Simplify the following text for a Grade 7 native English-speaking person.
Original: According to weight estimates by Ryosuke Motani a Stenopterygius weighed around 163 - whilst a Ophthalmosaurus icenicus weighed 930 - .
Simplified: According to weight estimates by Ryosuke Motani , a Stenopterygius weighed around 163 - whilst a Ophthalmosaurus icenicus weighed 930 â `` 950kg ( 1.03 â `` 1.04 short tons ) .