences in the morphology of the hand of basic Alvarezsauroidea and the derived members are characterized by digit reduction. In the evolution oftheropod dinosaurs, modifications of the hand were typical. The digital reduction, for instance, is a striking evolutionary phenomenon that is clearly exemplified intheropod dinosaurs.[2]
The enlargement of the manual digit II in alvarezsauroids and the concurrent reduction of the lateral digits, created one functional medial digit and two very small, and presumably vestigial, lateral digits. These morphological changes have been interpreted as adaptations for digging. One possible interpretation suggests that alvarezsauroids fed on insects, using their hands to search beyond the tree bark. This interpretation is consistent with their long, elongate snout and small teeth. Another interpretation suggests that they used their claws to break into ant and termite colonies, though the arm anatomy of an alvarezsaurid would require the animal to lie on its chest against a termite nest.[4]
In contrast to the digit reduction of the hand of derived alvarezsauroid to a claw used for digging,Haplocheirus was still able to grab things. However,Haplocheirus already shows the enlargement of the second manual digit. Important data on the evolution of the alvarezsauroid hand is also provided by the basalparvicursorineLinhenykus.[5] Another difference betweenAlvarezsauridae andHaplocheirus is the dentition. While alvarezsauroids show a simplified homogenous dentition,Haplocheirus on the other side possesses recurved serrated teeth. The dentition ofHaplocheirus and their basal phylogenetic position, suggest thatcarnivory was the primitive condition for the clade. Furthermore,Haplocheirus possesses more teeth on themaxilla than other alvarezsauroids.[2]
In 2025, Wang and colleagues identified hard tissue debris and possible soft tissues in the intestinal contents of theBannykus holotype (represented by the yellowish conglomerate) based onscanning electron microscopy, which was consistent with furtherRaman spectroscopy analysis that detected "characteristic peaks indicative of bone-derived material", suggesting thatBannykus probably had acarnivorous diet. The authors concluded that both the forelimb reduction and the dietary shift from carnivory to probablyinsectivory among alvarezsaurians occurred later in their evolution.[6]
The phylogenetic placement of Alvarezsauroidea is still unclear. At first, they were interpreted as a sister group ofAvialae (birds) or nested within the groupAvialae[2] and considered to be flightless birds,[5] because they share many morphological characteristics with them, such as a loosely sutured skull, a keeledsternum, fused wrist elements, and a posteriorly directedpubis.[2] But this association was reevaluated after the discovery of the primitive forms likeHaplocheirus,Patagonykus andAlvarezsaurus, which do not show all bird-like features as the first discovered speciesMononykus andShuvuuia.[4] This shows that bird-like characteristics were developed multiple times within theManiraptora. Furthermore, the Alvarezsauroidea had simplified homogenous dentition, convergent with that of some extant insectivorousmammals. More recently, they have been placed within theCoelurosauria basal to theManiraptora or as a sister taxon ofOrnithomimosauria within theOrnithomimiformes.
The cladogram below is the latest and most comprehensive analysis of alvarezsaurian evolution, following Meso et al. (2024).[7]
| Alvarezsauria | |
At first, alvarezsauroids were thought to have been originated inSouth America. However, the discovery ofHaplocheirus, and its basal phylogenetic position, as well as its early temporal position, suggests they derived inAsia rather thanSouth America. Xuet al. (2011) suggested that at least three dispersal events of alvarezsauroids took place; one from Asia toGondwana, one fromGondwana to Asia, and one from Asia toNorth America. This hypothesis is consistent with faunal interchanges.[5] On the other hand, sometheropod groups are inconsistent with thishypothesis. Xuet al. (2013) used event−based tree−fitting to perform a quantitative analysis of alvarezsauroid biogeography.[8] Their results showed an absence of statistical support for previous biogeographic hypotheses that favour pure vicariance or pure dispersal scenarios as explanations for the distributions of alvarezsauroids across South America, North America and Asia. They instead found that statistically significant biogeographic reconstructions suggest a dominant role for sympatric events ("within area" ones), combined with a mix of vicariance, dispersal and regional extinction. The Asian origin of alvarezsauroids is also bolstered by the discovery of alvarezsaurid specimens from the Turonian-ageBissekty Formation (some of which were namedDzharaonyx in 2022[9]) of Uzbekistan andBannykus,Tugulusaurus, andXiyunykus from the Early Cretaceous of China.[10][11]
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