Open Paleo

Alvarezsauroidea

Bonaparte, 1991 Type genus Alvarezsaurus

Its genera lived 165.3–66.0 million years ago Middle Jurassic–Late Cretaceous

Alvarezsauroidea is a group of small maniraptoran theropod dinosaurs that split into two grades separated chiefly by their hand anatomy. The basal, non-alvarezsaurid alvarezsauroids include long-armed forms such as Haplocheirus from the Late Jurassic of Asia, which retained grasping hands, recurved serrated teeth, and a carnivorous diet interpreted as the ancestral condition for the clade. The derived family Alvarezsauridae, ranging through the Cretaceous, shows a greatly enlarged single functional finger flanked by two tiny vestigial digits, along with a long snout and simplified, closely spaced teeth; these features have been read as adaptations for digging and for feeding on insects such as ants and termites. Most derived members were small, roughly 0.5 to 2 meters in body length, and the clade appears to have undergone a rare trend toward miniaturization. Fossils are best known from Asia but also occur in North and South America and possibly Europe, and current evidence points to an Asian origin followed by repeated dispersal events.

Under dispute

The position of Alvarezsauroidea within Coelurosauria is unsettled. It is variously recovered as an early-diverging maniraptoran, paired with therizinosaurs as the first-branching maniraptorans (Hartman and colleagues, 2019), or as close to Ornithomimosauria — a topology Hartman and others found only four steps longer. Meso and colleagues (2024) instead recover it as the earliest-diverging maniraptoriform clade, an arrangement they describe as poorly supported, although its place within Maniraptoriformes is well supported. The long-armed Jurassic Chinese members are themselves of contested affinity, with ornithomimosaur and compsognathid alternatives proposed for Haplocheirus and its kin (Hartman and colleagues, 2019; Agnolín and colleagues, 2021).

Diagnosis

  1. Elongate basipterygoid processes and a steeply (vertically) inclined basisphenoid
  2. Hypertrophied internal tuberosity of the humerus, separated from the humeral head by a distinct notch and lying level with or projecting above the head
  3. Enlarged humeral ectepicondyle
  4. Conical lateral distal condyle of the femur projecting further distally than the flattened medial condyle
  5. Greatly enlarged ungual on manual digit I, much larger than the unguals of the other digits
  6. First metacarpal with distal condyles lacking collateral ligament fossae

Clades within Alvarezsauroidea

Genera placed directly in Alvarezsauroidea

  • Alnashetri Makovicky et al., 2012 Cenomanian (100.5–93.9 Ma)Argentinaunder dispute
  • Bannykus Xu et al., 2018 Aptian (121.4–113.2 Ma)China
  • Haplocheirus Choiniere et al., 2010 Callovian–Oxfordian (165.3–154.8 Ma)Chinaunder dispute
  • Shishugounykus Qin et al., 2019 Callovian–Oxfordian (165.3–154.8 Ma)Chinaunder dispute
  • Tugulusaurus Dong, 1973 Valanginian–Albian (137.1–100.5 Ma)Chinaunder dispute
  • Xiyunykus Xu et al., 2018 Barremian–Aptian (125.8–113.2 Ma)China

References

  1. Bonaparte, J. F. (1991). Los vertebrados fósiles de la Formación Río Colorado, de la Ciudad de Neuquén y Cercanías, Cretácico Superior, Argentina [The vertebrate fossils of the Rio Colorado Formation, from the city of Neuquén and surrounding areas, Upper Cretaceous, Argentina]. Revista del Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" e Instituto Nacional de Investigación de las Ciencias Naturales: Paleontología 4(3): 17-123.

    Erected Alvarezsauridae on Alvarezsaurus; Alvarezsauroidea is coordinate with it.

    bonaparte1991a
  2. Choiniere, J. N.; Xu, X.; Clark, J. M.; Forster, C. A.; Guo, Y.; Han, F. (2010). A Basal Alvarezsauroid Theropod from the Early Late Jurassic of Xinjiang, China. Science 327(5965): 571-574. American Association for the Advancement of Science (AAAS). doi:10.1126/science.1182143

    Described Haplocheirus and recovered it as the earliest-diverging alvarezsauroid, confirming a basal position for the clade within Maniraptora.

    choiniere2010a
  3. Hartman, S.; Mortimer, M.; Wahl, W. R.; Lomax, D. R.; Lippincott, J.; Lovelace, D. M. (2019). A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight. PeerJ 7: e7247. PeerJ. doi:10.7717/peerj.7247

    Recovered Alvarezsauroidea with Therizinosauria as the first-branching maniraptorans; an ornithomimosaur sister-group topology is four steps longer.

    hartman2019a
  4. Agnolín, F. L.; Lü, J. C.; Kundrát, M.; Xu, L. (2021). Alvarezsaurid osteology: new data on cranial anatomy. Historical Biology 34(3): 443-452. doi:10.1080/08912963.2021.1929203

    Questioned the alvarezsauroid affinities of Haplocheirus and other Jurassic Chinese forms on cranial grounds, arguing ornithomimosaur resemblances; ran no cladistic analysis.

    agnolín2021b
  5. Meso, J. G.; Pol, D.; Chiappe, L.; Qin, Z.; Díaz-Martínez, I.; Gianechini, F.; Apesteguía, S.; Makovicky, P. J.; Pittman, M. (2024). Body size and evolutionary rate analyses reveal complex evolutionary history of Alvarezsauria. Cladistics 41(2): 135-155. Wiley. doi:10.1111/cla.12600

    Recovered Alvarezsauria within Maniraptoriformes with Bremer support of 3 and jackknife of 80%, and as its earliest-diverging clade with Bremer support of 1 and jackknife below 50%.

    meso2024a