Task: Question Answering
Input (English): Passage: Altar M this modest monument is the earliest known monument dedicated by K'ak' Tiliw Chan Yopaat. The importance of this monument lies in its text, in which this preeminent king of Quiriguá claimed the title of k'uhul ajaw, holy lord, and began his bid for independence from Copán. This rhyolite sculpture was dedicated on 15 September 734 and has the form of a monstrous head, possibly that of a crocodilian.
Altar N is another small rhyolite sculpture stylistically similar to Altar M. This sculpture has the form of a turtle shell with a skeletal head with a mirror on its forehead emerging sideways from one end and an elderly figure from the other. This is a representation of the bicephalic deity Pawatun (God N), a prominent underworld deity.
Zoomorph O is a crocodile-mountain hybrid monster, dedicated in 790 by king "Sky Xul". It is accompanied by an altar depicting a lightning god. It is located in the Ballcourt Plaza, just south of the ballcourt itself.
Zoomorph P (which explorer Maudslay nicknamed The Great Turtle) was dedicated in 795 by "Sky Xul" and is a masterpiece of Mesoamerican art. It weighs around 20 tons. On one side it depicts a larger-than-life portrait of "Sky Xul" himself seated cross-legged in the open jaws of an enormous crocodile-mountain hybrid monster. The design of this zoomorph is incredibly intricate and the whole monument is covered with skilfully executed sculpture. It is located in the Ballcourt Plaza, just south of the ballcourt. Zoomorph P is accompanied by an altar depicting an unidentified deity leaping from a split in the earth. A hieroglyphic text on the zoomorph describes the founding of Quiriguá under the supervision of the king of Copán. Traces of red pigment have been found on this monument, suggesting that it was originally painted red.
Altar Q and Altar R are two small rhyolite disks that probably served as ballcourt markers for the earliest ballcourt, the buried Structure 1B-sub.4. Together with a third stone they would have marked the central axis of the ballcourt. They both bear seated cross-legged figures carved in shallow relief.
Stela S is the earliest surviving monument of K'ak' Tiliw Chan Yopaat, it dates to 746. It was originally located in the northern half of the Great Plaza but was moved to an outlying group in ancient times. It is heavily eroded, some of the damage may have been inflicted by the process of moving it. It was fashioned from sandstone and bears the figure of K'ak' Tiliw Chan Yopaat on the front, the other three sides being covered by hieroglyphic text. Unfortunately, due to the heavy erosion most of the text is illegible. Stela S is 2.8 metres (9 ft) high (not including the part of the stela buried in the ground) and the dimensions of the base are 1.6 metres (5.2 ft) by 1.2 metres (3.9 ft), making it the earliest of the huge stelae that were to characterise Quiriguá, although it is significantly smaller than those that were to follow. 
Question: What small discs bear seated cross-legged figures carved in shallow relief and probably served as ballcourt markers?
Output (English): Altar Q.